担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1016/j.jsg.2023.104977

担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： https://doi.org/10.1029/2022GC010792

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Mineralogist  

Aluminum-rich and Si-poor calcium amphibole [~3.9 Al atoms per formula unit (apfu) and ~5.5 Si apfu for 23 O] occur in the quartz-bearing eclogites from the Donghai area, Sulu ultrahigh-pressure metamorphic belt, eastern China. Most of the aluminous amphibole phases are retrograde products from the exhumation and hydration stage and are texturally divided into a mantle phase around a porphyroblastic garnet and a crack-filling (vein) phase of a garnet. Less aluminous amphibole occurs as symplectite phase with plagioclase after omphacite. The formation process of the aluminous amphibole in the quartz-bearing samples is discussed on the basis of the analytical data by EPMA, FIB-TEM, and EBSD. The mantle amphibole occurs between garnet and symplectite or quartz. A set of plagioclase and aegirinediopside/argirine-hedenbergite thin monomineralic bands forms at the boundary between the mantle amphibole and matrix quartz. However, these monomineralic bands do not occur at the mantle amphibole-symplectite boundary. These textural diferences indicate that the recrystallization of the aluminous amphibole around garnet was controlled by significant local reactions, and the size of equilibrate domains was probably several tens of micrometers or less. The mantle amphibole is composed of inner (garnet-side) and outer (matrix-side) zones. The inner zone is compositionally homogeneous, and its atomic Al/Si value is ~0.63-0.66 and similar to that of garnet. Atomic Ca/Si value in the inner zone is also almost uniform and is generally identical to that of garnet. The outer zone exhibits a monotonic decrease in the Al/Si and Ca/Si values outward, and its composition at the outermost margin is similar to that of the symplectitic amphibole. The crack-filling amphibole has a composition similar to the inner zone of the mantle amphibole. The CPO pattern of the crack-filling amphibole is diferent from that of the adjacent mantle amphibole, showing that the crack-filling amphibole is cut by the mantle amphibole. The textural relationship between the mantle and crack-filling amphibole phases and their compositional characteristics imply that: (1) the mantle type is a slightly later stage product than the crack-filling type, and (2) the boundary between the inner and outer zones of the mantle aluminous amphibole probably corresponds to the initial surface of the porphyroblastic garnet. The inner zone is considered to have grown inward by simple substitution of garnet, using the tetrahedral and octahedral cations of the garnet as the basic framework. On the other hand, most of the outer zone of the mantle-type amphibole grew outward in the matrix from the initial surface of the garnet porphyroblast. The mantle amphibole shows a CPO similar to that of amphibole in the adjacent symplectite domain, suggesting that these two types of amphibole formed almost simultaneously, sharing crystallographic orientation with each other. The formation of crack-filling aluminous amphibole was probably promoted by the hydraulic microfracturing process at an early stage of exhumation and hydration. The mantle and symplectitic amphibole phases formation was promoted by the subsequent infiltration of metamorphic fluid. The aluminous amphibole in the SiO2 phase-bearing eclogites probably recrystallized with the formation of a localized SiO2-undersaturated reaction domain because of rapid exhumation and subsequent rapid cooling of the Sulu UHP metamorphic belt.

DOI： 10.2138/am-2022-7996

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

Geophysical surveys of the oceanic crust indicate that hydrothermal circulation universally occurs in seismic layer 2, which results in a low seismic velocity and high VP/VS due to the occurrence of cracks. However, the anomalously low VP/VS observed at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VS due to crack development. In this study, we present the first evidence that shows the low VP/VS in the oceanic crust is caused by epidotization due to upward fluid flow in hydrothermal systems. Simultaneous measurements of elastic wave velocity and porosity of epidosites collected by the Oman Drilling Project show that quartz precipitation and spheroidal pores results in low VP/VS, in contrast to diabases that contain thin cracks. The presence of spheroidal pores in epidosites is supported by CT imaging, and is consistent with predictions from the effective medium model.

DOI： 10.1029/2022GL098234

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Progress in Earth and Planetary Science  

Peridotites at water depths of 3430 to 5999 m have been discovered using the submersible Shinkai6500 (dives 6K-1397 and 6K-1398) on the southwestern slope of the 139°E Ridge (11°12′N, 139°15′E), a small ridge at the southwesternmost tip of the Mariana forearc near the junction with the Yap Trench and Parece Vela Basin. The peridotites studied consist of 17 residual harzburgites and one dunite and show various textures with respect to their depths. Peridotites with coarse-grained (> 1 mm) textures were sampled from the shallowest part (3705–4042 m) of the dive area, and peridotites with fine-grained (< 0.5 mm) textures were sampled deeper (5996 m). Olivine crystal-fabrics vary with grain size, with (010)[100] A-type patterns for the coarse-grained peridotites, {0kl}[100] D-type patterns for the fine-grained peridotites, and various indistinct patterns in samples of variable grain sizes. Fine-grained peridotites with D-type olivine crystal-fabrics could result from deformation under relatively higher flow stresses, suggesting that a ductile shear zone in the lithospheric mantle could occur in the deepest part of 139°E Ridge. Spinel Cr# range from relatively low (0.36) to moderately high (up to 0.57), and correlate with Ti contents (0.07–0.45 wt.%). The trace element patterns of clinopyroxene similarly exhibit steepening slopes from the middle to the light REEs regardless of textural variations. These mineralogical and geochemical features would result from melt-rock interactions under conditions of relatively shallow lithospheric mantle, which are much more comparable with the Parece Vela Basin peridotites than the Mariana forearc peridotites. Consequently, the Parece Vela Basin mantle is more likely exposed on the inner slope of the westernmost Mariana Trench, presumably due to the collision of the Caroline Ridge. [Figure not available: see fulltext.].

DOI： 10.1186/s40645-022-00476-5

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Minerals  

Dynamically recrystallizing quartz is believed to approach a steady-state microstructure, which reflects flow stress in dislocation creep. In a classic experimental study performed by Masuda and Fujimura in 1981 using a solid-medium deformation apparatus, two types of steady-state microstructures of quartz, denoted as S and P, were found under varying temperature and strain rate conditions. However, the differential stresses did not systematically change with the deformation conditions, and unexpectedly high flow stresses (over 700 MPa) were recorded on some experimental runs compared with the applied confining pressure (400 MPa). Internal friction in the sample assembly is a possible cause of reported high differential stresses. Using a pyrophyllite assembly similar to that used in the previous work and setting up paired load cells above and below the sample assembly, we quantified the frictional stress acting on the sample and corrected the axial stress. The internal friction changed in a complicated manner during pressurization, heating, and axial deformation at a constant strain rate. Our results suggest that Masuda and Fujimura overestimated the differential stress by about 200 MPa in their 800◦ C runs. Crystallographic fabrics in the previous experimental sample indicated that the development of elongated quartz grains, which are characteristics of Type-S microstructures, was associated with preferential growth of unfavorably oriented grains during dynamic recrystallization.

DOI： 10.3390/min12030329

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

A shear zone developed in the Teshima granite in the Ryoke metamorphic belt of Japan contains isolated K-feldspar inclusions showing various shapes within deformed host quartz grains. We present a shape-change model for isolated K-feldspar inclusions, which is a mathematical equation that considers stress, inclusion size, temperature, and the duration of deformation. To calculate the shape-change pattern (i.e., the aspect ratio and size of inclusions) with respect to deformation duration using the model, we used values for deformation temperature (~500–600 °C) and paleo-stress (90 ± 10 MPa) estimated from deformed quartz microstructures within the shear zone. Shape-change patterns were obtained by calculating the model at the estimated range of temperatures with an interval of 25 °C and at the estimated paleo-stress. Shape-change patterns were subsequently corrected by compensating for the effect of post-deformation annealing during exhumation of the shear zone. Deformation duration was estimated by identifying the shape-change pattern calculated by the model that most closely corresponded to the observed shape-change pattern for the shear zone and considering the cooling rate of the Ryoke belt. The resultant deformation duration of the shear zone is estimated as 16,500–8400 yr under conditions of T = 550 °C and σ = 90 ± 10 MPa. Shear strain in the shear zone is calculated as 50–260 by multiplying the shear strain rate by the deformation duration.

DOI： 10.1016/j.tecto.2022.229229

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25–40 km by reaction with CO2-rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400°–500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites—perhaps facilitated by fluid-rock reaction—in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, mainly at a serpentinization front.

DOI： 10.1029/2021jb022352

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JB022352

担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

Microstructures and olivine crystal fabrics were studied in amphibole-bearing peridotite samples obtained from the Marion Fracture Zone of the Southwest Indian Ridge by dredge D19 of the 1984 PROTEA Expedition Leg 5 cruise of the RV Melville. The peridotites show various textures ranging from extremely fine-grained well-layered ultramylonites to heterogeneously strained tectonites. Electron back-scatter diffraction analyses revealed that olivine crystal-preferred orientations (CPOs), which are developed primarily in coarse granular peridotites in the mantle, become weaker with an increasing degree of grain-size reduction from coarser to finer grains, for both porphyroclastic and matrix olivine grains. However, two well-layered ultramylonites are characterized by bimodal CPOs of (010)[001] (B type) and (001)[100] (E type) or a strong maximum of [010] normal to the foliation and girdle patterns of both [100] and [001] on the foliation plane (i.e., an axial [010] pattern or AG type). Moreover, spinel grains within these well-layered ultramylonites have not only been broken down to form olivine and amphibole by hydrous reactions, but have also been fractured and their fragments pulled apart in the fine-grained matrix. These features indicate that shear deformation occurred as increasing stress under hydrous conditions during the final stage of deformation, which enabled the local occurrence of low-temperature plastic deformation, resulting in the development of a CPO and a foliation within the ultramylonites.

DOI： 10.1016/j.tecto.2021.229206

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

Mantle deformation processes leading to seafloor spreading are often difficult to infer due to the highly serpentinized and weathered state of most abyssal peridotites. We investigated the development of high-temperature crystal-plastic deformation and lower temperature mylonitization processes in relatively fresh (<50% modal serpentine) and ultra-fresh (<1% serpentine) mantle peridotites derived from the heterogeneous mantle in the sparsely magmatic zone of ultraslow-spreading Gakkel Ridge system by analyzing 12 peridotites from two dredge sites (<1 km apart). Microstructurally, these 12 peridotites consist of seven high-T deformed samples and five mylonites. Modally, the 12 samples include harzburgites, lherzolites, an olivine websterite, and a plagioclase-bearing lherzolite. Based on their mineral major and trace element compositions, the lherzolites, harzburgites, and olivine websterite are residual peridotites. The lherzolites containing clinopyroxenes with flat REE patterns likely underwent refertilization with a high influx of melt. The plagioclase-bearing lherzolites probably formed by subsolidus reaction after the partial melting process. Microstructural observations support that high-T crystal-plastic deformation (most likely at temperatures exceeding 1000 °C) was active in the peridotites of the high-T deformation group, accommodating mantle flow beneath the Gakkel Ridge. The identified melt refertilization process may have contributed to the formation of [010]-fiber olivine fabrics in these peridotites. Mylonitic microstructures, decreasing fabric strength and grain-size reduction of olivine suggest that mylonitization occurred under relatively low-temperature mantle conditions (~800 °C) and probably accommodated strain localization. Water did not greatly affect the peridotites during the development of the shear zones, although amphibole with “dusty” zones developed in one mylonitic peridotite after mylonitization, indicating that late-stage metasomatism occurred locally within the shear zone. This low-T mylonitization is likely to have affected mantle peridotites of this region independently of petrogenetic processes. The development of these deformation processes in Gakkel Ridge suggests a shift from flow in the uppermost mantle to shear zone formation in the rift valley walls.

DOI： 10.1016/j.tecto.2021.229186

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Completely carbonated peridotites represent a window to study reactions of carbon-rich fluids with mantle rocks. Here, we present details on the carbonation history of listvenites close to the basal thrust in the Samail ophiolite. We use samples from Oman Drilling Project Hole BT1B, which provides a continuous record of lithologic transitions, as well as outcrop samples from listvenites, metasediments, and metamafics below the basal thrust of the ophiolite. 87Sr/86Sr of listvenites and serpentinites, ranging from 0.7090 to 0.7145, are significantly more radiogenic than mantle values, Cretaceous seawater, and other peridotite hosted carbonates in Oman. The Hawasina sediments that underlie the ophiolite, on the other hand, show higher 87Sr/86Sr values of up to 0.7241. δ13C values of total carbon in the listvenites and serpentinites range from −10.6‰ to 1.92‰. We also identified a small organic carbon component with δ13C as low as −27‰. Based on these results, we propose that during subduction at temperatures above >400°C, carbon-rich fluids derived from decarbonation of the underlying sediments migrated updip and generated the radiogenic 87Sr/86Sr signature and the fractionated δ13C values of the serpentinites and listvenites in core BT1B.

DOI： 10.1029/2021JB022704

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記述言語：英語   出版者・発行元：Earth and Planetary Science Letters  

Episodic tremor and slip (ETS) events in the forearc mantle wedge of a warm subduction zone may reflect mixed brittle–ductile deformation of serpentinite in association with high pore fluid pressures. To understand deformation mechanisms and processes occurring in the hydrated mantle wedge, we examined in the Sanbagawa metamorphic belt, SW Japan, an antigorite serpentinite shear zone derived from mantle wedge that was formed under pressure and temperature conditions that correspond to the ETS regions. The serpentinite underwent multiple extensional (mode I) and extensional–shear (mode I–II) failure events at supralithostatic pore fluid pressures (Pf). Such failure events led to drops in Pf (several MPa) and formation of a distributed ‘fault–fracture mesh’. Antigorite precipitation in the fracture openings contributed to an increase in Pf until the failure condition was reached again, and thereby antigorite kinetics controlled the recurrence interval of seismic events. We also suggest that under the low-Pf conditions that facilitate intracrystalline plasticity rather than cataclasis, the newly precipitated antigorite aggregates (localized along shear bands) deform by dislocation creep at a high strain rate and high shear stress, resulting in the transient, accelerated viscous creep that may characterize slow slip transients.

DOI： 10.1016/j.epsl.2021.117232

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

<title>Abstract</title>The hadal zone at trenches is a unique region where forearc mantle rocks are directly exposed at the ocean floor owing to tectonic erosion. Circulation of seawater in the mantle rock induces carbonate precipitation within the deep-sea forearc mantle, but the timescale and rates of the circulation are unclear. Here we investigated a peculiar occurrence of calcium carbonate (aragonite) in forearc mantle rocks recovered from ~6400 m water depth in the Izu–Ogasawara Trench. On the basis of microtextures, strontium–carbon–oxygen isotope geochemistry, and radiocarbon analysis, we found that the aragonite is sourced from seawater that accumulated for more than 42,000 years. Aragonite precipitation is triggered by episodic rupture events that expel the accumulated fluids at 10⁻²–10⁻¹ m s⁻¹ and which continue for a few decades at most. We suggest that the recycling of subducted seawater from the shallowest forearc mantle influences carbon transport from the surface to Earth’s interior.

DOI： 10.1038/s43247-021-00317-1

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その他リンク： https://www.nature.com/articles/s43247-021-00317-1

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

We quantified mineral proportions in listvenite (completely carbonated peridotite) in Hole BT1B drilled across the basal thrust of the Samail ophiolite by the International Continental Scientific Drilling Project Oman Drilling Project using 3D X-ray Computed Tomography (XCT). We analyzed >250,000 XCT images from a continuous ∼200 m listvenite core. Histograms of the intensity of X-ray attenuation of each XCT core-slice image were fitted assuming that the listvenites are composed of magnesite, quartz, and dolomite. The XCT mineral peaks were confirmed by comparison with chemical mapping data obtained using an X-ray fluorescence (XRF) core scanner. Listvenite matrix is composed almost entirely of magnesite and quartz, consistent with discrete XRD and XRF data. Veins are composed mostly of dolomite. The mean abundance of dolomite in listvenite from BT1B is 11 vol.%, whereas that in core sections within 15 m of the basal thrust is >50 vol.%, suggesting the basal thrust acted as a source of Ca- and CO2-rich fluids. The SiO2:MgO:CaO molar ratio in the entire core from BT1B is 42:52:6, similar to that of onboard XRF data for discrete samples (41:54:5), whereas average Oman peridotites have ratios of (39:60:1), indicating Ca addition perhaps during carbonation. P- and S-wave velocities and density of listvenite are close to those of peridotite and are higher than those of serpentinites. These results suggest that limited material transfer during carbonation and hydration of the Samail ophiolite, except for Ca, CO2, and H2O. Listvenites formed in the mantle wedge above subduction zones may be an overlooked reservoir for carbon in the Earth's interior.

DOI： 10.1029/2021jb022719

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JB022719

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

The Oman Drilling Project “Multi-Borehole Observatory” (MBO) samples an area of active weathering of tectonically exposed peridotite. This article reviews the geology of the MBO region, summarizes recent research, and provides new data constraining ongoing alteration. Host rocks are partially to completely serpentinized, residual mantle harzburgites, and replacive. Dunites show evidence for “reactive fractionation,” in which cooling, crystallizing magmas reacted with older residues of melting. Harzburgites and dunites are 65%–100% hydrated. Ferric to total iron ratios vary from 50% to 90%. In Hole BA1B, alteration extent decreases with depth. Gradients in water and core composition are correlated. Serpentine veins are intergrown with, and cut, carbonate veins with measurable 14C. Ongoing hydration is accompanied by SiO2 addition. Sulfur enrichment in Hole BA1B may result from oxidative leaching of sulfur from the upper 30 m, coupled with sulfate reduction and sulfide precipitation at 30–150 m. Oxygen fugacity deep in Holes BA3A, NSHQ14, and BA2A is fixed by the reaction 2H2O = 2H2 + O2 combined with oxidation of ferrous iron in serpentine, brucite, and olivine. fO2 deep in Holes BA1A, BA1D, and BA4A is 3–4 log units above the H2O-H2 limit, controlled by equilibria involving serpentine and brucite. Variations in alteration are correlated with texture, with reduced, low SiO2 assemblages in mesh cores recording very low water/rock ratios, juxtaposed with adjacent veins recording much higher ratios. The proportion of reduced mesh cores versus oxidized veins increases with depth, and the difference in fO2 recorded in cores and veins decreases with depth.

DOI： 10.1029/2021jb022729

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JB022729

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

The transition from the Semail ophiolite mantle to the underlying metamorphic sole was drilled at ICDP OmanDP Hole BT1B. We analyzed the bulk major, volatile and trace element compositions of the mantle-derived listvenite series and metamorphic rocks, with the aim to constrain chemical transfers associated with peridotite carbonation along the ophiolite basal thrust. The listvenite series comprise variously carbonated serpentinites and (fuchsite-bearing) listvenites. They have high CO2 (up to 43 wt.%) and variable H2O (0–12 wt.%). Yet, they have compositions close to that of the basal banded peridotites for most major and lithophile trace elements, with fuchsite-bearing listvenites overlapping in composition with amphibole-bearing basal lherzolites (e.g., Al2O3 = 0.1–2.2 wt.%; Yb = 0.05–1 x CI-chondrite). The protolith of the listvenite series was likely similar in structure and composition to serpentinized banded peridotites which immediately overlie the metamorphic sole elsewhere in Oman. The listvenite series are enriched in fluid mobile elements (FME) compared to Semail peridotites (up to ∼103–104 x Primitive Mantle), with concentrations similar to the underthrusted metabasalts and/or metasediments for Cs, Sr and Ca and sometimes even higher for Pb, Li, As, and Sb (e.g., Li up to 130 μg/g; As up to 170 μg/g). We also observe a decoupling between Sr-Ca enrichments and other FME, indicating interactions with several batches of deep CO2-rich fluids transported along the basal thrust. These results suggest that peridotite carbonation could represent one of the major trap-and-release mechanisms for carbon, water and FME along convergent margins.

DOI： 10.1029/2021JB022733

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Progress in Earth and Planetary Science  

This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling. [Figure not available: see fulltext.].

DOI： 10.1186/s40645-021-00454-3

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Seismic velocity structures of the oceanic lithosphere are variable due to the variation of alteration and heterogeneities in porosity. In order to interpret geophysically determined velocity structures, it is necessary to understand how the seismic velocity of the oceanic lithosphere is affected by alteration and porosity. We conducted petrological analysis and elastic wave velocity measurements at 200 MPa on mafic and ultramafic rocks drilled from the Samail ophiolite. The elastic wave velocity calculated from the mineral abundances of the ultramafic rocks was sensitive to serpentinization, whereas that of the mafic rocks was nearly independent of the alteration, suggesting that velocity changes with depth in the oceanic crust are mainly due to porosity reduction. Based on the effective medium theory and pressure dependence of the experimentally determined velocity, the change in the porosity was estimated to be 0.04%–0.42% at 200 MPa, indicating porosity reduction by the increase of lithostatic pressure in the oceanic crust in the fast-spreading system such as the Pacific plate. We estimated the degree of hydration in the mantle to account for the geophysical observations based on the laboratory experiments. Both the crustal porosity and mantle serpentinization increase towards the trench due to fracture related plate bending, indicating extensive hydration of the oceanic lithosphere close to the trench. From these estimates, water contents in the oceanic lithosphere were calculated to be as high as 3 wt.% in the shallow crust and 6 wt.% in the upper most mantle, which is significant for water transport into Earth's interior.

DOI： 10.1029/2021jb021923

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2021JB021923

担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics and Chemistry of Minerals  

Deformed polycrystalline rocks show various crystallographic textures reflecting their imposed deformation histories. However, the textures of ultrafine grains, which are in the submicrometer to nanometer order, may be overlooked depending on the analytical technique used. Thus, we report the first application of transmission Kikuchi diffraction (TKD), which is capable of high-spatial-resolution crystallographic texture analysis, to a fine-grained ultramylonitic peridotite sample in a scanning electron microscope. We successfully obtained TKD maps with an effective spatial resolution of ~ 80 nm and with highly reliable indexing using a conventional W-filament scanning electron microscope with a standard electron backscattered diffraction (EBSD) system. Olivine grains, which were clearly visualized by TKD, were slightly elongated in a direction subparallel to the macroscopic lineation texture. Their shapes were nonuniform with serrated grain boundaries, strongly indicating that the sample has been deformed dominantly by dislocation activity, even though the grain size is in the order of several micrometers or smaller. The combined TKD–transmission electron microscope (TEM) analysis indicated that a slip-system transition from the [100] slip to the [001] slip might have occurred, although the crystals’ preferred orientation patterns were not completely overwritten. The transition might have been sufficiently affected by water infiltration, high differential stress, or both along the transform fault. Thus, TKD efficiently analyzed the crystallographic textures and characterized the subgrain boundaries of polycrystalline rocks consisting of submicrometer-order grains. Moreover, combining the EBSD, TKD, and TEM methods allowed us to perform multiscale analyses of the crystallographic textures of ultrafine-grained deformed rocks, seamlessly linking the millimeter- to nanometer-order scales.

DOI： 10.1007/s00269-021-01161-7

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記述言語：英語   出版者・発行元：Tectonophysics  

To assess the geometry of cracks in highly altered peridotites, we analyzed the ultrasonic velocity of serpentinized dunites and harzburgites collected by the Oman Drilling Project (Holes BA1B, 3A, and 4A). First, we estimated the hydrated fraction from grain density to obtain the porosity-free matrix velocity, which indicated complete serpentinization at shallow depths and decreasing hydration at greater depths. We assume that the difference between the solid matrix and measured onboard ultrasonic velocity is attributed to cracks with a spheroidal shape in the samples. Application of the effective medium theory to onboard data, such as P-wave velocity and porosity, indicates that the average pore aspect ratio is mostly between 0.1 and 0.01, and crack density varies from 0.58 to 0.02. We found a positive relationship between aspect ratio and hydrated fraction, suggesting a change in crack shape related to dissolution–precipitation processes during hydration. The relatively high aspect ratio and hence high fluid flux at shallow depths are also consistent with the onboard resistivity data and present-day hydration processes inferred from the borehole fluid chemistry. The inversion of ultrasonic data provides a series of elastic moduli that can be used to make a rough approximation of Poisson's ratio from the onboard data, which is a key physical property for interpreting geophysical observations in the oceanic lithosphere.

DOI： 10.1016/j.tecto.2021.228978

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Lithos  

Interactions of hydrous fluid and melt with dry mantle rocks are known to result in metasomatic alteration of the lithospheric mantle. Here we investigate such interactions that occurred beneath the Hangay Dome, Mongolia, in 22 mantle xenoliths, which were recovered from Cenozoic basalts at the Tsagan, Zala, Horgo, and Shavaryn-Tsaram localities near the village of Tariat. The xenoliths are medium- to coarse-grained spinel lherzolites that exhibit variable degrees of reaction with silicate melt and fluid (indicated by their Fe- and LREE-enrichment and the presence of secondary clinopyroxene, amphibole, phlogopite, apatite, and sulfide). According to their normalized REE patterns and microstructures, the spinel lherzolites were divided into three groups. Group 1 lherzolites contain LREE-depleted clinopyroxene and whole-rock compositions, exhibit a greater number of preserved deformation textures, and are the least affected by metasomatism. These lherzolites are interpreted to represent the sub-continental lithosphere before the rejuvenation processes that occurred during the Cenozoic. Group 3 lherzolites are characterized by partial annealing of pre-existing textures, and LREE-enrichment in clinopyroxenes and whole-rock compositions compared to Group 1 lherzolites. Group 3 lherzolites are interpreted to be the result of the interaction of depleted lithospheric mantle with a basaltic melt during the Cenozoic. The lack of correlation between the intensity of metasomatism, the degree of annealing, and the calculated temperatures of the lherzolites suggests that the Cenozoic basaltic melt percolation postdates the static recrystallization. Group 2 lherzolites exhibit characteristics from both Groups 1 and 3. Numerical modelling of the interaction between depleted lherzolites and basaltic melts evidences that: (i) a single initial liquid may fractionate to produce a range of element patterns: the observed spectrum of REE compositions of the Tariat lherzolites cannot have resulted from simple mixing of basaltic melt with a depleted mantle rocks; instead, it can be explained by chromatographic fractionation during reactive porous melt flow; (ii) highly fractionated element patterns are derived from conventional initial melt compositions and do not imply the existence of exotic melts; and (iii) strong element fractionation can be produced along short distances even at the thin section and mineral scale; this opposes the view that long percolation distances are required to produce significant chromatographic effects: the clinopyroxene core-rim disequilibrium demonstrates that REE variations in clinopyroxene rims were acquired in response to interactions with a more evolved REE-rich melt.

DOI： 10.1016/j.lithos.2021.106407

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Minerals  

Deformation microstructures of peak metamorphic conditions in ultrahigh-pressure (UHP) metamorphic rocks constrain the rheological behavior of deeply subducted crustal material within a subduction channel. However, studies of such rocks are limited by the overprinting effects of retrograde metamorphism during exhumation. Here, we present the deformation microstructures and crystallographic-preferred orientation data of minerals in UHP rocks from the Dabie–Shan to study the rheological behavior of deeply subducted continental material under UHP conditions. The studied samples preserve deformation microstructures that formed under UHP conditions and can be distinguished into two types: high-strain mafic–ultramafic samples (eclogite and garnet-clinopyroxenite) and low-strain felsic samples (jadeite quartzite). This distinction suggests that felsic rocks are less strained than mafic–ultramafic rocks under UHP conditions. We argue that the phase transition from quartz to coesite in the felsic rocks may explain the microstructural differences between the studied mafic–ultramafic and felsic rock samples. The presence of coesite, which has a higher strength than quartz, may result in an increase in the bulk strength of felsic rocks, leading to strain localization in nearby mafic–ultramafic rocks. The formation of shear zones associated with strain localization under HP/UHP conditions can induce the detachment of subducted crustal material from subducting lithosphere, which is a prerequisite for the exhumation of UHP rocks. These findings suggest that coesite has an important influence on the rheological behavior of crustal material that is subducted to coesite-stable depths.

DOI： 10.3390/min11080842

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Lithos  

Mafic and ultramafic clasts (mostly ~1–5 cm in size) were recovered from three different serpentinite mud volcanos in the Mariana forearc during Integrated Ocean Drilling Program (IODP) Expedition 366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (~80 mol% jadeite), and aragonite as metamorphic minerals. In contrast, mafic clasts from drill sites closer to the trench are characterized by prehnite–pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at other mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite–blueschist-facies conditions in a subduction zone. Considering the consistency of the low-temperature metamorphic grade of mafic and ultramafic clast mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before the ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearc-mantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain Ca amphibole and talc, which indicates hot mantle hydration by metasomatic fluids released from the slab at subduction initiation.

DOI： 10.1016/j.lithos.2020.105941

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Structural Geology  

Microstructural investigations of mantle xenoliths from the Mt. Melbourne area were undertaken to reveal the origin of S-wave splitting beneath northern Victoria Land, Antarctica. The six analyzed peridotites contain various deformation features. The rotated olivine maxima of [100] and [010] into horizontal and vertical orientations, respectively, are classified into five samples with a D-type crystallographic preferred orientation (CPO) and one sample as an A-type CPO. The D-type olivine fabric can be explained by multiple slip systems of {0kl}[100] at low-temperature and high-stress conditions; therefore, both compressional and extensional regimes during subduction and rifting, respectively, could be applied in this study. With an assumption that olivine a-axes are aligned along the direction of mantle flow to form maximum S-wave splitting, the observed delay time of 0.9–1.3 s beneath northern Victoria Land can be partially explained by the anisotropy in the mantle peridotites. The remaining seismic anisotropy can be explained by the presence of melt pockets trapped along tectonic faults that developed perpendicular to the fast S-wave splitting direction. This study therefore demonstrates that the NE–SW-trending S-wave splitting beneath northern Victoria Land, Antarctica, results from the existence of both mantle peridotites as well as melt pockets trapped along the tectonic faults.

DOI： 10.1016/j.jsg.2020.104237

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本地質学会  

DOI： 10.5575/geosoc.2020.0050

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Mineralogical and Petrological Sciences  

Oxidation states within the planetary interior are intrinsically linked with the broad scale tectonism; however, it is difficult to estimate the actual oxidation conditions. Orthopyroxene-magnetite symplectite formed by olivine oxidation may provide a significant clue into such oxidation events. Here we report detailed mineralogical and petrological synthesis of such orthopyroxene-magnetite symplectites from olivine gabbros of Oman Ophiolite (Hole GT2A, ICDP Oman Drilling Project). In order to understand how oxidation affects different olivine compositions, we employed a phase equilibria approach and computed several temperature-composition diagrams at a fixed pressure (1 kbar). Our experiments predict the coexistence of olivine with Fo75-76 and Fo71 with the orthopyroxene (En79 and En76), respectively, which is remarkably similar to the mineral chemistry obtained from the Oman lower crustal gabbros. From the magnetite content, we also infer that the symplectite formation may have taken place over a range of temperatures (600-1000 °C) via subsolidus olivine oxidation and/or melt (oxidizing)-olivine interaction. The latter is more probable, considering the partial occurrence of orthopyroxene and clinopyroxene rim adjacent to the symplectites.

DOI： 10.2465/jmps.201130f

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

J-GLOBAL

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

J-GLOBAL

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

J-GLOBAL

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本地質学会  

DOI： 10.5575/geosoc.2020.0043

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geochemistry, Geophysics, Geosystems  

Oceanic core complexes (OCCs) represent tectonic windows into the oceanic lower crust and mantle; they are key structures in understanding the tectono-magmatic processes shaping the oceanic lithosphere. We present a petrological and geochemical study of gabbros collected at the Mado Megamullion, a recently discovered OCC located in the extinct Shikoku back-arc basin. Bathymetry of the Mado Megamullion reveals spreading-parallel corrugations extending 25 km from the breakaway to the termination. Samples from several locations include peridotites, gabbros, dolerite, and rare pillow basalts. Gabbros range from granular to varitextured olivine gabbros and oxide gabbros. The emplacement of these gabbroic rocks within the oceanic lithosphere was followed by a multiphase tectono-metamorphic evolution including (i) dynamic recrystallization within shear zones, developed under granulite- to upper-amphibolite-facies conditions, and (ii) intrusion of highly evolved melts forming felsic segregations. This tectono-metamorphic evolution recalls that of the lower crust from other OCCs worldwide, demonstrating that this OCC exposes deep-seated intrusions progressively exhumed by detachment faulting. Nonetheless, the Mado Megamullion lower crustal gabbros show an unusual crystal line of descent, different from what is reported from mid-ocean ridge lower crustal rocks. We infer that the water-bearing character of the primary melts in this back-arc basin triggered the early precipitation of clinopyroxene, soon followed by amphibole and Fe-Ti oxides. Such modifications in phase saturation are likely to be directly related to the back-arc setting of the Mado Megamullion. If so, the phase assemblages of oceanic gabbros may be a diagnostic for the tectonic setting of lower crustal rocks in ophiolites.

DOI： 10.1029/2020gc009199

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2020GC009199

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Mineralogical and Petrological Sciences  

Attenuated total reflection infrared (ATR-IR) spectroscopy allows measurements to be made directly from the surface of one-sided, diamond polished thin sections of geological samples. This method greatly reduces the sample preparation time when compared to other IR spectroscopy methods and opens the possibility of using infrared spectroscopy to study thin-section scale microstructures. ATR-IR spectroscopy of antigorite, chrysotile, and lizardite in samples from the Mt. Shiraga serpentinite body, central Shikoku, SW Japan, reveals clear spectral differences in the 650-1250 cm-1 region associated with the vibration of the Si-O bonds in SiO4 tetrahedra and in the 3300-3750 cm-1 region associated with the vibration of the O-H bond in MgO2(OH)4 octahedra. A data-processing algorithm developed in this study allows the absorbance intensity and wavenumber of a particular absorbance peak to be used to create serpentine mineral phase maps based on the highest intensity Si-O absorbance bands for antigorite, chrysotile, and lizardite. Our methodology can be used to map serpentinite microstructures in thin sections illustrating the potential of ATR-IR as a relatively un-explored analytical tool in petrological studies. A combination of ATR-IR and electron microprobe data shows that for antigorite the wavenumber of the O-H absorbance band is correlated with the Fe content. Metamorphic reactions of serpentine minerals play a key role in the hydrodynamics of the earth's lithosphere, and the new information on serpentine mineral hydroxyl group behavior obtained by applying the technique outlined in this study are of great potential interest to researchers in a wide range of different fields.

DOI： 10.2465/jmps.190807

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Permeability profiles in the crust-mantle sequences of the Samail ophiolite were constructed based on onboard measurements of the electrical resistivity of cores recovered during the Oman Drilling Project. For each sample, we measured dry and brine-saturated resistivity during the description campaign on the drilling vessel Chikyu. Owing to the conductive brine in the pore space, wet resistivity is systematically lower than dry resistivity. The difference between dry and wet resistivity is attributed to the movement of dissolved ions in brine that occupies the pore space. We applied effective medium theory to calculate the volume fraction of pores that contribute to electrical transport. Using an empirical cubic law between transport porosity and permeability, we constructed permeability profiles for the crust-mantle transition zone and the serpentinized mantle sections in the Samail ophiolite. The results indicate that (1) the gabbro sequence has a markedly lower permeability than the underlying mantle sequence; (2) serpentinized dunites have higher permeability than serpentinized harzburgites; and (3) discrete sample permeability is correlated with ultrasonic velocity, suggesting that the permeability variations predominately reflect crack density and geometry.

DOI： 10.1029/2019jb018698

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その他リンク： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2019JB018698

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  

The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes. Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life. This article is part of a discussion meeting issue ‘Serpentine in the Earth system’.

DOI： 10.1098/rsta.2018.0425

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掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Mineralogical and Petrological Sciences  

We used high-resolution X-ray computed tomography (HRXCT) combined with scanning electron microscopy (SEM) to obtain 3D and 2D images of multiphase solid inclusions within chromite from the Samail ophiolite to investigate post-entrapment modification of the inclusions. Results indicate that the parental melt of the chromitite was supersaturated in chromian spinel. Chromite continued to crystallize on the inner wall of the host chromite after the melt was trapped. Rapid growth caused crystallization of high-Cr#[= Cr/(Cr + Al)] chromite lining around inclusions. The necking-down of originally large melt inclusions probably produced various assemblages of daughter minerals among the inclusions. We report two observations that are consistent with rapid growth of the host chromite: the 3D distribution of inclusions in host chromite and the host chromite showing skeletal morphology. High-temperature homogenization experiment was conducted to obtain the parental melt composition of the inclusions. We found that the homogenized glass does not represent the parental melt trapped in the host chromite because of the remaining of high-Cr# chromite lining and possible residual phases in the experiments.

DOI： 10.2465/jmps.191008

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人日本地球化学会  

<p> マントルの微量元素の化学的特徴を明らかにすることは、沈み込みや火成活動などにともなうマントル―地殻相互作用を理解する上で重要である。様々な同位体比データを元にマントル対流様式について様々なモデルが提案されているが、未だ統一的な見解に至っておらず、マントルの不均質性やその要因を理解する上で、新たな切り口が必要である。 そこで、マントルの主要構成鉱物であるカンラン石の微量元素組成について着目した。LA-ICP-MSによる微量元素定量分析の報告はある(De Hoog et al., 2010)が、対象元素は限られている。特にカンラン石中の存在度の低いPb、Th、UはLA-ICP-MSでの定量が困難であり、その濃度範囲は不明である。本研究では、カンラン石中のPb、Th、Uを含む微量元素の定量分析を溶液法でICP-MSにより行った。</p>

DOI： 10.14862/geochemproc.67.0_131

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Drilling  

The architecture, formation, and modification of oceanic plates are fundamental to our understanding of key geologic processes of the Earth. Geophysical surveys were conducted around a site near the Hawaiian Islands (northeastern Hawaiian North Arch region; Hawaiian North Arch hereafter), which is one of three potential sites for an International Ocean Discovery Program mantle drilling proposal for the Pacific plate that was submitted in 2012. The Hawaiian North Arch site is located in 78-81 Ma Cretaceous crust, which had an estimated full spreading rate of 7-8 cm yr-1. This site fills a major gap in our understanding of oceanic crust. Previously drilling has been skewed to young or older crust (< 15 or > 110 Ma) and slow-spread crust. P-wave velocity structure in the uppermost mantle of the Hawaiian North Arch shows a strong azimuthal anisotropy, whereas Moho reflections below the basement are variable: strong and continuous, weak, diffuse, or unclear. We assume that the strength of the Moho reflection is related to the aging of the oceanic plate. The Hawaiian volcanic chain (200 km to the southwest of the proposed drill site) and the nearby North Arch magmatism on the proposed Hawaiian North Arch sites might also have affected recognition of the Moho via deformation and/or magma intrusion into the lower crust of the uppermost mantle. This workshop report describes scientific targets for 2 km deep-ocean drilling in the Hawaiian North Arch region in order to provide information about the lower crust from unrecovered age and spreading rate gaps from previous ocean drillings. Other scientific objectives to be achieved by drilling cores before reaching the target depth of the project are also described in this report.

DOI： 10.5194/sd-26-47-2019

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth, Planets and Space  

Elastic wave velocity and electrical conductivity in a brine-saturated granitic rock were measured under confining pressures of up to 150 MPa and microstructure of pores was examined with SEM on ion-milled surfaces to understand the pores that govern electrical conduction at high pressures. The closure of cracks under pressure causes the increase in velocity and decrease in conductivity. Conductivity decreases steeply below 10 MPa and then gradually at higher pressures. Though cracks are mostly closed at the confining pressure of 150 MPa, brine must be still interconnected to show observed conductivity. SEM observation shows that some cracks have remarkable variation in aperture. The aperture varies from ~ 100 nm to ~ 3 μm along a crack. FIB–SEM observation suggests that wide aperture parts are interconnected in a crack. Both wide and narrow aperture parts work parallel as conduction paths at low pressures. At high pressures, narrow aperture parts are closed but wide aperture parts are still open to maintain conduction paths. The closure of narrow aperture parts leads to a steep decrease in conductivity, since narrow aperture parts dominate cracks. There should be cracks in various sizes in the crust: from grain boundaries to large faults. A crack must have a variation in aperture, and wide aperture parts must govern the conduction paths at depths. A simple tube model was employed to estimate the fluid volume fraction. The fluid volume fraction of 10−4–10−3 is estimated for the conductivity of 10−2 S/m. Conduction paths composed of wide aperture parts are consistent with observed moderate fluctuations (< 10%) in seismic velocity in the crust.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-019-1112-9

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その他リンク： http://link.springer.com/article/10.1186/s40623-019-1112-9/fulltext.html

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geochemistry, Geophysics, Geosystems  

International Ocean Discovery Program Expedition 352 recovered sedimentary-volcaniclastic successions and extensional structures (faults and extensional veins) that allow the reconstruction of the Izu-Bonin forearc tectonic evolution using a combination of shipboard core data, seismic reflection images, and calcite vein microstructure analysis. The oldest recorded biostratigraphic ages within fault-bounded sedimentary basins (Late Eocene to Early Oligocene) imply a ~15 Ma hiatus between the formation of the igneous basement (52 to 50 Ma) and the onset of sedimentation. At the upslope sites (U1439 and U1442) extension led to the formation of asymmetric basins reflecting regional stretch of ~16–19% at strain rates of ~1.58 × 10−16 to 4.62 × 10−16 s−1. Downslope Site U1440 (closer to the trench) is characterized by a symmetric graben bounded by conjugate normal faults reflecting regional stretch of ~55% at strain rates of 4.40 × 10−16 to 1.43 × 10−15 s−1. Mean differential stresses are in the range of ~70–90 MPa. We infer that upper plate extension was triggered by incipient Pacific Plate rollback ~15 Ma after subduction initiation. Extension was accommodated by normal faulting with syntectonic sedimentation during Late Eocene to Early Oligocene times. Backarc extension was assisted by magmatism with related Shikoku and Parece-Vela Basin spreading at ~25 Ma, so that parts of the arc and rear arc, and the West Philippine backarc Basin were dismembered from the forearc. This was followed by slow-rift to postrift sedimentation during the transition from forearc to arc rifting to spreading within the Shikoku-Parece-Vela Basin system.

DOI： 10.1029/2019GC008329

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Lithos  

Multiple generations of amphibole may form in the lower crust due to magmatism and metamorphism during the development of oceanic core complexes. We investigated the occurrence and chemical compositions of amphibole in gabbro mylonites from the medial area of the Godzilla Megamullion along the Parece Vela Rift in the Philippine Sea. The samples contain brown and green amphiboles with a variety of different textures that may have different origins. The brown amphibole occurs mainly as blebs in clinopyroxene porphyroclasts (Bleb amphibole), the rims around clinopyroxene porphyroclasts (Coronitic amphibole), and as porphyroclasts and fine-grained amphibole within the matrix (Matrix amphibole). The trace element and Cl contents of the bleb and green amphiboles indicate magmatic and metamoprhic origins, respectively. The bleb amphibole is interpreted to have crystallized from a hydrous silicate melt derived from an oxide gabbro-forming melt prior to retrograde metamorphism. In contrast, the compositions of the coronitic amphibole and matrix amphibole vary between those of typical magmatic and metamorphic amphiboles, suggesting that the amphibole-forming reactions were continuously retrogressive. Retrograde metamorphism is generally interpreted to have involved seawater-derived fluids, but the trace element contents of the coronitic and matrix amphiboles do not differ significantly from those of the original minerals (i.e., clinopyroxene and plagioclase). One sample of gabbro mylonite (KH07–02-D18–1) contains amphiboles with high concentrations of light rare earth elements, indicating a large influx of externally derived LREE-enriched fluids. These fluids are interpreted to have formed from an interaction between hydrous silicate melt with LREE-enriched composition and seawater-derived fluid. Our results suggest that multiple phases of melt–fluid infiltration occurred during the development of the detachment fault at the Godzilla Megamullion.

DOI： 10.1016/j.lithos.2019.06.019

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本高圧力技術協会  

DOI： 10.11181/hpi.57.148

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Structural Geology  

Depth-sensing indentation tests were performed to obtain the loop energy (equivalent to the energy consumed to produce the indentation) and the residual depth of the indentation using a triangular pyramidal diamond indenter for the minerals in Mohs hardness scale except for diamond, as well as other minerals (apophyllite, forsterite, and tourmaline), at a maximum load ranging from 30 to 100 mN. A new graphic presentation is proposed that shows the hardness of minerals in log(penetration depth)−log(loop energy) space. The data for each mineral under different loads give a straight regression line with a slope of 2.6–2.9 (except for talc, which yields a slope of 2.2), while the data for different minerals under a given load yield a straight regression line with a slope of 1.1–1.2. A theoretical analysis of ideal materials, in terms of log(penetration depth)−log(loop energy) space, shows the existence of two series of parallel regression lines with slopes of 3 (data for each mineral at different loads) and 1 (data for different minerals under a given load). The results show a slight deviation between the measured and theoretical slopes, probably reflecting a progressive change in the mechanical properties of the minerals during the indentation tests.

DOI： 10.1016/j.jsg.2018.09.004

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Here we provide an appraisal of the Poisson's ratios (υ) for natural elements, common oxides, silicate minerals, and rocks with the purpose of searching for naturally auxetic materials. The Poisson's ratios of equivalently isotropic polycrystalline aggregates were calculated from dynamically measured elastic properties. Alpha-cristobalite is currently the only known naturally occurring mineral that has exclusively negative υ values at 20–1,500°C. Quartz and potentially berlinite (AlPO4) display auxetic behavior in the vicinity of their α-β structure transition. None of the crystalline igneous and metamorphic rocks (e.g., amphibolite, gabbro, granite, peridotite, and schist) display auxetic behavior at pressures of >5 MPa and room temperature. Our experimental measurements showed that quartz-rich sedimentary rocks (i.e., sandstone and siltstone) are most likely to be the only rocks with negative Poisson's ratios at low confining pressures (≤200 MPa) because their main constituent mineral, α-quartz, already has extremely low Poisson's ratio (υ = 0.08) and they contain microcracks, micropores, and secondary minerals. This finding may provide a new explanation for formation of dome-and-basin structures in quartz-rich sedimentary rocks in response to a horizontal compressional stress in the upper crust.

DOI： 10.1002/2017JB014606

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Energy Procedia  

In this invited review, we summarize the main results of ongoing research on "in situ" carbon mineralization in ultramafic rocks, including outcrop studies in Oman (e.g., [1, 2]), investigation of carbon mass transfer in subduction zones from the Oman Drilling Project (e.g., [3-7]), laboratory investigations (e.g., [8-12]) and numerical modeling (e.g., [13-17]) of the pressure of crystallization and reaction-driven cracking, and assessment of the rate, cost and capacity of various proposed methods for engineered carbon mineralization [18, 19].

DOI： 10.1016/j.egypro.2018.07.013

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geosphere  

The deep submergence research vehicle Shinkai 6500, diving on the Challenger segment of the Mariana forearc, encountered a superstructure of nascent arc crust atop a younger mantle with entrained fragments of metamorphosed crust. A plutonic block from this crust collected at 4900 m depth has a crystallization age of 46.1 Ma and mixed boninitic-arc tholeiitic geochemical signatures. A hornblende garnetite and two epidote amphibolites were retrieved from depths between 5938 m and 6277 m in an area dominated by peridotite. The garnetite appears to represent a crystal cumulate after melting of deep arc crust, whereas the amphibolites are compositionally similar to enriched mid-ocean ridge basalt (MORB). The initial isotopic compositions of these crustal fragments are akin to those of Eocene to Cretaceous terranes along the periphery of the Philippine plate. The garnetite achieved pressures of 1.2 GPa or higher and temperatures above 850 °C and thus could represent a fragment of the delaminated root of one of these terranes. This sample has coeval Sm-Nd, Lu-Hf, and 40Ar-39Ar ages indicating rapid ascent and cooling at 25 Ma, perhaps in association with rifting of the Kyushu-Palau arc. Peak P-T conditions were lower for the amphibolites, and their presence on the ocean floor near the garnetite might have resulted from mass wasting or normal faulting. The presence of relatively fusible crustal blocks in the circulating mantle could have contributed to the isotopic similarity of Mariana arc and backarc lavas with Indian Ocean MORB.

DOI： 10.1130/GES01536.1

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Reports  

Recent geophysical surveys indicate that hydration (serpentinization) of oceanic mantle is related to outer-rise faulting prior to subduction. The serpentinization of oceanic mantle influences the generation of intermediate-depth earthquakes and subduction water flux, thereby promoting arc volcanism. Since the chemical reactions that produce serpentinite are geologically rapid at low temperatures, the flux of water delivery to the reaction front appears to control the lateral extent of serpentinization. In this study, we measured the permeability of low-temperature serpentinites composed of lizardite and chrysotile, and calculated the lateral extent of serpentinization along an outer-rise fault based on Darcy's law. The experimental results indicate that serpentinization extends to a region several hundred meters wide in the direction normal to the outer-rise fault in the uppermost oceanic mantle. We calculated the global water flux carried by serpentinized oceanic mantle ranging from 1.7 × 1011 to 2.4 × 1012 kg/year, which is comparable or even higher than the water flux of hydrated oceanic crust.

DOI： 10.1038/s41598-017-14309-9

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立大学法人 静岡大学理学部地球科学教室  

DOI： 10.51053/shizuoka.44.0_31

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geochimica et Cosmochimica Acta  

The uppermost mantle in back arc regions is the site of complex interactions between partial melting, melt percolation, and fluid migration. To constrain these interactions and evaluate their consequences on geochemical cycles, we carried out an in situ trace element and water study of a suite of spinel peridotite xenoliths from two regions of the Japan back arc system, Ichinomegata (NE Japan) and Oki-Dogo (SW Japan), using LA-ICPMS and FTIR spectrometry, respectively. This study provides the first full dataset of trace element and hydrogen compositions in peridotites including analyses of all their main constitutive silicate minerals: olivine, orthopyroxene and clinopyroxene. The Ichinomegata peridotites sample a LREE-depleted refractory mantle (Mg# olivine = 0.90; Cr# spinel = 0.07–0.23; Yb clinopyroxene = 7.8–13.3 × C1-chondrite, and La/Yb clinopyroxene = 0.003–0.086 × C1-chondrite), characterized by Th-U positive anomalies and constant values of Nb/Ta. The composition of the studied Ichinomegata samples is consistent with that of an oceanic mantle lithosphere affected by cryptic metasomatic interactions with hydrous/aqueous fluids (crypto-hydrous metasomatism). In contrast, the Oki-Dogo peridotites have low Mg# olivine (0.86–0.93) and a broad range of compositions with clinopyroxene showing “spoon-shaped” to flat, and LREE-enriched patterns. They are also characterized by their homogeneous compositions in the most incompatible LILE (e.g., Rb clinopyroxene = 0.01–0.05 × primitive mantle) and HFSE (e.g., Nb clinopyroxene = 0.01–2.16 × primitive mantle). This characteristic is interpreted as resulting from various degrees of melting and extensive melt-rock interactions. FTIR spectroscopy shows that olivine in both Ichinomegata and Oki-Dogo samples has low water contents ranging from 2 to 7 ppm wt. H2O. In contrast, the water contents of pyroxenes from Ichinomegata peridotites (113–271 ppm wt. H2O for orthopyroxene, and 292–347 ppm wt. H2O for clinopyroxene) are significantly higher than in Oki-Dogo peridotites (9–35 ppm wt. H2O for orthopyroxene, and 15–98 ppm wt. H2O for clinopyroxene). This indicates a relationship between melt-rock interaction and water concentrations in pyroxenes. Our study suggests that the water content of the Japan mantle wedge is controlled by the late melt/fluid/rock interactions evidenced by trace element geochemistry: a mechanism triggered by magma-rock interactions may have acted as an efficient dehydrating process in the Oki-Dogo region while the Ichinomegata mantle water content is controlled by slab-derived crypto-hydrous metasomatism.

DOI： 10.1016/j.gca.2017.03.042

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：International Geology Review  

International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

DOI： 10.1080/00206814.2016.1276482

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geochemistry, Geophysics, Geosystems  

Most of the well-preserved ophiolite complexes are believed to form in suprasubduction zone (SSZ) settings. We compare physical properties and seismic structure of SSZ crust at the Izu-Bonin-Mariana (IBM) fore arc with oceanic crust drilled at Holes 504B and 1256D to evaluate the similarities of SSZ and oceanic crust. Expedition 352 basement consists of fore-arc basalt (FAB) and boninite lavas and dikes. P-wave sonic log velocities are substantially lower for the IBM fore arc (mean values 3.1–3.4 km/s) compared to Holes 504B and 1256D (mean values 5.0–5.2 km/s) at depths of 0–300 m below the sediment-basement interface. For similar porosities, lower P-wave sonic log velocities are observed at the IBM fore arc than at Holes 504B and 1256D. We use a theoretical asperity compression model to calculate the fractional area of asperity contact Af across cracks. Af values are 0.021–0.025 at the IBM fore arc and 0.074–0.080 at Holes 504B and 1256D for similar depth intervals (0–300 m within basement). The Af values indicate more open (but not necessarily wider) cracks in the IBM fore arc than for the oceanic crust at Holes 504B and 1256D, which is consistent with observations of fracturing and alteration at the Expedition 352 sites. Seismic refraction data constrain a crustal thickness of 10–15 km along the IBM fore arc. Implications and inferences are that crust-composing ophiolites formed at SSZ settings could be thick and modified after accretion, and these processes should be considered when using ophiolites as an analog for oceanic crust.

DOI： 10.1002/2016GC006638

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：International Journal of Refractory Metals and Hard Materials  

In this work, we examine the response of an ultra-fine grained (UFG) tungsten material to high-flux deuterium plasma exposure. UFG tungsten has received considerable interest as a possible plasma-facing material in magnetic confinement fusion devices, in large part because of its improved resistance to neutron damage. However, optimization of the material in this manner may lead to trade-offs in other properties. We address two aspects of the problem in this work: (a) how high-flux plasmas modify the structure of the exposed surface, and (b) how hydrogen isotopes become trapped within the material. The specific UFG tungsten considered here contains 100 nm-width Ti dispersoids (1 wt%) that limit the growth of the W grains to a median size of 960 nm. Metal impurities (Fe, Cr) as well as O were identified within the dispersoids; these species were absent from the W matrix. To simulate relevant particle bombardment conditions, we exposed specimens of the W-Ti material to low energy (100 eV), high-flux (> 1022 m− 2 s− 1) deuterium plasmas in the PISCES-A facility at the University of California, San Diego. To explore different temperature-dependent trapping mechanisms, we considered a range of exposure temperatures between 200 °C and 500 °C. For comparison, we also exposed reference specimens of conventional powder metallurgy warm-rolled and ITER-grade tungsten at 300 °C. Post-mortem focused ion beam profiling and atomic force microscopy of the UFG tungsten revealed no evidence of near-surface bubbles containing high pressure D2 gas, a common surface degradation mechanism associated with plasma exposure. Thermal desorption spectrometry indicated moderately higher trapping of D in the material compared with the reference specimens, though still within the spread of values for different tungsten grades found in the literature database. For the criteria considered here, these results do not indicate any significant obstacles to the potential use of UFG tungsten as a plasma-facing material, although further experimental work is needed to assess material response to transient events and high plasma fluence.

DOI： 10.1016/j.ijrmhm.2016.05.006

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

In order to constrain the interpretation of seismic data from receiver functions and deep profiles of the crust beneath southern China (Cathaysia and Yangtze blocks), we have measured S-wave velocities (Vs) and splitting as a function of hydrostatic confining pressure up to 650 MPa for 22 representative samples (i.e., granite, diorite, felsic gneiss and mylonite, amphibolite, schist, and marble) from the Yunkai metamorphic complex (China) that represent the crystalline basement beneath the region. The experimental data were combined with electron backscattering diffraction (EBSD) analysis of rock-forming minerals to constrain variations of Vp/Vs ratios and understand the origin of seismic anisotropy. The crusts beneath the Yangtze and Cathaysia blocks have different average thicknesses (H = 35.4 ± 6.3 km and 29.8 ± 1.8 km, respectively) but display almost the same Vp/Vs values (1.73 ± 0.08 and 1.74 ± 0.04, respectively). These ratios correspond to an average of bimodally distributed granitic and gabbroic lithologies which are dominant, respectively, in the upper and lower crusts, instead a homogeneous andesitic composition of the overall crust. Positive and negative correlations between H and Vp/Vs occur in west and east parts of southern China, respectively. The negative correlation indicates basaltic underplating from a partially molten mantle wedge above the subducting Pacific plate into the southern China crust, whereas the positive correlation implies that much larger thinning strain has taken place in the high temperature mafic lower crust (high temperature) than in the low temperature felsic upper crust during Mesozoic-Cenozoic tectonic extension.

DOI： 10.1016/j.tecto.2016.07.017

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geochemistry, Geophysics, Geosystems  

North Pond is an isolated sedimentary pond on the western flank of the Kane area along the Mid-Atlantic Ridge. Drill-hole U1382A of IODP Expedition 336 recovered peridotite and gabbro samples from a sedimentary breccia layer in the pond, from which we collected six fresh peridotite samples. The peridotite samples came from the southern slope of the North Pond where an oceanic core complex is currently exposed. The samples were classified as spinel harzburgite, plagioclase-bearing harzburgite, and a vein-bearing peridotite that contains tiny gabbroic veins. No obvious macroscopic shear deformation related to the formation of a detachment fault was observed. The spinel harzburgite with a protogranular texture was classified as refractory peridotite. The degree of partial melting of the spinel harzburgite is estimated to be ∼17%, and melt depletion would have occurred at high temperatures in the uppermost mantle beneath the spreading axis. The progressive melt–rock interactions between the depleted spinel harzburgite and the percolating melts of Normal-Mid Ocean Ridge Basalt (N-MORB) produced the plagioclase-bearing harzburgite and the vein-bearing peridotite at relatively low temperatures. This implies that the subsequent refertilization occurred in an extinct spreading segment of the North Pond after spreading at the axis. Olivine fabrics in the spinel and plagioclase-bearing harzburgites are of types AG, A, and D, suggesting the remnants of a mantle flow regime beneath the spreading axis. The initial olivine fabrics appear to have been preserved despite the later melt–rock interactions. The peridotite samples noted above preserve evidence of mantle flow and melt–rock interactions beneath a spreading ridge that formed at ∼8 Ma.

DOI： 10.1002/2016GC006429

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Crystallographic preferred orientations (CPOs) of olivine within natural peridotites are commonly depicted by pole figures for the [100], [010], and [001] axes, and they can be categorized into five well-known fabric types: A, B, C, D, and E. These fabric types can be related to olivine slip systems: A with (010)[100], B with (010)[001], C with (001)[001], D with {0kl}[100], and E with (001)[100]. In addition, an AG type is commonly found in nature, but its origin is controversial, and could involve several contributing factors such as complex slip systems, non-coaxial strain types, or the effects of melt during plastic flow. In this paper we present all of our olivine fabric database published previously as well as new data mostly from ocean floor, mainly for the convergent margin of the western Pacific region, and we introduce a new index named Fabric-Index Angle (FIA), which is related to the P-wave property of a single olivine crystal. The FIA can be used as an alternative to classifying the CPOs into the six fabric types, and it allows a set of CPOs to be expressed as a single angle in a range between -90° and 180°. The six olivine fabric types have unique values of FIA: 63° for A type, -28° for B type, 158° for C type, 90° for D type, 106° for E type, and 0° for AG type. We divided our olivine database into five tectonic groups: ophiolites, ridge peridotites, trench peridotites, peridotite xenoliths, and peridotites enclosed in high-pressure metamorphic rocks. Our results show that although our database is not yet large enough (except for trench peridotites) to define the characteristics of the five tectonic groups, the natural olivine fabrics vary in their range of FIA: 0° to 150° for the ophiolites, 40° to 80° for the ridge peridotites, -40° to 100° for the trench peridotites, 0° to 100° for the peridotite xenoliths, and -40° to 10° for the peridotites enclosed in high-pressure metamorphic rocks. The trench peridotites show a statistically unimodal distribution of FIA consisting of the high peak equivalent of the A type, but with some FIAs close to the AG and D types. The variations in the olivine fabrics in the trench peridotites could result from variations in deformation within the supra-subduction uppermost mantle, possibly related to evolution of the mantle since the subduction initiation of the Pacific plate.

DOI： 10.1016/j.epsl.2016.03.019

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

In order to constrain the effects of olivine fabric, melt-rock reaction, and hydration on the seismic properties and anisotropy of mantle rocks, we investigated serpentinized peridotites from the Luobusha ophiolite in the Indus-Tsangpo suture of the Tibetan Plateau. A-type and almost random olivine crystal-preferred orientations (CPO) occur in harzburgite and dunite samples, respectively. The dunite resulted from interactions of harzburgite with boninitic melt at ~800–970°C, yielding pyroxene dissolution and olivine precipitation. The olivine neoblasts formed from the melt-rock reaction show no evidence of dislocation creep and developed almost random CPO. Hence, the melt-rock reaction reduced seismic anisotropy. Our results together with those from the literature indicate that A-, B-, C-, D-, and E-type CPOs of olivine generally induce Vp anisotropy patterns with Vp(X) > Vp(Y) > Vp(Z), Vp(Y) > Vp(X) > Vp(Z), Vp(Z) > Vp(X) > Vp(Y), Vp(X) > Vp(Y) ≈ Vp(Z), and Vp(X) > Vp(Z) > Vp(Y), respectively. The effect of serpentinization was calibrated by the comparison of seismic velocities and anisotropy measured up to 600 MPa with the values calculated from the CPO data. Although the low-temperature (LT, <300°C) serpentinization (lizardite and chrysotile) decreases Vp by ~6–10% and Vs by ~12%, it does not change the anisotropy pattern because the mesh-texture characterized by serpentine veins perpendicular to the principal structural directions (X, Y, and Z) reduces the velocities in these orthogonal directions to almost equal extent. Thus, the magnitude of seismic anisotropy alone cannot be used as an indicator of the degree of LT serpentinization in the mantle rocks. Furthermore, Birch's law is found to hold when peridotites undergo serpentinization.

DOI： 10.1002/2015JB012579

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

Measurements of crystallographic preferred orientations (CPO) and calculations of P- and S-wave velocities (Vp and Vs) and anisotropy were conducted on three quartz–mica schists and one felsic mylonite, which are representative of typical metamorphic rocks deformed in the middle crust beneath the southeastern Tibetan plateau. Results show that the schists have Vp anisotropy (AVp) ranging from 16.4% to 25.5% and maximum Vs anisotropy [AVs(max)] between 21.6% and 37.8%. The mylonite has lower AVp and AVs(max) but slightly higher foliation anisotropy, which are 13.2%, 18.5%, and 3.07%, respectively, due to the lower content and CPO strength of mica. With increasing mica content, the deformed rocks tend to form transverse isotropy (TI) with fast velocities in the foliation plane and slow velocities normal to the foliation. However, the presence of prismatic minerals (e.g., amphibole and sillimanite) forces the overall symmetry to deviate from TI. An increase in feldspar content reduces the bulk anisotropy caused by mica or quartz because the fast-axis of feldspar aligns parallel to the slow-axis of mica and/or quartz. The effect of quartz on seismic properties of mica-bearing rocks is complex, depending on its content and prevailing slip system. The greatest shear-wave splitting and fastest Vp both occur for propagation directions within the foliation plane, consistent with the fast Pms (S-wave converted from P-wave at the Moho) polarization directions in the west Yunnan where mica/amphibole-bearing rocks have developed pervasive subvertical foliation and subhorizontal lineation. The fast Pms directions are perpendicular to the approximately E-W orienting fast SKS (S-wave traversing the core as P-wave) directions, indicating a decoupling at the Moho interface between the crust and mantle beneath the region. The seismic data are inconsistent with the model of crustal channel flow as the latter should produce a subhorizontal foliation where vertically incident shear waves suffer little splitting.

DOI： 10.1016/j.tecto.2016.04.024

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担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Island Arc  

DOI： 10.1111/iar.12157

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Island Arc  

We studied the contributions of plagioclase, clinopyroxene, and amphibole to the P-wave velocity properties of gabbroic mylonites of the Godzilla Megamullion (site KH07-02-D18) in the Parece Vela Rift of the central Parece Vela Basin, Philippine Sea, based on their crystal-preferred orientations (CPOs), mineral modes, and elastic constants and densities of single crystals. The gabbroic mylonites have been classified into three types based on their microstructures and temperature conditions: HT1, HT2 and medium-temperature (MT) mylonites. The P-wave velocity properties of the HT1 mylonite are dominantly influenced by plagioclase CPOs. Secondary amphibole occurred after deformation in the HT1 mylonite, so that its effect on P-wave velocity anisotropy is minimal due to weak CPOs. Although the HT2 mylonite developed deformation microstructures in the three minerals, the P-wave velocity properties of the HT2 mylonite are essentially isotropic, resulting from the destructive interference of different P-wave velocity anisotropy patterns produced by the distinct CPOs of the three constituent minerals (i.e., plagioclase, clinopyroxene, and amphibole). The P-wave velocity properties of the MT mylonite are influenced mainly by amphibole CPOs, whereas the effect of plagioclase CPOs on P-wave velocity anisotropy becomes very small with a decrease in the intensity of plagioclase CPOs. As a result, the gabbroic mylonites tend to have weak P-wave velocity anisotropy in seismic velocity, although their constituent minerals show distinct CPOs. Such weakness in the whole-rock P-wave velocity anisotropy could result from the destructive contributions of the different mineral CPOs with respect to the structural framework (foliation and lineation). These results show that amphibole has a high potential for P-wave velocity anisotropy by aligning both crystallographically and dimensionally during deformation in the hydrous oceanic crust. The results also suggest that the effect of a hydrous phase on P-wave velocity anisotropy within the detachment shear zone in a slow-spreading oceanic crust varies depending on the degree of deformation and on the timing of hydrothermal activity.

DOI： 10.1111/iar.12132

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Dislocation recovery experiments were conducted on pre-deformed olivine single crystals at 1450 to 1760 K, room pressure, and oxygen partial pressures near the Ni–NiO buffer to determine the annihilation rates for [100] and [001] dislocations on the (010) plane. Olivine single crystals were first deformed to activate the desired slip systems under simple shear geometry and then annealed at target conditions. The edge and screw dislocations with Burgers vectors, b, of [100] and [001], respectively, both elongated in the [001] direction were produced by the deformation. The dislocation annihilation rate constants of both types of dislocations are identical within 0.3 log unit. The activation energies for both dislocations are also identical, i.e., ∼400 kJ/mol, which is also identical to that of the Si self-diffusion coefficient. This correspondence suggests that olivine dislocation creep controlled by a diffusion-controlled process under low-stress and high-temperature conditions. This study offers a potential insight into the formation of AG-type fabric in olivine.

DOI： 10.1016/j.epsl.2016.02.029

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Petrology  

Dunite bands and dikes in ophiolitic mantle peridotites are interpreted as fossil melt channels within the suboceanic mantle. Concordant dunite bands (i.e. fossil melt channels transposed by outward transportation from the ridge axis via horizontal mantle flow) are particularly important as they possibly represent the melt channels through which the parental melts of mid-ocean ridge basalt (MORB) were transported to shallower depths beneath the paleo-ridge axis. We conducted field observations and sampling of concordant dunite bands (CDB) and their host harzburgite at selected outcrops covering a wide depth range in the mantle section along an inferred paleo-ridge segment in the northern to central part of the Oman ophiolite. The CDB increase in thickness and decrease in frequency upward. They are thicker and more frequent in the centre of the segment than near the segment ends when compared at the same stratigraphic level. The CDB consist mostly of olivine with minor spinel and very rare amounts of pyroxene. Clinopyroxene has a small grain size and an interstitial position relative to olivine. The constituent minerals in the CDB and their host harzburgite were analyzed by electron microprobe for major elements and by laser ablation inductively coupled plasma mass spectrometry for trace elements. Most of the CDB have refractory major element mineral compositions, such as high Fo [100 � Mg/(Mg + Fe)] in olivine (>90�5), high Cr# [Cr/(Cr + Al)] in chromian spinel (>0�50), and low Al2O3 (<3�5 wt %) in clinopyroxene. Chondrite-normalized trace element patterns of clinopyroxene in the host harzburgites consistently show a gentle decrease from heavy REE (HREE) to middle REE (MREE) and a sharp decrease from MREE to light REE (LREE) (= highly depleted), but those in the CDB show weaker LREE depletion, which is more variable depending on the stratigraphic level and position along the paleo-ridge segment. In contrast, the HREE concentrations in clinopyroxene in the CDB are higher than or similar to those of the host harzburgites. Trace element compositions of clinopyroxene in the CDB and their host harzburgites are evaluated with a one-dimensional, steady-state, open-system decompressional melting-reaction model. The modeling results suggest that an LREE-enriched melt generated at high pressure was transported upwards through melt channels to the shallow mantle (up to the Moho transition zone), where it mingled with highly depleted melts accumulated from fractionally melted peridotites to generate normal (N)-MORB-like melts. The mantle started upwelling (= melting) in the garnet stability field in the segment centre, but either in the garnet or in the spinel stability field near the segment ends. This suggests a variation of geothermal gradient along the paleo-ridge segment: higher in the segment centre and lower near the segment ends. This inference is supported by the presence of thicker (up to 250 cm) CDB as well as more frequent occurrence of CDB in the segment centre than near the segment end and by the geochemical evidence for chromatographic N-MORB-like melt percolation into the host peridotite only in the uppermost horizons near the segment ends.

DOI： 10.1093/petrology/egw020

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

We calibrated the magnitude and symmetry of seismic anisotropy for 132 mica- or amphibole-bearing metamorphic rocks to constrain their departures from transverse isotropy (TI) which is usually assumed in the interpretation of seismic data. The average bulk Vp anisotropy at 600MPa for the chlorite schists, mica schists, phyllites, sillimanite-mica schists, and amphibole schists examined is 12.0%, 12.8%, 12.8%, 17.0%, and 12.9%, respectively. Most of the schists show Vp anisotropy in the foliation plane which averages 2.4% for phyllites, 3.3% for mica schists, 4.1% for chlorite schists, 6.8% for sillimanite-mica schists, and 5.2% for amphibole schists. This departure from TI is due to the presence of amphibole, sillimanite, and quartz. Amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation, forming orthorhombic anisotropy with Vp(X)>Vp(Y)>Vp(Z). Effects of quartz are complicated, depending on its volume fraction and prevailing slip system. Most of the mica- or amphibole-bearing schists and mylonites are approximately transversely isotropic in terms of S wave velocities and splitting although their P wave properties may display orthorhombic symmetry. The results provide insight for the interpretation of seismic data from the southeast Tibetan Plateau. The N-S to NW-SE polarized crustal anisotropy in the Sibumasu and Indochina blocks is caused by subvertically foliated mica- and amphibole-bearing rocks deformed by predominantly compressional folding and subordinate strike-slip shear. These blocks have been rotated clockwise 70-90° around the east Himalayan Syntaxis, without finite eastward or southeastward extrusion, in responding to progressive indentation of India into Asia.

DOI： 10.1002/2015JB012209

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

In this work, we examine how deuterium becomes trapped in plasma-exposed tungsten and forms near-surface platelet-shaped precipitates. How these bubbles nucleate and grow, as well as the amount of deuterium trapped within, is crucial for interpreting the experimental database. Here, we use a combined experimental/theoretical approach to provide further insight into the underlying physics. With the Tritium Plasma Experiment, we exposed a series of ITER-grade tungsten samples to high flux D plasmas (up to 1.5 × 1022m-2s-1) at temperatures ranging between 103 and 554 °C. Retention of deuterium trapped in the bulk, assessed through thermal desorption spectrometry, reached a maximum at 230 °C and diminished rapidly thereafter for T > 300 °C. Post-mortem examination of the surfaces revealed non-uniform growth of bubbles ranging in diameter between 1 and 10 μm over the surface with a clear correlation with grain boundaries. Electron back-scattering diffraction maps over a large area of the surface confirmed this dependence; grains containing bubbles were aligned with a preferred slip vector along the <111> directions. Focused ion beam profiles suggest that these bubbles nucleated as platelets at depths of 200 nm-1 μm beneath the surface and grew as a result of expansion of sub-surface cracks. To estimate the amount of deuterium trapped in these defects relative to other sites within the material, we applied a continuum-scale treatment of hydrogen isotope precipitation. In addition, we propose a straightforward model of near-surface platelet expansion that reproduces bubble sizes consistent with our measurements. For the tungsten microstructure considered here, we find that bubbles would only weakly affect migration of D into the material, perhaps explaining why deep trapping was observed in prior studies with plasma-exposed neutron-irradiated specimens. We foresee no insurmountable issues that would prevent the theoretical framework developed here from being extended to a broader range of systems where precipitation of insoluble gases in ion beam or plasma-exposed metals is of interest.

DOI： 10.1063/1.4928184

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：International Ocean Discovery Program: Preliminary Reports  

The objectives for Expedition 352 were to drill through the entire volcanic sequence of the Bonin fore arc to 1. Obtain a high-fidelity record of magmatic evolution during subduction initiation and early arc development, 2. Test the hypothesis that fore-arc basalt lies beneath boninite and understand chemical gradients within these units and across the transition, 3. Use drilling results to understand how mantle melting processes evolve during and after subduction initiation, and 4. Test the hypothesis that the fore-arc lithosphere created during subduction initiation is the birthplace of suprasubduction zone (SSZ) ophiolites. Expedition 352 successfully cored 1.22 km of igneous basement and 0.46 km of over-lying sediment, providing diverse, stratigraphically controlled suites of fore-arc basalts (FAB) and boninite related to seafloor spreading and earliest arc development. FAB were recovered at the two deeper water sites (U1440 and U1441) and boninites at the two sites (U1439 and U1442) drilled upslope to the west. FAB lavas and dikes are depleted in high-field strength trace elements such as Ti and Zr relative to mid-ocean-ridge basalt but have relatively diverse concentrations of trace elements bezcause of variation in degrees of melting and amount of subducted fluids involved in their genesis. All FAB magmas underwent significant crystal fractionation in a persistent magma chamber system. Holes U1439C and U1442A yielded entirely boninitic lavas. We defined three boninite differentiation series based on variations in MgO, SiO2, and TiO2 concentrations of the parental magmas. Lavas in both pairs of holes have compositions that generally become more primitive and have lower TiO2 concentrations upward. The presence of dikes at the base of the sections at Sites U1439 and U1440 provides evidence that boninitic and FAB lavas are both underlain by their own conduit systems and that FAB and boninite group lavas are likely offset more horizontally than vertically. We thus propose that seafloor spreading related to subduction initiation migrated from east to west after subduction initiation and during early arc development. Initial spreading was likely rapid, and an axial magma chamber was present. Melting was largely decompressional during this period, but subducted fluids affected some melting. As subduction continued and spreading migrated to the west, the embryonic mantle wedge became more depleted, and the influence of subducted constituents dramatically increased, causing the oceanic crust to be built of boninitic rather than tholeiitic magma. The general decrease in fractionation upward reflects the eventual disappearance of persistent magma chambers, either because spreading rate was decreasing with distance from the trench or because spreading was succeeded by off-axis magmatism trenchward of the ridge. The extreme depletion of the sources for all boninitic lavas was likely related to the incorporation of mantle residues from FAB generation. This mantle depletion continued during generation of lower silica boninitic magmas, exhausting clinopyroxene from the mantle such that the capping high-Si, low-Ti boninites were generated from harzburgite. Additional results of the cruise include recovery of Eocene to recent deep-sea sediment that records variation in sedimentation rates with time resulting from variations in climate, the position of the carbonate compensation depth, and local structural control. Three phases of highly explosive volcanism (latest Pliocene to Pleistocene, late Miocene to earliest Pliocene, and Oligocene) were identified, represented by 132 graded air fall tephra layers. Structures found in the cores and reflected in seismic profiles show that this area had periods of normal, reverse, and strike-slip faulting. Finally, basement rock P-wave velocities were shown to be slower than those observed during logging of normal ocean crust sites.

DOI： 10.14379/iodp.pr.352.2015

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

A series of simple-shear experiments on blueschist was performed at 400-500°C and 1-2.5 GPa to understand the deformation of seismically active, subducting oceanic crust. The experiments show that brittle microstructures are mainly found at pressures of 1-2 GPa, whereas ductile microstructures form at 2.5 GPa. J-indices (a measure of fabric intensity) of glaucophane crystal preferred orientations change systematically with changing shear strain and confining pressure, and the angle between the slip plane and the shear direction of samples deformed at >2 GPa is similar to that of a strain ellipsoid. These results, together with the variable orientations of fine grains in a selected area electron diffraction image at 2 GPa, indicate that the brittle-ductile transition for glaucophane occurs at ∼2 GPa. In contrast to this, lawsonite in the experiments show abundant fracturing in most specimens and a poor correlation between the J-index, shear strain, and confining pressure. This demonstrates that lawsonite deformed by brittle failure under all experimental conditions. In the case of a starting material that has a strong fabric and deformed under dry experimental conditions, the brittle-ductile transition zone of glaucophane will be much shallower than 2 GPa. Therefore, our initial experimental results on the deformation behavior of blueschist indirectly support the dehydration embrittlement of subducting oceanic crust (glaucophane) as an important factor in the origin of intraplate earthquakes.

DOI： 10.1002/2014JB011528

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

DOI： 10.1016/j.epsl.2014.10.026

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

We tested plagioclase flow laws for rheological properties of the detachment shear zone developed in an oceanic core complex. Gabbroic mylonites occur extensively over the ~125-km length of the Godzilla Megamullion, an enormous oceanic core complex situated in an extinct Philippine Sea back-arc basin. The mylonites were produced in the detachment shear zone at temperatures of 650-850 °C in the lower crust over a period of approximately 4 million years, corresponding to a slow spreading rate of 2.54±0.21 cm/yr (i.e. 8.05×10-10 m/s). Applying the rheological parameters for plagioclase flow laws, combined with the geochronological spreading rate, we calculated deformation mechanism maps of plagioclase as shear strain rates of 10-12 to 10-8 s-1, corresponding to shear zones in thickness of ~0.1 to ~1000 m. Our results show that, assuming a constant stress condition defined by a shear strain rate of 10-12 s-1 for grain size of 1000 μm in a temperature range between 650 and 850 °C, petrofabric parameters such as crystal-preferred orientations and dynamically recrystallized grain sizes along with estimated equilibrium temperatures can be possibly explained by the deformation mechanism maps of plagioclase for shear strain rates mostly of 10-11 to 10-9 s-1. It suggests that even if the entire thickness of the detachment shear zone may lie several hundred meters below the spreading center, the shear zone could be stratified, comprising many anastomosing narrow zones. It implies that, during the development of the detachment fault, strain localization would occur in the lower crust over a broad (~1000 m) zone at high temperatures.

DOI： 10.1016/j.epsl.2014.10.005

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus, the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured P and S wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 MPa. Anorthosites and gabbroic anorthosites containing >80 vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns: Vp(Z)/Vp(Y) ≥ 1 and Vp(Z)/Vp(X) ≥ 1 for anorthosites while 0.8 < Vp(Z)/Vp(Y) ≤ 1 and 0.8 < Vp(Z)/Vp(X) ≤ 1 for gabbros. Amphibolites lie in the same domain as gabbros, but show a significantly stronger tendency of Vp(X) > Vp(Y) than the gabbros. Laminated anorthosites with Vp(X) ≈ Vp(Y) Vp(Z) display a strong crystal preferred orientation (CPO) of plagioclase whose (010) planes and [100] and [001] directions parallel to the foliation. For the gabbros and amphibolites characterized by Vp(X) ≈ Vp(Y) > Vp(Z) and Vp(X) > Vp(Y) > Vp(Z), respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poisson's ratio calculated using the average P and S wave velocities from the three principal propagation-polarization directions (X, Y, and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively.

DOI： 10.1002/2014JB011352

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geotectonica et Metallogenia  

The Yushugou massif in the Kumishi region at the nouthern margin of the southern Tianshan (Xinjiang) consists of extensively deformed peridotites (lherzolite, harzburgite and wehrlite) associated with foliated and lineated mafic composition-dominated granulites, together carried up and emplaced into greenschist-lower amphibolitefacies metasediments at 380-420 Ma. The granulite-peridotite units are interpreted as tectonic slices of continental mantle and lower crustal materials immediately adjacent the Moho. The high pressure metamorphic slices were extruded by tectonic compression from a detachment in the crust-mantle boundary up into the greenschist-lower amphibolite-facies midde crust along intracontinental thrust shear zones. Such compressional uplifts of the crust-mantle transition zone were similar to the Ivrea-Verbano zone in the southern Alps of Italy and the Jijal-Patan-Chilas seuqence in the Kohistan arc of Pakistan. The boundary between mafic granulites and peridotites, which represents an exposed example of the Moho, is a high strain zone: in the mantle rocks olivine defomed by diffusion-accommodated grain bounadry sliding while pyroxenes deformed by disloaction creep, and in the lower crustal mafic granulites both pyroxenes and plagioclase deformed by dislocation creep. Thus, the Yushugou massif provides important clues to the nature and seismic reflecitivity of the continental Moho.

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Seismic velocity is one of the most important sources of information about the Earth's interior. For its proper interpretation, we must have a thorough understanding of the dependence of seismic velocity on microstructural elements, including the modal composition, the crystal preferred orientation (CPO), the grain shape, the spatial distribution of mineral phases, etc. The conventional Voigt, Reuss and Hill averaging schemes take into account only the modal composition and the CPO. The information about the Earth's interior is thus poorly constrained. For a better interpretation, it is critical to have a calculation method which accounts for the grain shape and the spatial distribution of mineral phases, etc. We propose a calculation method which accounts for the grain shape of strongly dimensionally anisotropic minerals like micas and serpentines. Our method can be applied to a distributed geometrical orientation of mineral grains. Comparison was made between calculated and measured velocities in three antigorite-serpentinite mylonites. Judging from the root mean square relative error, our method provides velocities closer to measured values than the Voigt, Reuss and Hill averaging schemes. The input of the grain shape considerably improves the prediction of seismic properties. However, large discrepancies (>0.1 km/s) between measured and calculated velocities can be seen in some directions. The discrepancies might come from microstructural elements which were not considered in the calculation (layer structures and cracks). © 2014 Elsevier B.V.

DOI： 10.1016/j.epsl.2014.01.025

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

The present study, which is a follow-up of the Journal of Geophysical Research paper by Ji et al. (2013a), provides a new calibration for both seismic and fabric properties of antigorite serpentinites. Comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic-preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high T serpentinite, which is essentially controlled by the antigorite c axis fabric, is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock. Extrapolation of the experimental data with both pressure and temperature suggests that Vp anisotropy decreases but shear wave splitting (ΔVs) and V p/Vs increase with increasing pressure in either cold or hot subduction zones. For a cold, steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening or trench-parallel movements, forming trench-parallel seismic anisotropy. For a hot, shallowly subducting slab, however, antigorite is most likely deformed by simple shear or transpression. Trench-normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30°. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneity in Vp, V s and attenuation, shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks, hydrated zones of subducting lithospheric mantle, and the crust-mantle boundary if the temperature is below 700°C in the region of interest. ©2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013JB010661

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Lithos  

Spinel peridotite xenoliths from the Ichinomegata Volcano (NE Japan) have distinct foliations defined by compositional layering between olivine-rich and pyroxene-rich layers as well as lineations defined by elongated spinel grains. Crystallographic preferred orientations (CPOs) of olivine are consistent with slip on (010)[100] and {0kl}[100]. The angles between the foliation and the olivine slip planes decrease with increasing values of the J-index (i.e. CPO strength). Such composite planar relationships within the peridotite xenoliths could result from shearing in the uppermost mantle, so that shear strains can be estimated by the angles between the foliation and the olivine slip plane in terms of simple shear strain (0.31-4.26). From these observations, we argue that a suite of the peridotite xenoliths recorded a rare snapshot of uppermost-mantle flow related to back-arc spreading during the opening of the Japan Sea. The peridotite xenoliths with higher J-indices (i.e. higher shear strain) tend to have slightly lower minimum temperatures, possibly defining a vertical strain gradient in the uppermost mantle section at the time of the volcano's eruption. The CPO data have been used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and are compared to field geophysical data from the literature. Our results are consistent with a roughly EW-oriented fastest P-wave propagation direction in the uppermost mantle beneath the northeast part of the Japan arc. Average samples are calculated based on three different structural reference frames; horizontal plane parallel to 1) foliation, 2) the plane containing the maximum concentration of olivine [100], and 3) P-wave maximum direction. S-wave anisotropy deduced from CPOs requires a reasonable thickness of the anisotropic layer (24.1-26.6. km), and the structural reference frame does not have significant effect on the estimation of thickness. Consequently, Ichinomegata peridotites record a long and complicated tectonic history; they preserve deformation 'frozen in' during back-arc spreading and this deformation also has an effect on present-day mantle flow. © 2013.

DOI： 10.1016/j.lithos.2013.10.035

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Microstructures of a layered peridotite ultramylonite from the Oman ophiolite are compared with that of experimentally deformed samples. Average grain sizes and grain size ratios of olivine and pyroxene from each layer are compared with respect to the fraction of pyroxene (fpx) in the layer. Grain size of the pyroxene is almost constant among different fpx layers, whereas olivine grain size decreases significantly with increasing fpx, both of which were characteristic features found in forsterite + enstatite aggregates after grain growth experiments (Tasaka and Hiraga, 2013). Furthermore, the Zener relationship (log dol/dpx versus log fpx) found in the ultramylonite is remarkably comparable to that observed in our experiments. These observations indicate effective pinning of olivine grain growth due to the presence of pyroxene grains during the deformation of the rocks. Olivine grains in layers with fpx ≥ 0.03 do not exhibit lattice-preferred orientation (LPO), whereas the grains in layers with fpx <0.03 exhibit LPO, indicating that deformation proceeded via diffusion- And dislocation-accommodated creep in the former and the latter layers, respectively. We simulated the evolution of grain size and viscosity in the shear zone based on our grain growth and flow laws obtained for diffusion creep of forsterite + enstatite (Tasaka and Hiraga, 2013; Tasaka et al., 2013) and successfully reproduced the observed grain sizes in the ultramylonite. We therefore conclude that the relative values of the kinetic parameters, some of which are functions of the fpx, are applicable to nature. © 2014. American Geophysical Union.

DOI： 10.1002/2013JB010619

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form as a result of solid-state deformation. Instead, the good correspondence between the original antigorite orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within the foliation-commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep. © 2013 Elsevier B.V.

DOI： 10.1016/j.epsl.2013.11.025

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Solid Earth  

DOI： 10.5194/se-5-509-2014

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Planets  

A large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase. We have measured P and S wave velocities (Vp and V s) and anisotropy of 17 amphibole-rich rock samples containing 34-80 vol % amphibole at hydrostatic pressures (P) up to 650 MPa. Combined petrophysical and geochemical analyses provide a new calibration for mean density, average major element contents, mean Vp-P and V s-P coefficients, intrinsic Vp and Vs anisotropy, Poisson's ratios, the logarithmic ratio Rs/p, and elastic moduli of amphibole-rich rocks. The Vp values decrease with increasing SiO2 and Na2O + K2O contents but increase with increasing MgO and CaO contents. The maximum (≤0.38-0.40 km/s) and minimum S wave birefringence values occur generally in the propagation direction parallel to Y and normal to foliation, respectively. Amphibole plays a critical role in the formation of seismic anisotropy, whereas the presence of plagioclase, quartz, pyroxene, and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations (CPOs). The CPO variations cause different anisotropy patterns illustrated in the Flinn diagram of V p(X)/Vp(Y)-Vp(Y)/Vp(Z) plots. The results make it possible to distinguish, in terms of seismic properties, the amphibolites from other categories of lithology such as granite-granodiorite, diorite, gabbro-diabase, felsic gneiss, mafic gneiss, eclogite, and peridotite within the Earth's crust. Hence, amphibole, aligned by dislocation creep, anisotropic growth, or rigid-body rotation, is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs, which contains rather little phyllosilicates such as mica or chlorite. Key Points New calibration of seismic velocities and anisotropy of amphibole-rich rocksAmphibolites are abundant in the crust beneath continents and oceanic islandsAmphibole plays a critical role in the formation of seismic anisotropy © 2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgrb.50352

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics of the Earth and Planetary Interiors  

Investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust, and here we report on our systematic fabric analyses of glaucophane, lawsonite, and epidote in naturally deformed blueschists from the Diablo Range and Franciscan Complex in California, and the Hida Mountains in Japan. Glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations (CPOs) characterized by a (100)[001] pattern. These characteristics, together with a bimodal distribution of grain sizes from some samples, possibly indicate the occurrence of dynamic recrystallization for glaucophane. Although lawsonite and epidote display high aspect ratios and a strong CPO of (001)[010], the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism. The P-wave (AVP) and S-wave (AVS) seismic anisotropies of glaucophane (AVP=20.4%, AVS=11.5%) and epidote (AVP=9.0%, AVS=8.0%) are typical of the crust; consequently, the fastest propagation of P-waves is parallel to the [001] maxima, and the polarization of S-waves parallel to the foliation can form a trench-parallel seismic anisotropy owing to the slowest VS polarization being normal to the subducting slab. The seismic anisotropy of lawsonite (AVP=9.6%, AVS=19.9%) is characterized by the fast propagation of P-waves subnormal to the lawsonite [001] maxima and polarization of S-waves perpendicular to the foliation and lineation, which can generate a trench-normal anisotropy. The AVS of lawsonite blueschist (5.6-9.2%) is weak compared with that of epidote blueschist (8.4-11.1%). Calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench-parallel and trench-normal seismic anisotropy beneath NE Japan, but not to that beneath the Ryukyu arc. Our results demonstrate, therefore, that lawsonite has a strong influence on seismic velocities in the oceanic crust, and that lawsonite might be the cause of complex anisotropic patterns in subduction zones. © 2013 Elsevier B.V.

DOI： 10.1016/j.pepi.2013.06.011

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV  

Harzburgites obtained from the oldest crust-mantle section in the Philippine Sea plate (similar to 52 Ma) along the landward slope of the southern lzu-Ogasawara Trench, preserve mantle fabrics formed during the infancy of the subduction zone; that is during the initial stages of Pacific plate subduction beneath the Philippine Sea plate. The harzburgites have relatively fresh primary minerals despite of their heavy serpentinizations, and show inequigranular interlobate textures, and crystal preferred orientation patterns in olivine (001)1100] and Opx (100)1001]. The harzburgites have the characteristics of residual peridotites, whereas the dunites, obtained from the same location as the harzburgites, provide evidence for the earliest stages of arc volcanism during the inception of subduction. We propose that the (001)1100] olivine patterns began forming in immature fore-arc mantle with an increase in slab-derived hydrous fluids during the initial stages of subduction in in situ oceanic island arc. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.eps1.2013.06.031

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

The Fizh massif, Oman Ophiolite, contains a ductile shear zone at the Moho Transition Zone. The dunites in the shear zone are classified based on microstructures into coarse granular texture, medium-grained texture, mylonite, and ultramylonites toward a gabbro contact. The average grain size of olivine decreases toward the shear zone, which contains a zone of high strain (~15 m wide). The proportion of hydrous minerals (amphibole and chlorite) in the shear zone show an increase toward the gabbro contact, suggesting that water infiltrated the shear zone from the gabbro contact. Equilibrium temperatures indicate a higher deformation temperature (~900°C) outside of the high strain zone compared with inside this zone (~750°C). Under these geochemical and temperature conditions, the temporal evolution of olivine crystal-preferred orientations (CPO) indicates the following continuous deformation scenario. First, deformation by dislocation creep under higher temperatures resulted in slip by D-type {0. kl}[100] and then weak E-type (001)[100] slip. Next, deformation by dislocation creep under lower temperatures and higher stress conditions produced a C-type (100)[001] CPO. Finally, superplastic deformation by grain boundary sliding resulted in a random CPO. © 2013 Elsevier B.V.

DOI： 10.1016/j.epsl.2013.07.009

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立大学法人 静岡大学理学部地球科学教室  

DOI： 10.51053/shizuoka.40.0_13

CiNii Research

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

Intense serpentinization within the Machinoyama ultramafic body, eastern-most part of the Yakuno ophiolite in the Paleozoic Maizuru belt, SW Japan, resulted in the development of block-in-matrix structure in the brittle regime, possibly associated with exhumation along a fault. Various microstructures are heterogeneously distributed throughout peridotite blocks in a serpentinite matrix. The microstructures are classified into four domains according to olivine grain size: coarse (1.0-1.5. mm), medium (0.5-1.0. mm), small (0.2-0.5. mm), and fine-grained (0.01. mm). Even in a single peridotite block, the medium-, small- and fine-grained domains occur at various scales. In particular, the fine-grained domain occurs in thin zones that cut across the other domains. These observations suggest that deformation in the peridotites before the serpentinization was partitioned into anastomosing zones during progressive inhomogeneous non-coaxial shear, resulting in the simultaneous development of a range of microstructures from the coarse-grained domain to the small-grained domain. Olivine crystal-preferred orientations tend to be weaker with decreasing olivine grain size, indicating a change in deformation mechanism from dislocation creep to grain-size-sensitive creep. Moreover, the fine-grained domains contain much more second phase than the small-grained domain even in the same sample, suggesting the role of the second phases on strain localization during dynamic recrystallization of olivine. © 2012 Elsevier B.V.

DOI： 10.1016/j.tecto.2012.07.008

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

DOI： 10.1016/j.tecto.2012.11.027

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Island Arc  

Deformation microstructures for a lawsonite blueschist from the New Idria serpentinite body, Diablo Range, are investigated to clarify rheological behaviors of glaucophane and lawsonite, which are main mineral assemblages of subducting oceanic crust at relatively cold geotherm. Developments of crystal-preferred orientations (CPOs) with small grain size, irregular grain boundary and high aspect ratio of glaucophane indicate deformation mechanism as recovery and dynamic recrystallization possibly accommodated by dislocation creep, while lawsonite deforms by rigid body rotation based on euhedral grains with angular or straight grain boundaries. Higher aspect ratios, lower angle to foliation, and stronger CPOs of both minerals in the glaucophane-rich layer rather than those in the lawsonite-rich layer suggest the strain localization into the glaucophane-rich layer. Additionally fabric strength (the degree of crystal alignment) and seismic anisotropy are higher in the glaucophane-rich layer than that of the lawsonite-rich layer, which is consistent with the microstructural analyses. All our results imply, therefore, the dominant role of glaucophane rather than lawsonite for rheological behavior and seismic anisotropy of blueschist. © 2012 Wiley Publishing Asia Pty Ltd.

DOI： 10.1111/iar.12003

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Solid Earth  

This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow-and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs. © 2013 Author(s).

DOI： 10.5194/se-4-511-2013

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Drilling  

DOI： 10.2204/iodp.sd.15.10.2013

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立大学法人 静岡大学理学部地球科学教室  

DOI： 10.51053/shizuoka.39.0_7

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：静岡大学地球科学教室  

DOI： 10.14945/00007130

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

We have investigated K-feldspar recrystallisation in granitoid mylonites within a ductile shear zone from the Ryoke metamorphic belt, SW Japan. Fine-grained K-feldspar (20μm on average) occurs in the matrix and in pull-apart areas within fractured K-feldspar porphyroclasts. These fine grains are elongated and oriented parallel to the main foliation in the matrix, and their grain surfaces, observed with the scanning electron microscope, are not smooth, but rough due to the development of very fine (<1μm) round grains of K-feldspar on the surface of each grain. In pull-apart areas, the crystallographic orientation of fine-grained K-feldspar, as measured by electron backscatter diffraction (EBSD), is strongly controlled by that of the host porphyroclast, and shows rotations with shear components parallel to fractures. In the matrix, the crystallographic orientation of fine-grained K-feldspar is not consistent with intracrystalline plasticity, but rather with a growth rate that is slightly controlled by nearby porphyroclasts. All this, together with the growth features on grains, suggests that solution-precipitation of K-feldspar from K-rich aqueous fluid occurred during progressive deformation. Infrared (IR) mapping was performed to evaluate the distribution of water in pull-apart areas and the matrix. Water is heterogeneously distributed within K-feldspar porphyroclasts, which contain 150-2200ppm H 2O. In contrast, the water content is low (150-300ppm H 2O) and homogeneously distributed in fine-grained K-feldspar in the matrix and pull-apart areas, even though included in these analyses are grain boundaries that can generally contain abundant aqueous fluid. The results of EBSD analysis and IR mapping indicate that water is released during solution-precipitation of K-feldspar under mid-crustal conditions. The solution-precipitation process under a water-rich environment in the middle crust results in the formation of fine grains, possibly deforming dominantly by grain-size-sensitive creep, and with the release of aqueous fluid involved in the process. © 2012 Elsevier B.V.

DOI： 10.1016/j.tecto.2012.01.033

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics and Chemistry of Minerals  

Annealing experiments on agate were performed to investigate grain growth kinetics and the effect of crystallographic anisotropy on normal grain growth of quartz. The experiments were conducted using a piston-cylinder apparatus at 700-800°C and 0.5 GPa for 0-66 h. The grain growth rate was expressed by D n-D 0n = kt with k = k 0exp(-H*/RT) where D 0 is the initial grain size at t = 0, with n = 4.4 ± 0.3, and H* = 191.3 ± 11.0 kJ/mol is the activation enthalpy and logk 0 = 19.8 ± 1.4. While the grain aspect ratios are nearly constant at ~0.7 (short/long) during grain growth, the longest axis in individual grains tends to be oriented parallel to their c-axis, indicating that a primary crystal-preferred orientation of c-axis of the agate could result in the development of a weak shape-preferred orientation during grain growth. © 2011 Springer-Verlag.

DOI： 10.1007/s00269-011-0476-6

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記述言語：英語  

DOI： 10.1073/pnas.1112005109

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

Peridotite xenoliths from Northeast Tasmania, Australia, consist of spinel lherzolites and minor dunites that originated from the uppermost mantle at depths of less than 40. km, based on the geothermal gradient beneath Tasmania, where the depth of the Moho is 29. km. The crystal preferred orientations (CPOs) of olivine and pyroxene were measured in 5 of 13 collected samples (1 dunite and 4 lherzolites) that were large enough to enable measurements. Olivine CPOs indicate the dominance of (010) [100] slip. Geothermobarometric analyses of the xenoliths yield temperatures of 950-1050. °C and pressures of 0.7-1.1. GPa. Using these data, we calculated changes in the seismic properties of a rock with ideal volume fractions of olivine and pyroxene under conditions of 1000. °C and 1. GPa, for various orientations of the structural reference frame in the case of three geodynamic models (extension, shear, and upwelling). The model of horizontal extension performs best in explaining the structure of the uppermost lithospheric mantle beneath Northeast Tasmania. © 2011 Elsevier B.V.

DOI： 10.1016/j.tecto.2011.12.002

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人　日本鉱物科学会  

The understanding of olivine fabric has dramatically been progressed during the last~10 years by both natural and experimental studies along with the major technological improvement. Crystal-preferred orientations (CPO) are the expression of crystallographic fabrics of grains within the rock with respect to the structural frame (X-,Y- and Z-axes). CPO patterns can be interpreted based on the plane of plastic flow and the flow direction. Olivine fabrics, which are olivine CPO patterns, have been classified into five types: A, B, C, D and E types by a series of experimental studies. An additional AG type has also been proposed in recognition of its common occurrence in nature. New results have already required major modiications to the geodynamic interpretation of the upper mantle, although some uncertainties still remain regarding the olivine fabrics and their development. © 2012, Japan Association of Mineralogical Sciences. All rights reserved.

DOI： 10.2465/gkk.120712b

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Peridotite xenoliths from the Kilbourne Hole maar, New Mexico, consist of spinel lherzolite, harzburgite, and dunite. Because Kilbourne Hole erupted at approximately 10. ka, these xenoliths represent essentially current conditions beneath the Rio Grande rift. In this study, we present detailed petrofabric data and seismic properties obtained from peridotite xenoliths from Kilbourne Hole to illuminate the origin and significance of shear-wave splitting in the uppermost mantle beneath this active rift. Using phase relations and the temperature of equilibration, we infer that these xenoliths were derived from the uppermost mantle, from depths of 35-60. km. Their crystallographic preferred orientations indicate the preservation of olivine b-axis fiber fabrics with a strong concentration of [010] with girdles of [100] and [001]. We consider three geodynamic models for the source region of these xenoliths: horizontal extension, lateral shear, and upwelling. After calculating seismic properties using a volume fraction of olivine, orthopyroxene and clinopyroxene appropriate to each model, we conclude that these xenoliths are derived from a lateral shear zone (vertical foliation (XY plane) and horizontal lineation within the plane of the foliation (X-axis)). However, the degree of seismic anisotropy generated by peridotite xenoliths alone is limited, so that the existence of melt in thin cracks or dikes could be required to cause a significant increase; the orientation of such melt pockets parallel to the XY plane in either model would result in an increase in anisotropy. These results indicate that the shear-wave splitting observed in the Rio Grande rift is a reflection of the lithospheric fabric and the presence of melts as thin cracks or dikes. © 2011 Elsevier B.V.

DOI： 10.1016/j.epsl.2011.09.013

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立大学法人 静岡大学理学部地球科学教室  

DOI： 10.51053/shizuoka.38.0_21

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Structural Geology  

Antigorite (Atg) is stable throughout large parts of the wedge mantle of most subduction zones. Atg shows strong acoustic anisotropy and crystallographic preferred orientation (CPO) patterns of this mineral may contribute significantly to seismic anisotropy in convergent margins. Atg CPO patterns from the Higashi-Akaishi (HA) forearc mantle body of southwest Japan adds to the data set suggesting the most common Atg CPO pattern has a c-axis perpendicular to the foliation and a b-axis parallel to the stretching lineation. Statistical analysis using the eigenvector method of Atg CPO from two mutually perpendicular directions in the same sample (YZ-section and XZ-section) shows no significant differences implying sample preparation has no significant affect on the resulting Atg CPO. Reuss (uniform stress) averages of anisotropy for the Higashi-Akaishi samples are approximately treble the values for Voigt (uniform strain) averages. When comparing calculated anisotropy of hydrated mantle peridotite samples-such as the Higashi-Akaishi unit-with observed S-wave delay times in convergent margins, the appropriate averaging method needs to be considered. © 2011.

DOI： 10.1016/j.jsg.2011.08.006

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：静岡大学地球科学教室  

DOI： 10.14945/00006209

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge. Copyright 2011 by the American Geophysical Union.

DOI： 10.1029/2010JB007931

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Lithos  

Microstructural and petrologic analyses of 7 gabbroic rocks sampled from the medial area of the Godzilla Megamullion (site KH07-02-D18), located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a high-temperature ductile shear zone associated with hydrothermal metamorphism in the lower crust. The deformed gabbroic rocks are petrographically classified into mylonites and an ultramylonite, and are characterized by porphyroclastic textures consisting mainly of coarse plagioclase and clinopyroxene/amphibole porphyroclasts in a fine-grained matrix. Plagioclase crystallographic-preferred orientations vary from (010)[100] and (001)[100] patterns in the mylonites to a weak (001)[100] pattern in the some mylonites and ultramylonite, suggesting a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep with increasing intensity of deformation. The chemical composition of matrix plagioclase is generally more sodic than that of porphyroclasts. Secondary amphibole is ubiquitous, consisting mainly of pargasite and magnesiohornblende (brown hornblende) and actinolite (green hornblende). The mineral assemblage is consistent with the hydrothermal metamorphic reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. Compared with deformed gabbroic rocks from the breakaway and termination areas of the Godzilla Megamullion, the samples record ductile shearing under high temperature conditions, possibly related to the development of the Godzilla Megamullion, although hydrothermal activity in the medial area appears to have been less intense than in both the breakaway and termination areas. © 2011 Elsevier B.V.

DOI： 10.1016/j.lithos.2011.02.001

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Island Arc  

Relicts of deformed lithospheric mantle have been identified within serpentinites and weathered peridotites recovered from nine dredge sites and one submersible dive site from across the Godzilla Megamullion, which was emplaced at the now-extinct Parece Vela Rift in the Parece Vela Basin, a back-arc basin in the Philippine Sea. The serpentinites consist dominantly of lizardite±chrysotile and magnetite with minor relict primary minerals that include pyroxene, spinel, and rare olivine. The weathered peridotites consist of pyroxene, spinel, lizardite±chrysotile, and magnetite as well as weathering products of olivine. These rocks were classified in hand specimen into three types with different structures: massive, foliated, and mylonitic. In thin-section the serpentine minerals show no sign of deformation, whereas relict primary minerals show evidence of plastic deformation such as undulose extinction, kink bands, dynamic recrystallization, and weak to moderate crystallographic preferred orientations. Therefore, the serpentinites and weathered peridotites result from the static replacement and weathering of previously ductile-deformed peridotite. Given their location close to or on the detachment surface that exposed them, the relicts of peridotite provide evidence of deformation in the lithospheric mantle that could be related to the formation and emplacement of the Godzilla Megamullion in the Parece Vela Rift. © 2011 Blackwell Publishing Asia Pty Ltd.

DOI： 10.1111/j.1440-1738.2011.00759.x

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geological Journal  

Water contents estimates based on infrared spectroscopy and deformation microstructures are used to constrain the origin of textural variations observed in mantle xenoliths from Kimberley, South Africa. Infrared spectra indicate water contents of 10-370ppm (H2O by weight) in olivine, 20-370ppm in orthopyroxene, up to 340ppm in garnet and 30-550ppm in clinopyroxene, although no systematic differences are detected among texturally distinct xenoliths (i.e. granular and sheared peridotites). In contrast, active slip systems in olivine, as inferred from tilt boundaries, are different between the granular and sheared peridotites; the former deformed via the [100](010) slip system, which is commonly active under hot, dry conditions, whereas the latter deformed via the [001](100) slip system, which is commonly activated in water-rich environments. The discrepancy in water contents estimates based on infrared spectroscopy and deformation microstructures may reflect different stages of the evolution of water content: the present water contents, as analysed by infrared spectroscopy, may have been modified by the most recent events (e.g. eruption by kimberlite magma), whereas the water contents inferred from tilt boundaries may have resulted from the various deformation processes in the deep cratonic mantle. Accordingly, we suggest that the observed heterogeneity in water content within cratonic roots facilitates local deformation, and gives rise to the range of textures seen in cratonic mantle xenoliths. © 2010 John Wiley & Sons, Ltd..

DOI： 10.1002/gj.1216

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Petit-spots, the late Miocene alkali basaltic volcanoes on the Early Cretaceous NW Pacific Plate, originate at the base of the lithosphere. The petit-spot volcanic rocks enclose fragments of tholeiitic basalt, dolerite, gabbro, and mantle peridotite, providing a unique window into the entire section of subducting oceanic lithosphere. We provide here the first direct observations on the deep structure of the Pacific lithosphere using microstructural analyses of a petit-spot peridotite xenolith. The xenolith is a lherzolite that consists mainly of coarse- and medium-grained olivine, orthopyroxene, and clinopyroxene, as well as fine-grained aggregates of spinel and orthopyroxene that probably represent replaced pyrope-rich garnet. A strong deformational fabric is marked by a parallel alignment of millimeter-sized elongate minerals and their crystallographic preferred orientation. The olivine displays a [010] fiber pattern with a girdle of [100] axes and a maximum of [010] perpendicular to the foliation, a pattern which is consistent with a transpressional deformation in high temperature conditions at the base of oceanic lithosphere. Our microstructural observations and seismic data indicate that the lower part of the NW Pacific lithosphere possess an early stage structure of mantle flow at the asthenosphere. This interpretation is compatible with a conventional model in which oceanic lithosphere is thickened during cooling and plate convection. A discrepancy between the weak anisotropy in the petit-spot peridotite and the strong azimuthal anisotropy from the seismic data in the NW Pacific plate implies the existence of a highly anisotropic component in the deep oceanic lithosphere. © 2010 Elsevier B.V.

DOI： 10.1016/j.epsl.2010.12.011

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担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics of the Earth and Planetary Interiors  

Garnet clinopyroxenites occur within foliated dunite in the Higashi-akaishi peridotite mass, located within the subduction-type high-pressure/low-temperature Sanbagawa metamorphic belt. The garnet clinopyroxenites contain 3-80% garnet, and garnet and clinopyroxene are homogeneously distributed. Garnet crystals contain extensive, regular dislocation arrays and dislocation networks, suggesting that dislocation creep was the dominant deformation mechanism. Analyses of crystallographic orientation maps indicate similar grain sizes and aspect ratios for garnet and clinopyroxene, regardless of modal composition, indicating that these minerals deformed with similar degree of plasticity. Moreover, indexes of crystallographic fabric intensity (i.e., J-index and M-index) for both garnet and clinopyroxene tend to increase with increasing modal composition of garnet. Fourier-transform infrared spectroscopy analysis revealed that water content in garnet is ∼60. ppm, whereas that in clinopyroxene is ∼70. ppm. Olivine crystal-preferred orientations in the Higashi-akaishi peridotite mass, characterized by [0. 0. 1] (0. 1. 0), are thought to have developed during deformation under wet conditions. Consequently, we argue that the presence of water could act to enhance garnet plasticity during deformation. The results reveal contrasting influences of water on the deformation of garnet and diopside: under wet conditions compared with dry, the strain rate increases by two orders of magnitude for garnet but by an order of magnitude for diopside. Given the influence of water on the creep strength of garnet, garnet within the Higashi-akaishi mass may have become significantly as weak as clinopyroxene during deformation. © 2010 Elsevier B.V.

DOI： 10.1016/j.pepi.2010.10.008

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）  

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Island Arc  

Gabbroic rocks and amphibolites were collected from the KR03-01-D10 dredge site located on the West Arm Rise of the Godzilla Megamullion, close to the Parece Vela Rift which appears to correspond to the termination area of a detachment fault, the Philippine Sea. The gabbroic rocks and amphibolites reveal the occurrence of a high hydrothermal activity in the lower crust close to a paleo-ridge. In the gabbroic rocks, plagioclase compositions of both porphyroclasts and matrix were transformed into sodium-rich compositions close to albite. Amphiboles are of secondary rather than igneous origin based on their microstructural occurrences. In the amphibolites, anorthite contents of porphyroclasts and matrix plagioclase are relatively lower than those of the gabbroic rocks, whereas the chemical compositions of amphibole within the amphibolites are similar to those of amphibole within the gabbroic rocks. Amphibolites represent the product of retrograde metamorphism associated with hydrothermal alteration of the gabbroic body by the reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. The estimated temperatures of the amphibolites derived from the amphibole thermobarometer and the gabbroic rocks derived from the hornblende-plagioclase geothermometer show ~700-950°C and 650-840°C, respectively. The hydrothermal alteration recorded in the gabbroic rocks possibly occurred under high-T conditions; the rocks were then metamorphosed to the amphibolites during a retrogressive stage. Our study indicates that amphibolitization took place with various degrees of deformation. It may imply that the hydrothermal activity increased as the Godzilla Megamullion developed as an oceanic core complex in the paleo-ridge. © 2010 Blackwell Publishing Asia Pty Ltd.

DOI： 10.1111/j.1440-1738.2010.00741.x

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

We conducted a microstructural study of samples from a natural serpentinite shear zone in the Ohmachi Seamount, Izu-Bonin frontal arc. The serpentinite samples consist mainly of columnar antigorite grains that show marked variations in texture from two approximately orthogonal sets of grains (interpenetrating) to aligned (schistose) forms. Because the two types of grains have similar compositions, these textural differences are interpreted to reflect the existence of a strain gradient toward a plate interface in a subduction zone. The crystal-preferred orientation (CPO) of antigorite with interpenetrating texture is almost randomly oriented, whereas in the case of schistose texture the CPO shows a typical [010](001) pattern. We also found that with increasing intensity of schistosity, the polarization plane of Vs1 for antigorite grains becomes aligned parallel to the flow plane, consistent with a plane oriented normal to the maximum concentration of slow antigorite c-axes. This configuration results in seismic anisotropy that is approximately five times higher than that for olivine grains. These findings indicate that if a serpentinite layer on the plate interface attains large bulk shear strains (γ>~2), the resultant alignment of antigorite grains within the layer strongly influences the orientation and magnitude of seismic anisotropy in the mantle wedge, depending on the dip angle of the subducting slab. © 2010 Elsevier B.V.

DOI： 10.1016/j.epsl.2010.08.035

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

Peridotite xenoliths from southern Texas consist of spinel lherzolite, harzburgite and minor dunite. Based on phase relations and temperature of equilibration, Knippa xenoliths come from the uppermost mantle, 40-70 km deep. Knippa xenoliths provide rare snapshots of upper mantle processes and compositions beneath south-central Laurentia. They preserve olivine a-axis fiber fabrics with a strong concentration of [100] and girdles of [010] and [001]. Assuming a lithospheric mantle having a horizontal flow direction parallel to fast directions, the mantle lithospheric fabric revealed by the xenoliths mostly explains the magnitude of observed shear-wave splitting observed along the southern margin of the Laurentian craton. © 2010 by the American Geophysical Union.

DOI： 10.1029/2010GL044538

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

To identify the dominant deformation mechanism in continental middle crust at an arc-trench system, we used an SEM-EBSD system to measure the lattice-preferred orientations of quartz grains in fine-grained metachert from the low-grade (chlorite and chlorite-biotite zones) part of the low-P/high-T Ryoke metamorphic belt, SW Japan. Quartz c-axis fabrics show no distinct patterns related to dislocation creep, although the strain magnitudes estimated based on deformed radiolarian fossils are high enough that a distinct fabric might be expected to have formed during deformation. Fabric intensities are very low, indicating a random distribution of quartz c-axes. Quartz grains are equant in shape and polygonal, and free of intracrystalline plasticity. These observations suggest that the dominant deformation mechanism in the metacherts was grain-size-sensitive flow (diffusion creep accompanied by grain-boundary sliding) rather than dislocation creep, possibly reflecting the relatively low strain rate or low flow stress compared with that in high-strain zones. The development of grain-size-sensitive flow in metamorphic tectonites at mid-crustal conditions would result in a significant decrease of the rocks strength of the continental middle crust. © 2010 Elsevier B.V.

DOI： 10.1016/j.tecto.2010.06.002

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Contributions to Mineralogy and Petrology  

The effect of grain growth on the cation exchange between synthesized forsterite aggregates (i. e., dunite) and nickel-rich aqueous fluid was evaluated experimentally at 1. 2 GPa and 1,200°C. The grain boundary (GB) migration caused nickel enrichment in the area swept by the GBs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to ~80 times that was calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected fluid networks was found to be 6.5 × 10-19-6.7 × 10-18 m3/s, which is 4-5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper mantle. © 2010 Springer-Verlag.

DOI： 10.1007/s00410-009-0481-7

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立大学法人 静岡大学理学部地球科学教室  

DOI： 10.51053/shizuoka.37.0_13

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立大学法人 静岡大学理学部地球科学教室  

DOI： 10.51053/shizuoka.37.0_27

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：静岡大学  

DOI： 10.14945/00005289

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Metamorphic Geology  

Calcite-dolomite solvus geothermometry is a versatile method for the estimation of metamorphic temperature because of its simplicity. However, in medium- to high-grade metamorphic rocks the accuracy of estimating temperature by the integration of unmixed dolomite and calcite is hampered by the heterogeneous distribution of unmixed dolomite, difficulties in distinguishing between preexisting and exsolved dolomite and demarcating grain boundaries. In this study, it is shown that calcite-dolomite solvus thermometry can be applied to calcite inclusions in forsterite and spinel for the estimation of peak metamorphic temperature in granulite facies marbles from Skallevikshalsen, East Antarctica. The marbles are comprised of a granoblastic mineral assemblage of calcite + dolomite + forsterite + diopside + spinel + phlogopite ± apatite, characteristic of granulite facies metamorphic conditions. Forsterite, spinel and apatite frequently contain 'negative crystal' inclusions of carbonates that display homogeneously distributed dolomite lamellae. On the basis of narrow ranges of temperature (850-870 °C) recorded from carbonate inclusions compared with the range from matrix carbonate it is regarded that the inclusion carbonates represent a closed system. Furthermore, this estimate is consistent with dolomite-graphite carbon isotope geothermometry, and is considered to be the best estimate of peak metamorphic temperature for this region. Matrix calcite records different stages of retrograde metamorphism and re-equilibration of calcite that continued until Mg diffusion ceased at ∼460 °C. Electron backscattered diffraction (EBSD) results together with morphological features of unmixed coarse tabular dolomite suggest anisotropic diffusion and mineral growth are influenced by crystallographic orientation. Identification of sub-grain boundaries and formation of fine-grained unmixing in calcite rims suggest the presence of grain boundary fluids in the late retrograde stages of metamorphic evolution. These results, thus, demonstrate the usefulness of carbonate inclusion geothermometry in estimating the peak metamorphic temperatures of high-grade terranes and the application of EBSD in understanding the unmixing behaviour of minerals with solid solutions. © 2010 Blackwell Publishing Ltd.

DOI： 10.1111/j.1525-1314.2010.00877.x

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Petrology  

The Mohelno peridotite is a medium-sized ultramafic body (2 km×4 km in size) enclosed in the Gföhl granulites in the eastern part of the BohemianMassif. It consists mainly of coarse spinel peridotite (harzburgite and dunite); garnet peridotite occurs only in the sheared and deformed margins of the body. To decipher the origin and history of this mantle-derived peridotite, we determined the mineral chemistry by electron microprobe analysis and olivine fabric patterns by the electron backscattered diffraction method for each rock type.We found two distinct types of olivine fabric (crystal-preferred orientation; CPO) in the peridotite, which can be correlated with the mineralogy and thermal history of each. The olivine CPO in coarse-grained spinel peridotite shows a strong concentration of [100] slightly oblique to the lineation and [010] and [001] girdles normal to the lineation (which is the so-called {0kl}[100] pattern typical of medium-temperature deformation). Olivine in coarse-grained garnet peridotite, on the other hand, shows a strong concentration of [010] normal to the foliation and a concentration of [100] parallel to the lineation (which is the so-called (010)[100] pattern typical of high-temperature deformation). These fabric patterns become diffuse as the grain size is reduced for each mineralogical type.We interpret the development of these contrasting fabric patterns and mineralogical types based on the pressure-temperature history of each rock type determined by applying published geothermometers and geobarometers to the constituent minerals. Starting from a high-temperature (>1200°C) spinel peridotite, during exhumation and cooling in contact with surrounding granulites, the marginal part of the body was transformed to garnet peridotite, whereas the interior remained in the spinel-peridotite facies because cooling was slower inside the body. Because of the slow cooling and continuous deformation in the interior of the body, the original high-temperature fabric pattern in the spinel peridotite was converted to a lower-temperature type.The high-temperature fabric was preserved only at the margin of the body where cooling was more rapid. Reduction of grain size that occurred during later, low-temperature, deformation partly obliterated the high-temperature fabric patterns for both garnet and spinel peridotites. The initial rapid cooling at high temperatures associated with deformation probably occurred after the mantle peridotite was emplaced within the crustal granulites, which implies that the spinel- to garnet-peridotite transformation took place in the continental crust. © The Author 2010. Published by Oxford University Press.

DOI： 10.1093/petrology/egp092

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担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）  

J-GLOBAL

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記述言語：英語  

DOI： 10.1038/nature08513

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

Peridotite xenoliths derived from the low velocity zone beneath the Avacha frontal volcano, Kamchatka, preserve a-axis slip fabrics, comparable with those in xenoliths from the back-arc region of the NE Japan. Although low-velocity zones are commonly attributed to zones of partially melted mantle, migration of the melt does not erase the existing olivine fabrics and related Scismic anisotropics. These anisotropics may counteract the anisotropics associated with c-axis slip fabrics, if they exist, along the slab or in the high-pressure zone. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2009GL038527

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geochemistry, Geophysics, Geosystems  

[1] Two N-S fault zones in the southern Mariana fore arc record at least 20 km of left-lateral displacement. We examined the eastward facing slope of one of the fault zones (the West Santa Rosa Bank fault) from 6469 to 5957 m water depth using the submersible Shinkai 6500 (YK06-12 Dive 973) as part of a cruise by the R/V Yokosuka in 2006. The dive recovered residual but still partly fertile lherzolite, residual lherzolite, and dunite; the samples show mylonitic, porphyroclastic, and coarse, moderately deformed secondary textures. Crystal-preferred orientations of olivine within the peridotites show a typical [100](010) pattern, with the fabric intensity decreasing from rocks with coarse secondary texture to mylonites. The sampled peridotites therefore represent a ductile shear zone within the lithospheric mantle of the overriding plate. Peridotites were probably exposed in association with a tear in the subducting slab, previously inferred from bathymetry and seismicity. Furthermore, although the dive site is located in the fore arc close to the Mariana Trench, spinel compositions within the sampled peridotites are comparable to those from the Mariana Trough back arc, suggesting that back-arc basin mantle is exposed along the West Santa Rosa Bank fault. © 2009 by the American Geophysical Union.

DOI： 10.1029/2008GC002197

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

P- (Vp) and S-wave (Vs) velocities of garnet-free (two-pyroxene granulite) and garnet-bearing (garnet granulite and garnet pyroxenite) lowermost crustal rocks collected from the Kohistan arc, northern Pakistan, were measured at 0.1-1.0 GPa and 25-400 °C. Garnet granulite had higher Vp (+ 0.31 km/s) and Vs (+ 0.27 km/s) than two-pyroxene granulite. Although Vp and Vs increased with increasing volume percent of garnet, plagioclase-free garnet pyroxenite showed significantly higher Vp and Vs than plagioclase-rich garnet granulite mainly due to the low Vp and Vs of plagioclase. In contrast, we observed two quasi-linear relationships between Vp (Vs) and SiO2 content for the garnet-bearing and garnet-free rocks. The garnet-bearing rocks had relatively higher Vp and Vs and stronger SiO2 dependences than the garnet-free rocks. The stronger SiO2 dependences of Vp and Vs in the garnet-bearing rocks suggest that the garnet formation in mafic to ultramafic rocks (e.g., pyroxenite and hornblendite), having relatively lower SiO2, leads to more pronounced increases in Vp and Vs than that of relatively felsic rocks (e.g., felsic-to-mafic granulite). Indeed, the Vp and Vs of the garnet pyroxenite were significantly higher than those of garnet granulite but comparable to those of dunite. The significantly high Vp and Vs of the garnet pyroxenite yielded high reflection coefficients between the garnet granulite and garnet pyroxenite of up to 0.13 for P-waves and 0.14 for S-waves, comparable to values expected for Moho reflection. Thus the lithological boundary between plagioclase-rich garnet granulite and plagioclase-free garnet pyroxenite in the lowermost crust of the Kohistan arc corresponds to the seismic Moho discontinuity. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2008.12.010

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：一般社団法人 日本地質学会  

In this study of peridotite xenoliths from Ichinomegata volcano, Oga peninsula, NE Japan, we used electron backscattered diffraction to assess the validity of two methods of identifying the olivine slip system that operated during deformation. The xenoliths are harzburgite in composition and contain a distinct foliation and lineation defined by aligned spinel and pyroxene grains. Using the first method, the dominant slip system in olivine was estimated based on the crystallographic preferred orientation (CPO) of olivine with respect to the foliation and lineation. The second method was based on an analysis of subgrain rotation: given that intracrystalline subgrain boundaries were observed within several olivine grains, we measured the misorientation angles across these boundaries to define sets of slip planes, slip directions, and rotation axes. The olivine slip systems estimated using the two methods are essentially identical. The CPO pattern provides an important constraint on the dominant slip system that operated during deformation, whereas the analysis of subgrain rotation yields the slip system responsible for the development of individual subgrain boundaries composed of edge dislocations. The two methods are therefore complementary.

DOI： 10.5575/geosoc.115.288

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Analysis of an in situ fault zone within the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues to the relevant deformation mechanisms and their temporal evolution within oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies to a final depth of 1415 m below the sea floor (mbsf), with very high recovery rates of up to 100% (generally ∼ 80%). We identified an intra-crustal fault zone between 720 and 780 mbsf in a section of massive gabbro, olivine gabbro, oxide gabbro units, and minor diabase intrusions. Of particular interest is the section between 744 and 750 mbsf, which unfortunately was marked by low recovery rates (17%). Electrical borehole-wall images show a 1-m-thick zone of east-dipping fractures within this interval, which is otherwise dominated by N-S dipping structures. Despite the high fracture density in this section, the hole walls are smooth, with rare breakouts, suggesting that the low recovery rate was due to a change in lithology rather than well conditions. The recovered rocks include ultracataclasite and possibly incohesive fault gouge that formed in the upper amphibolite regime, with mostly amphibole infill. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, consistent with increased amounts of amphibole found in the core. Equilibration temperature conditions of about 640 °C were obtained for plagioclase clasts and aluminous actinolite, assuming a pressure of 200 MPa. The permeability of the fault zone is in the range of 10- 19 to 10- 17 m2. Although the permeability appears to be high within the fault zone relative to other parts of the section, it is no higher than that in typical lower crustal material. As a consequence, because brittle failure occurred at high temperatures, the fault zone was subsequently completely sealed by hydrous minerals, thereby preventing further fluid circulation and preserving water in the crust. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2008.08.033

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

Microstructural and petrological analyses of gabbroic rocks sampled from the Godzilla Mullion, located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a ductile shear zone in the lower crust. The shear zone is interpreted to represent a detachment fault within an oceanic core complex. Microstructures indicative of intense deformation, characterized by porphyroclastic textures consisting dominantly of coarse plagioclase porphyroclasts and lesser clinopyroxene porphyroclasts in a fine-grained matrix, are observed within samples of gabbroic rocks dredged near the breakaway area of the Godzilla Mullion (dredge site D6). Samples are classified into three types based upon the grain-size of fine-grained plagioclase in the matrix: coarse (80-130 μm), medium (25 μm), and fine (∼ 10 μm). Although the chemical composition of plagioclase porphyroclasts is consistently An 40-50 among all sample types, the compositions of fine grains in the matrix vary with decreasing grain-size, being An 40-50 for the coarse-type, An 30-40 for the medium-type, and An 5-30 for the fine-type. This finding implies that the composition of fine-grained plagioclase in the matrix is related to the following retrograde reaction that occurred during deformation: clinopyroxene + plagioclase + Fe-Ti oxide + fluid → hornblende + plagioclase. Plagioclase crystal-preferred orientations also show a gradual change with grain-size, varying from a (010)[100] pattern for the coarse-type, (010)[100] and (001)[100] patterns for the medium-type, and a weak (001)[100] pattern or random orientations for the fine-type. These patterns are interpreted to result from a change in the deformation mechanism of plagioclase from dislocation creep to grain-size-sensitive creep with decreasing temperature, thereby leading to strain softening and localization during cooling. Although secondary amphibole occurs ubiquitously within all samples, the chemical composition of amphibole varies from pargasitic hornblende (i.e. brown hornblende) to actinolite (i.e. green hornblende) within each of the sample types. However, amphibole in the coarse-type shows no evidence of deformation, whereas brown hornblende in the medium- and fine-types is plastically deformed. As a consequence, we argue that the microstructural development of the gabbroic rocks occurred during uplift-related cooling of the gabbro body and that a primary shear zone developed near the breakaway area at depth under anhydrous conditions at high temperatures above 850 °C; the shear zone subsequently evolved during progressive retrogression in association with hydration of the shear zone, possibly resulting in the development of the detachment fault that gave rise to the Godzilla Mullion. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2008.06.009

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Applied Physics  

Various ZnO morphologies with hexagonal cross sections were fabricated by a thermal oxidation method. Depending on the growth temperature, the lateral lengths of the hexagonal cross section varied from several hundred nanometers to the micrometer order. Comparing the cathodoluminescence (CL) of the synthesized ZnO to that of commercial ZnO:Zn phosphor revealed that a strong luminescence can be realized in undoped ZnO by fabricating one-dimensional structures. Specifically, in the micropyramid and towerlike structures, the CL intensity is locally concentrated near the hexagonal boundary. The enhanced green emission in these structures can be qualitatively explained by whispering gallery resonance modes produced in the hexagonal microcavity. © 2008 American Institute of Physics.

DOI： 10.1063/1.2990050

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担当区分：責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg + Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr + Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites, as their formation requires hot, hydrous, shallow mantle (> 1250 °C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880-1030 °C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2008.06.014

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：国立大学法人 静岡大学理学部地球科学教室  

DOI： 10.51053/shizuoka.35.0_45

CiNii Research

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：東京地学協会  

&emsp;Peridotites derived from the uppermost mantle consist dominantly of olivine and subsequently of pyroxene, spinel, garnet, and plagioclase. Crystal-plastic flow of mantle rocks results in various types of structure within peridotite being developed to varying degrees, depending upon the structure sensitivity of the different mineral phases. Plastic deformation leads to the simultaneous development of shape-preferred orientations and crystal-preferred orientations. A shape-preferred orientation is the expression of the average orientation of flattening (foliation) and elongation (lineation) directions, as defined by the orientations of individual grains. A crystal-preferred orientation (CPO) is the expression of crystallographic orientations of grains within the rock, as developed via dislocation creep and recrystallization. During intense homogeneous plastic deformation of a peridotite composed of minerals with a dominant slip system, the preferred orientation of the slip plane and slip direction tends to coincide with the plane of plastic flow and the flow direction, respectively. Recently, a new olivine CPO classification (A, B, C, D, and E types) has been proposed by Karato and co-workers to illustrate the roles of stress and water content as controlling factors of olivine slip systems. An additional CPO type (AG) has also been proposed in recognition of its common occurrence in nature. Given that olivine and the other constituent minerals in peridotites contain intrinsic elastic anisotropies, the development of CPO within peridotite during plastic deformation gives rise to seismic anisotropy in the upper mantle. Thus, the anisotropic properties of mantle rocks derived from the upper 100 km of the mantle, such as Ichinomegata peridotite xenoliths from the northeast Japan arc, have been calculated and applied with the aim of understanding the seismic anisotropy of the Earth's mantle.

DOI： 10.5026/jgeography.117.93

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

We retrieved samples of peridotite from a dredge haul (KH92-1-D2) collected during Cruise KH92-1 undertaken by the research vessel (R/V) Hakuho in 1992 at the landward trench slope of the southern Mariana Trench (11°41.16′N, 143°29.62′E; depth 6594-7431 m), which is the deepest ocean in the world. Ten of 30 retrieved samples possessed both a foliation and lineation, as assessed from 46 thin sections of various orientations and observations of hand samples. The samples showed marked variation in microstructure, ranging from coarse (> 5 mm) equigranular and intensely elongated textures to finer (< 1 mm) porphyroclastic and fine-grained equigranular textures. Olivine fabrics also varied among the different samples, with (010)[100] and (010)[001] patterns (termed A- and B-type, respectively) observed in samples with coarse textures and no clear patterns observed in samples with fine textures. Even though the peridotite samples were retrieved from a single dredge site, some contain primary tectonic microstructures and some contain secondary microstructures. Recent bathymetric and topographic analyses indicate that the lithosphere in this region is as thin as 20 km. Such a thin lithosphere may have been intensely deformed, even perhaps in the ductile regime, during fore-arc extension; consequently, the observed variations in microstructure within the peridotite samples probably reflect the complex tectonic evolution of the southern Mariana region. © 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2007.08.010

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

Oceanic core complexes expose gabbroic rocks on the seafloor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown. Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of the section to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. We propose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridobte: emplacement of a large intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic core surrounded by deformed serpentinite. Oceanic core complexes may therefore reflect processes associated with relatively enhanced periods of mafic intrusion within overall magma-poor regions of slow- and ultra-dow-spreading ridges. ? 2007 The Geological Society of America.

DOI： 10.1130/G23531A.1

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Structural Geology  

Microstructural analyses of shear band cleavages in a centimeter-scale shear zone within a metasomatic biotite band in the Teshima granite, Ryoke metamorphic belt, southwest Japan, show that strain partitioning occurred between quartz and biotite-feldspar domains within the shear zone. Pre-tectonic hydrothermal alteration within the granite caused biotite replacement of both plagioclase and K-feldspar, resulting in the development of biotite-feldspar domains where K-feldspar mantles dominantly biotite-plagioclase aggregates. Subsequently, the altered granite was plastically deformed in simple shear, so that intra-layer shear band cleavages were passively developed within the biotite-feldspar domains, whereas intense dynamic recrystallization occurred in the quartz domains. The rotation and orientation of the intra-layer shear band cleavages can be explained by a finite strain ellipse model, which shows that strain in the biotite-feldspar domain requires only 10-20% of the bulk simple shear strain for the development of such cleavages, so that most of strain could be accommodated by deformation in the quartz domain. Consequently, the model suggests that the development of the shear zone resulted in strain partitioning between the quartz and the biotite-feldspar domains due to compositional variations via hydrothermal alteration within the granite. © 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jsg.2007.02.003

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Metamorphic Geology  

In this study, the chemistry and microstructure of garnet aggregates within a metamorphic vein are investigated. Garnet-bearing veins in the Sanbagawa metamorphic belt, Japan, occur subparallel to the foliation of a host mafic schist, but some cut the foliation at low angle. Backscattered electron image and compositional mapping using EPMA and crystallographic orientation maps from electron-backscattered diffraction (EBSD) reveal that numerous small garnet (10-100 lm diameter) coalesce to form large porphyroblasts within the vein. Individual small garnet commonly exhibits xenomorphic shape at garnet/ garnet grain boundaries, whereas it is idiomorphic at garnet/quartz boundaries. EBSD microstructural analysis of the garnet porphyroblasts reveals that misorientation angles of neighbourpair garnet grains within the vein have a random distribution. This contrasts with previous studies that found coalescence of garnet in mica schist leads to an increased frequency of low angle misorientation boundaries by misorientation-driven rotation. As garnet nucleated with random orientation, the difference in misorientation between the two studies is due to the difference in the extent of grain rotation. A simple kinetic model that assumes grain rotation of garnet is rate-limited by grain boundary diffusion creep of matrix quartz, shows that (i) the substantial rotation of a fine garnet grain could occur for the conditions of the Sanbagawa metamorphism, but (ii) the rotation rate drastically decreased as garnet grains formed large clusters during growth. Therefore, the random misorientation distribution of garnet porphyroblasts in the Sanbagawa vein is interpreted as follows: (i) garnet within the vein grew so fast that substantial grain rotation did not occur through porphyroblast formation, and thus (ii) random orientations at the nucleation stage were preserved. The extent of misorientation-driven rotation indicated by deviation from random orientation distribution may be useful to constrain the growth rate of constituent grains of porphyroblast that formed by multiple nucleation and coalescence. © 2006 Blackwell Publishing Ltd.

DOI： 10.1111/j.1525-1314.2006.00642.x

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Geophysical Research Letters  

A dense network of seismic stations has been deployed across the northeast Japan arc to investigate mantle wedge structures. To attain independent petrophysical constraints, we determined the seismic properties of Ichinomegata mantle xenoliths from the back-arc region that were brought to the surface from the mantle lithosphere by volcanic eruptions. We calculated the seismic properties of the xenoliths from olivine and pyroxene crystal-preferred orientations and single crystal elastic constants. The small magnitude of measured S-wave splitting (delay time of 0.22 s in the area where the xenoliths were entrained) can be explained by the average seismic properties of mantle xenoliths for an approximately 20-km thick horizontal anisotropic layer, indicating that the mantle lithosphere could be one of the dominant sources of seismic anisotropy this layer is possibly related to deformation in the uppermost mantle lithosphere due to back-arc spreading along the northeast Japan arc. Copyright 2006 by the American Geophysical Union.

DOI： 10.1029/2006GL025812

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：構造地質研究会  

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担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Earth and Planetary Science Letters  

Subhorizontal mantle structures subparallel to the Moho are rotated into NW-SE subvertical orientations across a shear zone in a sinistral sense of shear within the northern Fizh mantle section of the Oman ophiolite. Dynamic recrystallization resulted in grain size reduction of olivine and the development of porphyroclastic texture. Mean olivine grain size stabilized at ∼0.7 mm within the shear zone center; this may reflect the steady-state grain size of dynamically recrystallized olivine, as determined by the deviatoric stress, which in this case was as low as 10 MPa. Crystal-preferred orientation (CPO) patterns of olivine are consistently [100]-fiber or partial fiber texture, indicating that olivine slip systems did not change during shearing. Dynamic recrystallization causes a weakening of olivine fabric intensity toward the shear zone center, but this weakening is counterbalanced by CPO strengthening due to dislocation glide. This process resulted in an abrupt decrease in seismic anisotropy at the center of the shear zone, in contrast to a gradual decrease in olivine fabric intensity and mean grain size. The measured seismic anisotropy patterns did not change in ways that would be significantly measurable by seismological observations. Despite the development of the shear zone, dispersion of both P- and S-waves in the shear zone may be of little effect with respect to the overall seismic anisotropy. This is not only because the shear zone occurs substantially in a narrow region but also because the seismic anisotropy is weaker in the shear zone than the high-T structure region. It suggests that a record of simple systematic seismic anisotropy observed in the upper mantle may indicate a simplified mantle flow structure, as localized structures may be obscured in the region of the observation. © 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2006.02.019

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）  

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Geophysical Research: Solid Earth  

Interfacial tension (γ) and differential stress (σ) affect the shape of a mineral grain included within a crystalline host. We present a simple model that predicts the progressive change in aspect ratio of an ellipsoidal inclusion. Three processes are considered in the model: dislocation creep, interface diffusion creep, and rounding by interface diffusion. The model reveals that (1) the inclusion aspect ratio (L) evolves toward a steady state value, (2) the time taken to achieve a steady state aspect ratio increases with increasing grain size (R), and (3) the dominant deformation mechanism varies from diffusion creep to dislocation creep with increasing grain size and/or differential stress. The L-R distribution pattern of garnets in a granulite-facies quartzite from the Lützow-Holm Complex, East Antarctica is compared with the model result. The garnet aspect ratio systematically varies with respect to grain size, and the most elongate ones are of intermediate grain size (∼250 μm). A two-stage deformation with high and low differential stresses (stage A and B) best explains for the observed L-R pattern of garnets. The duration (tA) at stage A is determined only when the differential stress (σA) is assumed due to the size independency of dislocation creep. Assuming γgrtqtz = 1.0 N m-1, Dgb,A = 1.0 × 10-12 m2 s-1, Dgb,B = 1.0 × 10-11 m2 s-1, and σA = 1.0 MPa, the garnet data are fitted to a theoretical curve under condition of σB = 1.4 × 10-2 MPa, tA = 0.5 Myr, and tB = 14.4 Myr. Copyright 2005 by the American Geophysical Union.

DOI： 10.1029/2004JB003526

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Scientific Drilling  

DOI： 10.2204/iodp.sd.1.05.2005

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Tectonophysics  

The Woodroffe thrust, central Australia, is a > 1.5-km-wide mylonitized shear zone marked by large volumes of mm- to cm-scale pseudotachylyte veins. The pseudotachylytes display typical melt-origin features, including rounded and embayed clasts, spherulitic and dentritic microlites, and flow structures within a fine-grained matrix. Three types of pseudotachylyte are identified on the basis of deformation texture, vein morphology, and host-rock lithology: cataclasite-related (C-Pt), mylonite-related (M-Pt), and ultramylonite-related (Um-Pt). The M-Pt and Um-Pt veins intrude into mylonite and ultramylonite and are themselves overprinted by subsequent mylonitization. These pseudotachylytes contain an internal foliation defined by flattened porphyroclasts and layering of the fine-grained vein matrix, and the foliation is generally oriented parallel to foliation in the surrounding mylonite and ultramylonite. These observations constrain the timing and environment of M-Pt and Um-Pt pseudotachylyte formation to a protracted period of deformation and mylonitization within the ductile regime of the crust. The M-Pt and Um-Pt veins, as well as the host mylonite, are overprinted by cataclasis and multiple generations of late-stage C-Pt veins that were generated in the brittle-dominated regime of the upper crust during uplift and exhumation of the shear zone. The coexistence of multiple generations of voluminous C-Pt, M-Pt, and Um-Pt veins indicates that the pseudotachylyte veins represent a large number of large earthquakes and accompanying seismic slip over an extended period of seismicity on the Woodroffe thrust. The timing and distribution of pseudotachylyte indicate that the earthquakes nucleated at the base of the brittle-dominated seismogenic zone and propagated down through the brittle-ductile transition into the ductile-dominated regime of the crust. © 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2004.10.016

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Integrated Ocean Drilling Program  

Integrated Ocean Drilling Program Expedition 305, a joint science program with Expedition 304, was designed to investigate the processes that control formation of oceanic core complexes, as well as the exposure of ultramafic rocks in very young oceanic lithosphere. Prior studies indicated that two main drill sites on Atlantis Massif, on the western rift flank of the Mid-Atlantic Ridge (MAR) at 30°N, could provide key constraints on the structure of the detachment fault zone, rock types exposed at shallow structural levels in the footwall, and their alteration history, as well as that of the volcanic succession in the hanging wall. Expedition 305 deepened Hole U1309D in the footwall of Atlantis Massif to 1415.5 meters below seafloor, with high recovery (average = 74.8%) of dominantly gabbroic rocks. Hole U1309D was logged twice, providing the opportunity for unprecedented core-logging integration for a deep borehole in the oceanic lithosphere. The recovered rocks range from dunitic troctolite, troctolite, (olivine) gabbro, and gabbronorite to evolved oxide gabbro that locally contains abundant zircon and apatite, and diabase. The texture of the dunitic troctolite suggests a cumulate origin. The gabbroic suite from Hole U1309D is among the most primitive recovered from the MAR, with Mg# ranging from 67 to 87. Although alteration mineral assemblages record cooling of gabbroic rocks from magmatic conditions to zeolite facies, a low-temperature phase that reflects alteration at temperatures <500°C is most significant. The overall trends in alteration and the changes in secondary mineralogy downhole suggest that there may be two separate secondary processes that have affected the footwall in the vicinity of Hole U1309D. In the upper ∼840 m, seawater-rock interactions may pervade the gabbroic sequence. Below that depth, the nature of and the fluctuations in degree and style of metamorphism are related to fluids of a different composition percolating along fault/ductile deformation zones. Hence, the core records an extensive history of gabbroic rock-fluid interaction, possibly including magmatic fluids. One of the prominent features of the rocks from Hole U1309D is the lack of extensive amphibolite facies alteration and deformation. This contrasts strongly with the gabbroic suite recovered from Ocean Drilling Program Hole 735B, at the Southwest Indian Ridge. The rocks recovered in Hole U1309D show very little deformation, and any deformation related to a major detachment fault system must have occurred at low temperature and must be strongly localized in the very upper part of the hole. This, together with very minor deformation in the amphibolite facies, is not consistent with the classical "core complex" interpretation of the corrugated, domal massifs on the seafloor resulting from surface exposure of a detachment fault that roots deeply at the base of the lithosphere. In addition, shipboard paleomagnetic measurements indicate there has been no significant net tectonic rotation (5°) of the footwall. This seems to preclude a rolling hinge model for the uplift of the core of Atlantis Massif along a single concave, normal fault. The ∼ 1.4 km sequence of dominantly gabbroic rocks is inconsistent with the initial prediction that the footwall was composed of an uplifted mantle section where serpentinization was responsible for lower densities/seismic velocities in the upper few hundred meters. A more complex model than that put forward before Expeditions 304 and 305 will be required. The fact that we did not reach fresh mantle peridotite, together with the known exposures of serpentinized mantle along the southern ridge of the massif, supports models of complicated lateral heterogeneity in slow-spreading oceanic crust. We have, however, placed a constraint on the magnitude of this heterogeneity - gabbro bodies in this setting can exceed 1.5 km in thickness.

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記述言語：日本語   出版者・発行元：東京 : 海洋調査技術学会  

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その他リンク： http://id.ndl.go.jp/bib/028614624

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記述言語：日本語   出版者・発行元：東京 : 日本化学会  

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その他リンク： http://id.ndl.go.jp/bib/027458336

記述言語：英語   掲載種別：記事・総説・解説・論説等（学術雑誌）   出版者・発行元：IODP-MI  

The objectives for Expedition 352 were to drill through the entire volcanic sequence of the Bonin fore arc to 1. Obtain a high-fidelity record of magmatic evolution during subduction initiation and early arc development, 2. Test the hypothesis that fore-arc basalt lies beneath boninite and understand chemical gradients within these units and across the transition, 3. Use drilling results to understand how mantle melting processes evolve during and after subduction initiation, and 4. Test the hypothesis that the fore-arc lithosphere created during subduction initiation is the birthplace of suprasubduction zone (SSZ) ophiolites. Expedition 352 successfully cored 1.22 km of igneous basement and 0.46 km of over-lying sediment, providing diverse, stratigraphically controlled suites of fore-arc basalts (FAB) and boninite related to seafloor spreading and earliest arc development. FAB were recovered at the two deeper water sites (U1440 and U1441) and boninites at the two sites (U1439 and U1442) drilled upslope to the west. FAB lavas and dikes are depleted in high-field strength trace elements such as Ti and Zr relative to mid-ocean-ridge basalt but have relatively diverse concentrations of trace elements bezcause of variation in degrees of melting and amount of subducted fluids involved in their genesis. All FAB magmas underwent significant crystal fractionation in a persistent magma chamber system. Holes U1439C and U1442A yielded entirely boninitic lavas. We defined three boninite differentiation series based on variations in MgO, SiO2, and TiO2 concentrations of the parental magmas. Lavas in both pairs of holes have compositions that generally become more primitive and have lower TiO2 concentrations upward. The presence of dikes at the base of the sections at Sites U1439 and U1440 provides evidence that boninitic and FAB lavas are both underlain by their own conduit systems and that FAB and boninite group lavas are likely offset more horizontally than vertically. We thus propose that seafloor spreading related to subduction initiation migrated from east to west after subduction initiation and during early arc development. Initial spreading was likely rapid, and an axial magma chamber was present. Melting was largely decompressional during this period, but subducted fluids affected some melting. As subduction continued and spreading migrated to the west, the embryonic mantle wedge became more depleted, and the influence of subducted constituents dramatically increased, causing the oceanic crust to be built of boninitic rather than tholeiitic magma. The general decrease in fractionation upward reflects the eventual disappearance of persistent magma chambers, either because spreading rate was decreasing with distance from the trench or because spreading was succeeded by off-axis magmatism trenchward of the ridge. The extreme depletion of the sources for all boninitic lavas was likely related to the incorporation of mantle residues from FAB generation. This mantle depletion continued during generation of lower silica boninitic magmas, exhausting clinopyroxene from the mantle such that the capping high-Si, low-Ti boninites were generated from harzburgite. Additional results of the cruise include recovery of Eocene to recent deep-sea sediment that records variation in sedimentation rates with time resulting from variations in climate, the position of the carbonate compensation depth, and local structural control. Three phases of highly explosive volcanism (latest Pliocene to Pleistocene, late Miocene to earliest Pliocene, and Oligocene) were identified, represented by 132 graded air fall tephra layers. Structures found in the cores and reflected in seismic profiles show that this area had periods of normal, reverse, and strike-slip faulting. Finally, basement rock P-wave velocities were shown to be slower than those observed during logging of normal ocean crust sites.

DOI： 10.14379/iodp.pr.352.2015

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記述言語：日本語   出版者・発行元：日野 : 海洋出版  

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その他リンク： http://id.ndl.go.jp/bib/023630425

記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：英語   掲載種別：記事・総説・解説・論説等（学術雑誌）   出版者・発行元：日本地質学会 = Geological Society of Japan  

DOI： 10.5575/geosoc.118.11.IX_X

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：日本語   出版者・発行元：海洋出版  

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記述言語：英語   掲載種別：記事・総説・解説・論説等（学術雑誌）   出版者・発行元：日本地質学会  

Hypocenter of the Chi-Chi earthquake is located on the detachment fault distributed approximately 10 km of depth, near the boundary between continental basement and overlying continental shelf sediments. Exhumed section of the basement rocks underwent greenschist facies (350-450℃) metamorphism. Overlying Eocene-Miocene sediments (termed as Slate Belt) also underwent sub-greenschist facies (&lt;350℃) metamorphism. We found an outcrop-scale duplex structure exposed in the Slate Belt, suggesting an outcrop of the detachment fault. The outcrop consists of alternation of sandstone and slate layers. It is notable that pegmatitic, V-shaped quartz veins in the sandstone concentrate beneath the detachment fault, suggesting cracks filled with fluid by hydrofaulting. At this time, overlying slate layers play a major role as an impermeable cap-rock. If the stored fluid beneath the detachment was expelled to the pre-existing link thrust, cracks in the fault zone would propagate and facilitate the rupture process of the earthquake.

DOI： 10.5575/geosoc.111.2.III_IV

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開催年月日： 2020年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2020年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2020年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2020年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2020年

記述言語：英語   会議種別：口頭発表（一般）  

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開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

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開催年月日： 2019年

記述言語：英語   会議種別：口頭発表（一般）  

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開催年月日： 2018年9月

記述言語：日本語  

開催地：北海道大学、札幌（北海道胆振東部地震のため中止）   国名：日本国  

開催年月日： 2018年9月

記述言語：日本語  

開催地：高知大学、高知   国名：日本国  

開催年月日： 2018年9月

記述言語：日本語  

開催地：北海道大学、札幌（北海道胆振東部地震のため中止）   国名：日本国  

開催年月日： 2018年9月

記述言語：日本語  

開催地：北海道大学、札幌（北海道胆振東部地震のため中止）   国名：日本国  

開催年月日： 2018年9月

記述言語：日本語  

開催地：北海道大学、札幌（北海道胆振東部地震のため中止）   国名：日本国  

開催年月日： 2018年9月

記述言語：日本語  

開催地：北海道大学、札幌（北海道胆振東部地震のため中止）   国名：日本国  

開催年月日： 2018年9月

記述言語：日本語  

開催地：北海道大学、札幌（北海道胆振東部地震のため中止）   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：英語  

国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年5月

記述言語：日本語  

開催地：幕張メッセ、千葉   国名：日本国  

開催年月日： 2018年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2018年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2018年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2018年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2018年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2018年

記述言語：英語   会議種別：口頭発表（一般）  

researchmap

開催年月日： 2018年

記述言語：英語   会議種別：口頭発表（一般）