Publisher：Chemical Geology  

In sedimentary rocks rich in Fe-oxide, such as red beds, white bleached spots that are free of Fe-oxide minerals are often observed. The spot formation has been explained by localized reduction reactions in relation to organic substances, such as fluids including hydrocarbon and organic debris as a precursor, and the recent major prevalent approach is the microbial activity in sediments. However, the evidence for microbial activities within the spots is rarely found, and it remains debatable whether the entire spots are of microorganism origin. We discovered bleached spots in zebra rock, which is the sedimentary rock from northern Australia with rhythmic Fe-oxide bands, and explained that the spots were formed by pH changes induced by the decomposition of primary pyrite. Our comprehensive petrological studies further demonstrate that the spots developed during the infiltration of the acidic fluid in relation to the Fe-oxide band formation, and the primary pyrite as a precursor is currently preserved as a pseudomorph composed of dickite and aggregation of hematite and goethite. This study demonstrates that the bleached spots in zebra rock were formed by inorganic chemical reactions, indicating that the proposal in other studies that spots of this kind can be used as a biomarker to find life on Mars is not always available.

DOI： 10.1016/j.chemgeo.2022.121049

Web of Science

Scopus

Language：Japanese   Publisher：The Geological Society of Japan  

<p>Fe oxide Liesegang bands have often been observed in sedimentary and igneous rocks, and they are formed during weathering and alteration by water-rock interactions. In this study, micro-X-ray fluorescence (μ-XRF) mapping was used to study the Fe bands in an Fe oxide concretion from the Jurassic Navajo Sandstone in Utah, USA, to estimate the duration of their formation. Most of the peaks in Fe concentration are steeper on the inner side than on the outer side, which indicates a supply of ferrous ions (Fe²⁺) from outside the concretion. The precipitation of Fe oxide was controlled by pH buffering that resulted from a reaction between acidic water and alkaline pore water that formed through the dissolution of an earlier calcite concretion. The reaction rate within the Fe oxide concretion was estimated from the width of the Fe peaks and the expected diffusion coefficient for Fe through the rock matrix, and it was found to be no more than years to decades-faster than previously estimated. This demonstrates that μ-XRF mapping is a useful technique to extract quantitative information about water-rock interactions from rocks.</p>

DOI： 10.5575/geosoc.2022.0008

CiNii Research

Publisher：Chemical Geology  

Zebra rock, found in the eastern Kimberley region of northern Western Australia, is a Late Proterozoic sedimentary rock with a rhythmic Liesegang-type Fe-oxide banding. The striped rhythmic pattern in sedimentary rocks is an important key to infer chemical conditions of water-rock reactions. Although past studies have discussed the zebra rock formation for decades, the process remains unclear. Here, we introduce a new formation model, suggesting that zebra rock formed in an acidic hydrothermal system and that pH buffering of Fe2+-bearing acidic fluid, in a neutralization reaction with primary carbonate minerals, induced rhythmic Fe-precipitation. The Fe profile clearly shows a reaction front, indicating unidirectional diffusive fluid migration along bedding planes. The two types of zebra rock distinguished by color and by clay mineral assemblages correspond with typical acidic hydrothermal alteration zonation. Although a specific volcanic event related to the hydrothermal activity cannot be identified presently, this study provides a new model for the zebra rock, recording both hydrothermal alteration and post-Late Proterozoic fluid migration.

DOI： 10.1016/j.chemgeo.2021.120699

Web of Science

Scopus