研究者詳細 - 道林　克禎

Rutile exsolution lamellae of garnet in quartz eclogite from the Sanbagawa Belt, Mt. Gongen, central Shikoku, Japan 査読有り Open Access

TAKEBAYASHI Tomohiro, KOUKETSU Yui, MICHIBAYASHI Katsuyoshi

Journal of Mineralogical and Petrological Sciences 118 巻 ( 1 ) 頁： n/a 2023年

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

Needle-shaped rutile inclusions occur in garnet within the quartz-eclogite at Mt. Gongen in the Sanbagawa belt, central Shikoku. They are approximately 5-25 µm along the long axis and are typically oriented along three directions, each intersecting at 120°. This indicates that the needle-shaped rutile is a lamella exsolved from the garnet. Garnet with needle-rutile inclusions is restricted to the quartz-poor domain of the quartz eclogite sample, which consist of quartz, garnet, omphacite, phengite, epidote, kyanite, and hornblende. Garnet grains with rutile lamellae show a composition of the almandine-pyrope series with 14-21 mol% grossular content. Rutile exsolution lamellae were concentrated in the range of 27-34 mol% pyrope of garnet crystals. The garnet host with rutile lamellae has a higher TiO2 content (TiO2 = 0.06-0.19 wt%) than those in rutile-free areas. These chemical compositional characteristics suggest that Ti was incorporated into the crystal structure during garnet growth and subsequently partially exsolved as rutile lamellae during the retrograde stage. Rutile lamellae in garnet have generally been regarded as indicators of ultrahigh-pressure metamorphism, but the present report from quartz-eclogite of the Sanbagawa belt, where no coesite has been found, provides evidence in a natural sample that the appearance of rutile exsolution lamellae is not necessarily under ultrahigh-pressure conditions.

DOI： 10.2465/jmps.221219d

Variation in olivine crystal-fabrics and their seismic anisotropies in the Horoman peridotite complex, Hokkaido, Japan 査読有り

CiNii Research

Relationships between intensity of deformation induced Cr-Al chemical zoning and geometrical properties of spinel: An approach applying machine learning analyses 査読有り

Uhmb, T., Michibayashi, K.

Journal of Structural Geology 177 巻 2023年

Matsuyama, K., Michibayashi, K.

Journal of Geodynamics 158 巻 2023年

Paleo-permeablity structure of the crustal section of the Samail Ophiolite based on automated detetion of veins in X-ray CT core images from the Oman Drilling Project 査読有り Open Access

Akamatsu. Y., Katayama, I., Okazaki, K., Michibayashi, K.

Geochemistry, Geophysics, Geosystems 24 巻 2023年

Formation process of Al-rich calcium amphibole in quartz-bearing eclogites from the Sulu Belt, China 査読有り

Enami M., Taguchi T., Kouketsu Y., Michibayashi K., Nishiyama T.

American Mineralogist 107 巻 ( 8 ) 頁： 1582 - 1597 2022年8月

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

Simultaneous Measurements of Elastic Wave Velocity and Porosity of Epidosites Collected From the Oman Ophiolite: Implication for Low V<inf>P</inf>/V<inf>S</inf> Anomaly in the Oceanic Crust 査読有り Open Access

Nagase K., Hatakeyama K., Okazaki K., Akamatsu Y., Abe N., Michibayashi K., Katayama I.

Geophysical Research Letters 49 巻 ( 11 ) 2022年6月

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

Peridotites with back-arc basin affinity exposed at the southwestern tip of the Mariana forearc 査読有り国際共著 Open Access

Oya Shoma, Michibayashi Katsuyoshi, Ohara Yasuhiko, Martinez Fernando, Kourim Fatma, Lee Hao-Yang, Nimura Kohei

PROGRESS IN EARTH AND PLANETARY SCIENCE 9 巻 ( 1 ) 2022年3月

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

Steady-State Microstructures of Quartz Revisited: Evaluation of Stress States in Deformation Experiments Using a Solid-Medium Apparatus 査読有り Open Access

Shimizu I., Michibayashi K.

Minerals 12 巻 ( 3 ) 2022年3月

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

A shape-change model for isolated K-feldspar inclusions within a shear zone developed in the Teshima granite, Ryoke metamorphic belt, Japan: Estimation of the duration of deformation in a natural shear zone 査読有り Open Access

Uhmb T.H., Michibayashi K.

Tectonophysics 824 巻 2022年2月

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

Listvenite Formation During Mass Transfer into the Leading Edge of the Mantle Wedge: Initial Results from Oman Drilling Project Hole BT1B 査読有り国際共著 Open Access

Kelemen P.B., Carlos de Obeso J., Leong J.A., Godard M., Okazaki K., Kotowski A.J., Manning C.E., Ellison E.T., Menzel M.D., Urai J.L., Hirth G., Rioux M., Stockli D.F., Lafay R., Beinlich A.M., Coggon J.A., Warsi N.H., Matter J.M., Teagle D.A.H., Harris M., Michibayashi K., Takazawa E., Al Sulaimani Z., Kelemen P., de Obeso J.C., Matte J.M., Teagle D., Coggon J., Bompard N., Zihlmann B., Warsi N., Harris M., Morris T., Bennett E., Boulanger M., France L., Früh-Green G., Garbe-Schönberg D., Ildefonse B., Jesus A., Manning C., Mock D., Müller S., Noël J., Nothaft D., Perez A., Pezard P., Zeko D.

Journal of Geophysical Research: Solid Earth 127 巻 ( 2 ) 2022年2月

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

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

Heterogeneity in texture and crystal fabric of intensely hydrated ultramylonitic peridotites along a transform fault, Southwest Indian Ridge 査読有り国際共著 Open Access

Kakihata Y., Michibayashi K., Dick H.J.B.

Tectonophysics 823 巻 2022年1月

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

Deformation beneath Gakkel Ridge, Arctic Ocean: From mantle flow to mantle shear in a sparsely magmatic spreading zone 査読有り国際共著

Harigane Y., Michibayashi K., Morishita T., Tamura A., Hashimoto S., Snow J.E.

Tectonophysics 822 巻 2022年1月

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

Web of Science

Deep Sourced Fluids for Peridotite Carbonation in the Shallow Mantle Wedge of a Fossil Subduction Zone: Sr and C Isotope Profiles of OmanDP Hole BT1B 査読有り国際共著 Open Access

de Obeso J.C., Kelemen P.B., Leong J.M., Menzel M.D., Manning C.E., Godard M., Cai Y., Bolge L., Matter J., Teagle D., Coggon J., Harris M., Bennett E., Bompard N., Boulanger M., France L., Früh-Green G., Garbe-Schönberg D., Ildefonse B., Jesus A., Koepke J., Koornneef L., Lafay R., Lissenberg J., MacLeod C., Mock D., Morris T., Müller S., Noël J., Nothaft D., Perez A., Pezard P., Warsi N., Zeko D., Zihlmann B., Bechkit M.A., Brun L., Célérier B., Henry G., Paris J., Lods G., Robert P., Al Amri S., Al Shukaili M., Al Qassabi A., Moe K., Yamada Y., Takazawa E., Michibayashi K., Abe N., Akitou T., AlShahri S.A., Hamed Shames Al-Siyabi H., Alhumaimi S.M., AlRawahi M.H., Al Sarmi M.S., Alwaeli B.H., Beinlich A., Carter E., Cheadle M., Cloos M., Cooper M., Crispini L., Deamer L., Deans J., Faak K., Greenberger R., Harigane Y., Hatakeyama K., Horst A., Hoshide T., Ishii K., Johnson K., Kettermann M., Kim H., Kondo K., Kotowski A., Kourim F., Kusano Y., Menzies C., Morishita T., Nguyen D.K., Nozaka T., Okazaki K., Picazo S., Senda R., Tateishi Y., Till J., Umino S., Urai J., Usui Y., D’Andres J.

Journal of Geophysical Research: Solid Earth 127 巻 ( 1 ) 2022年

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

海底から地球の未来を探求するー海洋底掘削科学の新展開

道林克禎

科学 92 巻頁： 886 - 887 2022年

Cataclastic and crystal-plastic deformation in shallow mantle-wedge serpentinite controlled by cyclic changes in pore fluid pressures 査読有り国際誌 Open Access

Hirauchi, K; Nagata, Y; Kataoka, K; Oyanagi, R; Okamoto, A; Michibayashi, K

EARTH AND PLANETARY SCIENCE LETTERS 576 巻頁： 117232 - 117232 2021年12月

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

Web of Science

Hadal aragonite records venting of stagnant paleoseawater in the hydrated forearc mantle 査読有り Open Access

Ryosuke Oyanagi, Atsushi Okamoto, Madhusoodhan Satish-Kumar, Masayo Minami, Yumiko Harigane, Katsuyoshi Michibayashi

Communications Earth & Environment 2 巻 ( 1 ) 2021年12月

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

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

Major Mineral Fraction and Physical Properties of Carbonated Peridotite (Listvenite) From ICDP Oman Drilling Project Hole BT1B Inferred From X-Ray CT Core Images 査読有り国際共著 Open Access

Okazaki K., Michibayashi K., Hatakeyama K., Abe N., Johnson K.T.M., Kelemen P.B.

Journal of Geophysical Research: Solid Earth 126 巻 ( 12 ) 2021年12月

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

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

Initial Results From the Oman Drilling Project Multi-Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon 査読有り国際共著 Open Access

Kelemen P.B., Leong J.A., Carlos de Obeso J., Matter J.M., Ellison E.T., Templeton A., Nothaft D.B., Eslami A., Evans K., Godard M., Malvoisin B., Coggon J.A., Warsi N.H., Pézard P., Choe S., Teagle D.A.H., Michibayashi K., Takazawa E., Al Sulaimani Z.

Journal of Geophysical Research: Solid Earth 126 巻 ( 12 ) 2021年12月

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

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

Geochemical Profiles Across the Listvenite-Metamorphic Transition in the Basal Megathrust of the Semail Ophiolite: Results From Drilling at OmanDP Hole BT1B 査読有り国際共著 Open Access

Godard M., Carter E.J., Decrausaz T., Lafay R., Bennett E., Kourim F., de Obeso J.C., Michibayashi K., Harris M., Coggon J.A., Teagle D.A.H., Kelemen P.B.

Journal of Geophysical Research: Solid Earth 126 巻 ( 12 ) 2021年12月

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