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.
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.
•Bleached spots are discovered in a sedimentary rock with rhythmic Fe-oxide bands.•Pseudomorphs consisted of dickite and Fe3+ minerals after pyrite are observed at the center of spots.•Bleached spots were formed during the infiltration of acidic fluid related to the Fe-oxide band formation.•This study shows that the bleached spots in zebra rock were formed by pH changes induced by the decomposition of pyrite.
Raman CM geothermometry applied to 126 samples of pelitic schists collected over an area of 11 km × 7 km reveals the thermal structure of the Asemigawa region of the Sanbagawa metamorphic belt, ...southwest Japan in unprecedented detail. In general, the estimated temperatures gradually increase from south to north in the range of 288–553°C. However, a temperature gap from ~380 to ~440°C is identified near the boundary between the chlorite and garnet zones. This temperature region matches the depth of the continental Moho of the Sanbagawa subduction zone. The temperature gradient in the higher‐temperature domain is higher than that in the lower‐temperature domain, and large‐scale tight folds that affect the thermal structure are developed in the high‐grade units and in the vicinity of the temperature discontinuity. These geological structures probably reflect that the exhumed slab units was dammed at the Moho depth due to the upward movement being impeded by increase in the coupling strength of the overlying rocks associated with exhumation from beneath serpentinite rocks to a shallower domain overlain by crustal rocks. Changes in the coupling strength along the subduction boundary led the strong folding at the higher‐temperature domain and the pre‐formed foliation developed at the Moho depth may have acted as the tectonic boundary, resulting in a temperature discontinuity. These results will contribute to elucidating various geological phenomena occurring in the forearc regions of modern subduction zones.
Large parts of the shallow mantle wedge are thought to be hydrated due to release of fluids from the subducting slab and serpentinization of the overlying mantle rocks. If serpentinization proceeds ...under low SiO2 activity, brucite can be a major phase in the low-temperature (<450°C) part of the serpentinized mantle wedge, but only very few natural examples have been documented. A combined petrological, geochemical, and geological study shows that brucite is widely distributed in the wedge mantle-derived Shiraga metaserpentinite body in the Sanbagawa metamorphic belt of SW Japan. Thermodynamic modeling combined with bulk rock composition and point counting indicates that the original fully hydrated shallow parts of the Sanbagawa mantle wedge contained ~10–15vol.% brucite before the onset of exhumation of the Shiraga body and before peak metamorphic conditions. A distinct zone of brucite-free essentially monomineralic antigorite serpentinite occurs limited to a 100-m-thick marginal zone of the body. This indicates a limited degree of Si-metasomatism by slab-derived fluids in the shallow mantle wedge. The presence of brucite may strongly affect the H2O budget and mechanical properties of serpentinite; these should be taken into consideration when examining the behavior of the shallow mantle wedge.
•~10–15vol.% brucite in the shallow mantle wedge of a warm subduction zone•Complete hydration of peridotite mainly dunite in the shallow mantle wedge under low SiO2 activity to form antigorite–brucite serpentinite•Limited Si-metasomatism of the wedge mantle
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.
‘Liesegang patterns’, rinds and bands are commonly observed in nature and form by self-organised periodic precipitation of Fe-oxyhydroxide following a nonlinear reaction-diffusion process. Although ...strictly Liesegang patterns consist of bands that increase in width with increasing distance from the source of the Fe that precipitated as Fe-oxyhydroxide, regular banded patterns are also sometimes observed that are otherwise similar to Liesegang patterns. However, the detailed process and time scale of regular Fe-oxyhydroxide bands development is still not fully understood. Here we describe an example of regular Fe-oxyhydroxide bands formed within dacite cobbles. Iron was provided to the outer surfaces of the cobbles by acidic water that diffused towards the cobbles' cores. The spatial distributions of Ca and Fe within the Fe-oxyhydroxide bands across the cobbles show that the rhythmic Fe-oxyhydroxide precipitation was controlled by pH buffering. The width of each band (L) and the expected diffusion coefficient of the rock matrix (D) provide the rate of reaction (V) and allow us to estimate the duration of Fe-oxyhydroxide band formation. A ‘diffusion-reaction cross plot’ implies that the rhythmic Fe-oxyhydroxide patterns formed very rapidly, within an order of 102–103 years, considerably faster than previously estimated. The simplified model can be applied to estimate the reaction time in any similar rock if regular Fe-oxyhydroxide bands are observed.
•Liesegang like patterns are pervasive in rocks but yet poorly understood.•Circular Fe-oxyhydroxide bands in cobbles used for reaction time estimation•Rhythmic bands formed considerably faster than previously estimated.•Simplified time estimation model is applicable in any type of rock.
Both igneous and sedimentary rocks affected by water–rock interaction commonly form rhythmic Fe‐oxide bands that provide information on palaeo‐groundwater. This study investigated the formation of ...Fe‐oxide banded patterns of concentrically banded concretions from the Jurassic Navajo Sandstone (United States) and banded claystone ‘zebra rock’ from the Neoproterozoic Johnny Cake Shale Member (Western Australia). Micro‐X‐ray fluorescence imaging of both samples revealed uniformly banded patterns of asymmetrical Fe content peaks. This study proposes that their formations were caused by diffusion, with a pH buffering reaction of Fe2+ and O2 due to dissolution of earlier materials by acidic fluids. Based on the model, it is suggested that the acid fluids for Navajo diffused towards the inner side of the concretion, and that for Kimberley they penetrated oblique or perpendicular to bedding. In addition, the Fe‐oxide precipitation has resulted from pH rises of acidic fluids because of diagenetic alteration of earlier calcite concretion in Navajo and that of earlier pyrite‐bearing claystone in Kimberley. The interpretation for Kimberley is based on the δ34S values (<−20‰) of zebra rock, suggesting that the alunite and Fe‐oxide resulted from the supply of SO42− and Fe2+ produced as a result of pyrite oxidation in earlier rocks. The formation rate of the Fe‐oxide band was estimated from the constant width of the Fe peaks and the expected diffusion coefficient for Fe2+ through the rock matrix, and it was found to be on the order of years for Navajo and on the order of tens of years for Kimberley.
It is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip (ETS) zones. In order to ...examine the petrological controls on the large-scale structure, we compared recent geophysical observations in the Shikoku area, southwest Japan with petrological models of the hanging wall mantle wedge. As a result, we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the Shikoku area: Short-term ETSs take place in the antigorite + olivine stability field and silent long-term slow slip events (SSEs) take place in the antigorite+brucite stability field. Based on observations of natural antigorite serpentinites, we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes. The serpentinization reaction in the antigorite+brucite stability field (olivine + H2O → antigorite + brucite) proceeds at the expense of water. This is consistent with moderately elevated pore pressures inferred for long-term SSEs. The existence of weak brucite enhances the development of shear zones oblique to the main foliation. The resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes. In contrast, progress of the serpentinization reaction in the antigorite + olivine stability field (olivine + H2O + SiO2 → antigorite) results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short-term SSEs. Our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3-D models for strain evolutions along plate boundaries.
•Two types of antigorite serpentinite can be developed in mantle wedge.•Serpentinization in the Ol + Atg stability causes elevated pore fluid pressure.•Brc-bearing serpentinite effectively reduces fluid pressure along subduction boundary.•The nature of antigorite serpentinites can explain two types of slow slip event.
High-performance and low-power microprocessors are key to PDA applications. A dynamic voltage and frequency management (DVFM) scheme with leakage power compensation effect is introduced in a ...microprocessor with 128-bit wideband 64-Mb embedded DRAM. The DVFM scheme autonomously controls clock frequency from 8 to 123 MHz in steps of 0.5 MHz and also adaptively controls supply voltage from 0.9 to 1.6 V in steps of 5 mV, achieving 82% power reduction in personal information management scheduler application and 40% power reduction in MPEG4 movie playback. This low-power embedded microprocessor, fabricated with 0.18-/spl mu/m CMOS embedded DRAM technology, enables high-performance operations such as audio and video applications. As process technology shrinks, this adaptive leakage power compensation scheme will become more important in realizing high-performance and low-power mobile consumer applications.
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