Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.lithos.2019.105176

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2018jb016858

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2018JB016858

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

To understand the similarity and diversity of serpentinization processes in different rock systems, gabbroic rocks recovered from IODP Site U1415 at the Hess Deep Rift were examined and compared with peridotites from the adjacent ODP Site 895. Textural observations, micro-Raman spectroscopic analyses and electron microprobe analyses indicated that most of the olivine-replacing serpentine in the gabbroic rocks lack the mixing with brucite, which is common in peridotites. At least three stages of serpentinization are observable in the gabbroic rocks; each generation is characterized by different submicroscopic mixtures or solid solutions of sheet silicates: 1) Mg-Fe2+ lizardite + ferri-lizardite + chlorite, 2) Mg-Fe2+ lizardite + ferri-lizardite, and 3) Mg-Fe2+ lizardite + ferri-lizardite + saponite. The first and third generations of serpentine and mixed minerals are relatively Fe-rich, whereas the second generation is Fe-poor and associated with abundant magnetite and pyrrhotite. The major difference between the alteration of gabbroic and peridotitic systems is probably best explained by the iron content and modal abundance of primary olivine and by rock-dominated fluid compositions with a high silica activity due to the alteration of plagioclase in gabbroic rocks. The mineralogical variations between the reported three generations of mixed sheet silicates in gabbroic rocks can be ascribed to variations of silica and/or oxygen activities in the associated fluids under decreasing temperature conditions. The abrupt increase of magnetite crystallization during serpentinization in gabbroic rocks could be caused by oxidation at a relatively high 5102 activity without the olivine-serpentine-brucite buffering assemblage. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.lithos.2016.12.032

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Aluminous spinel, corundum and diaspore are reported from intensely altered parts of primitive troctolites recovered from IODP Site U1415 at the Hess Deep Rift. The spinel is green-colored, has an irregular shape, has low Cr concentrations, and is so distinct from primary igneous chromite. Corundum and diaspore occur mainly at the rims of green spinel grains with a texture suggesting a sequential replacement of spinel by corundum, and then corundum by diaspore. The green spinel is associated with anorthite and pargasite, which is overgrown by tremolite that forms coronitic aggregates with chlorite around olivine. These petrographic observations are supported by pressure-temperature pseudosections, which predict spinel + pargasite stability field, and tremolite/hornblende + chlorite field at lower temperature conditions. From these pseudosections and simplified system phase diagrams, estimated formation temperature conditions calculated at 2 kbar are 650-750 degrees C for spinel + pargasite, 410-690 degrees C for tremolite/hornblende + chlorite, 400-710 degrees C for corundum, and <400 degrees C for diaspore. Because the aluminous spinel occurs in the domains that were previously occupied by magmatic plagioclase, and because spinel-bearing rocks characteristically have high Al2O3/CaO and Al2O3/SiO2 ratios, it is likely that the stabilization of spinel was caused by the loss of Ca2+ and SiO2(aq) in high-temperature hydrothermal fluids. The results of this study suggest that (1) the concentrations of aluminous phases in the lower oceanic crust are presently underestimated, and (2) chemical modification of the lower oceanic crust due to high-temperature hydrothermal metasomatic reactions could be common near spreading axes.

DOI： 10.1007/s00410-016-1266-4

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DOI： 10.1016/j.epsl.2014.10.023

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In contrast to the widely recognized aspects of serpentinization, initial stages of hydration and tectonic processes of unserpentinized peridotites are still unclear, but have important implications for understanding the lithospheric architecture of supra-subduction zones. This study provides petrological evidence from the Oeyama ophiolite, SW Japan, of the effects of high-temperature metasomatic hydration immediately before the cooling and ductile deformation of forearc peridotites. Key findings in this study are: 1) complex association of high-temperature metasomatic minerals: tremolitic amphibole, cummingtonite, phlogopite, chlorite, olivine and orthopyroxene in veins and in mylonites; 2) the systematic variation in Si and Na K contents of the tremolitic amphibole, corresponding to its mode of occurrence and mineral association; and 3) the presence of thin (<0.7 mm) veins of fine-grained olivine accompanied by a narrow diffusion zone of the host primary olivine. On the basis of petrography and mineral chemistry, the temporal sequence of hydration and deformation of the Oeyama ophiolite is considered as follows: 1) infiltration of slab-derived fluids, causing decomposition of primary pyroxene and chemical modification of primary olivine, 2) metasomatic formation of variable modal amounts of amphibole, phlogopite, chlorite, vein-forming olivine and secondary orthopyroxene at 650-750 degrees C; 3) early-stage mylonitization of the hydrous peridotites in localized shear zones; and 4) syntectonic serpentinization at 400-600 degrees C to form serpentinite mylonites. Paragenesis and amphibole compositions suggest comparable temperature conditions for metasomatism and early-stage mylonitization. Mylonitization occurred exclusively in hydrous peridotites, and the peridotite mylonites were preferentially overprinted by syntectonic serpentinization. Diffusion profiles of olivine cut by a vein suggest rapid cooling immediately after the metasomatic fluid infiltration. From these observations and calculations, it is concluded that the exhumation of the forearc peridotites was closely related to the infiltration of high-temperature metasomatic fluids and hydration occurred under a wide range of temperature conditions. (C) 2013 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.lithos.2013.11.012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies(1) and ancient analogues (ophiolites)(2-5) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges(3,6). Geochemical analysis of these primitive lower plutonic rocks-in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas-provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle(7) and mid-ocean-ridge basalt differentiation(8,9). The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

DOI： 10.1038/nature12778

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Association of Mineralogical Sciences  

Serpentinization of peridotites involves the production of hydrogen, which is a source of vital energy for chemosynthetic communities and abiotic methane or other hydrocarbons. Serpentinite-hosted hydrothermal vent fields that discharge fluids with hydrogen have been widely noticed as a possible environment for the generation of life on the early Earth and other terrestrial planets. In this context, it is important for us to understand petrological constraints on serpentinization processes related to hydrogen production. Magnetite formation by oxidation of iron in olivine is the most effective process for hydrogen production during serpentinization. Recent petrological studies have revealed that the magnetite formation is controlled by silica activity and Fe-Mg diffusion rate in olivine crystal, as well as temperature and water/rock ratio during serpentinization. Without local elevation of silica activity via fluid infiltration, magnetite forms at temperatures ranging approximately from 150 to 350 °C with most favorable condition at around 300 °C, but fails to form because of increasing diffusion rate in olivine crystal at higher temperatures and Fe-serpentine or Fe-brucite formation at lower temperatures. It should be kept in mind, however, that the formation of oxidized serpentine could produce hydrogen as well.<br>

DOI： 10.2465/gkk.120608

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Other Link： https://jlc.jst.go.jp/DN/JALC/10012046026?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We report superconductivity in novel iron-based compounds Ca-10(PtnAs8)(Fe2-xPtxAs2)(5) with n = 3 and 4. Both compounds crystallize in triclinic structures (space group P (1) over bar), in which Fe2As2 layers alternate with PtnAs8 spacer layers. Superconductivity with a transition temperature of 38K is observed in the n = 4 compound with a Pt content of x similar or equal to 0: 36 in the Fe2As2 layers. The compound with n = 3 exhibits superconductivity at 13 K.

DOI： 10.1143/JPSJ.80.093704

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

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 degrees 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 similar to 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.

DOI： 10.1029/2010JB007931

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Anthophyllite or another species of Mg-amphibole commonly occurs in an intervening zone between the higher grade orthopyroxene zone and lower grade talc zone in progressively metamorphosed peridotites. However, the anthophyllite zone is absent in some of the thermally metamorphosed peridotite complexes in SW Japan despite the existence of the other zones. A comparative study presented here reveals similarities in rock composition and metamorphic pressure-temperature conditions at high-grade zones between the metaperidotite complexes, and differences in the following respects. The metaperidotite complex that contains an anthophyllite zone has less abundant magnetite and olivine that is more homogeneous than the complex where the anthophyllite zone is absent. It is likely that the degree of cation diffusion in olivine crystals depends on duration of heat retention in metaperidotites during thermal metamorphism, which is supported by the variation in mineralogy of intrusive rocks and pelitic hornfelses surrounding the metaperidotites, and by calculations based on a simplified model of thermal conduction. The long duration of heat retention looks to be a necessary condition for the formation of anthophyllite crystals, which have a sluggish nucleation rate. In addition, the circulation of reducing fluids during prolonged metamorphism likely promoted the decomposition of magnetite and the growth of anthophyllite, into which iron is preferentially distributed. This study cautions about kinetic controls and redox conditions for anthophyllite formation in metaperidotites.

DOI： 10.1111/j.1525-1314.2010.00921.x

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

Incipient-stage alteration products in relatively fresh oceanic gabbros from deep boreholes provide critical information on hydration processes in the oceanic lower crust and their effect on lithosphere dynamics. We present the results of a petrographic study on the alteration of olivine-bearing gabbroic rocks recovered from the deeper parts of Integrated Ocean Drilling Program (IODP) Hole U1309D in the Atlantis Massif near the Mid-Atlantic Ridge at 30 degrees N. In these rocks, alteration is localized in proximity to fluid-infiltration veins or igneous contacts. It is most conspicuous in halos surrounding amphibole+chlorite veins or leucocratic veins in olivine-bearing gabbros, where coronitic fringes of tremolite, chlorite and talc occur around discrete olivine grains. Many of the halos exhibit a zonal pattern with systematic changes in mineral assemblage, generally consisting of three zones: tremolite+chlorite around relict olivine-plagioclase contacts; talc pseudomorphs after olivine; and tremolite pseudomorphs after olivine. The tremolite+chlorite assemblage appears in increasing amounts and talc grows unevenly with increasing thickness toward the veins. The alteration minerals have highly magnesian compositions, reflecting the compositions of the precursor igneous phases. Within the zone closest to the veins, green hornblende with a relatively high Al content occurs, showing textures suggestive of its later formation than the coronitic tremolite and chlorite. Considering the mode of occurrence and chemical composition of the minerals combined with thermodynamic calculations of silica and water activities in a simplified system, we conclude that the zoned halos were caused by metasomatism owing to protracted or sequential infiltration of hydrothermal fluids at amphibolite-facies conditions (450-750 degrees C, 1.5-2 kbar). Textural relationships clearly indicate that zoned halos formed earlier than serpentinization and clay mineral formation, and suggest that the high-temperature, amphibolite-facies alteration took place in a near-axis region before the exhumation of the lower crustal rocks. Recent results of seafloor drilling have provided supporting evidence for the predominance of gabbroic rocks in oceanic core complexes. The similarity in mineral association between zoned halos and schistose fault rocks suggests that preferential formation of talc and/or chlorite, rather than serpentine, at contacts between gabbroic rocks and peridotite plays an essential role in detachment faulting and tectonic exhumation of oceanic core complexes from lower crustal levels.

DOI： 10.1093/petrology/egq098

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Olivine that has well-developed parting similar to cleavage, i.e., so-called "cleavable olivine", occurs in peridotites at many localities of orogenic belts and the seafloor. Some conflicting hypotheses for the genesis of the parting have been proposed but not yet fully proved or disproved. We present new data of structural, petrological and mineralogical analyses of cleavable olivine and host ultramafic rocks in the Oeyama ophiolitic complexes, SW Japan. The following are our key findings to understand the genesis of cleavable olivine. I) Cleavable olivine is distributed in the ultramafic complexes regardless of metamorphic grade of contact aureoles. 2) Cleavable olivine from contact aureoles has variable chemical compositions by the effect of thermal metamorphism. 3) Cleavable olivine commonly occurs in or near serpentinite mylonites. 4) Antigorite blades commonly occur along the parting planes of olivine, and the parting planes along with antigorite blades are locally bent to the direction of foliation. 5) Poles of the parting planes of olivine tend to be distributed around a plane vertical to the foliation of host serpentinite mylonite. From these facts we conclude that cleavable olivine was produced during a sequence of localized plastic deformation and alteration of peridotites at temperatures around 600 degrees C or lower. The parting is likely to have derived from dislocation arrangement by recovery processes after plastic deformation of hydrous peridotites and have been brought into prominence during syntectonic serpentinization. The preferred orientation of the parting planes suggests that cleavable olivine is a potential indicator of regional tectonics of the upper mantle at supra-subduction zones.

DOI： 10.2465/jmps.100408

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Low-temperature alteration products in gabbros from the ocean floor have significant implications for incipient processes of seawater-rock interaction and exhumation tectonics of the lower-crustal rocks. In this paper we report mode of occurrence and mineralogical characteristics of clay minerals in gabbroic rocks recovered from Integrated Ocean Drilling Program (IODP) Hole U1309D at an oceanic core complex, Atlantis Massif, Mid-Atlantic Ridge at 30 degrees N. The clay minerals were identified by optical microscope, electron microprobe, Raman spectrometer, and transmission electron microscope as mainly composed of mixed-layer saponite-talc, saponite, and vermiculite. They are characteristically rich in iron that is significantly oxidized and distributed into the tetrahedral site, suggesting a relatively high-temperature condition for oxidation. They are restricted to domains near the contacts between olivine and talc or form pseudomorphs after olivine near microcracks filled with zeolite or clay minerals. These facts suggest the infiltration of oxidative seawater and reactions to variable fluid/rock ratios at variable temperatures. Close association of vermiculite with microcracks radiated from serpentinized olivine suggests that the deep infiltration of seawater at an off-axis region was caused by fracturing resulting from serpentinization and enhanced by relatively abundant olivine-rich lithology at Atlantis Massif. Compared with gabbroic rocks of an oceanic core complex at ultraslow-spreading ridge (ODP Hole 735B), those of Atlantis Massif substantially lack mixed-layer smectite-chlorite. Mixed-layer smectite-chlorite is a product of prehnite-actinolite to greenschist facies alteration and looks to preserve a record of ambient thermal structure through which the massif passed on rising to a shallow level. The absence of pervasive formation of mixed-layer smectite-chlorite under relatively reducing conditions suggests low permeability and/or limited fluid-rock reactions on the way to shallow levels. From the observation and consideration of the characteristics of clay minerals, sequence and distribution of static alteration related to fracturing, original lithology, and tectonic settings of the oceanic core complexes, we conclude that Atlantis Massif was more rapidly exhumed to the oxidative subseafloor environment than Atlantis Bank. The difference of exhumation rate possibly reflected either the disparity in spreading rate between the whole ridge systems or regional variation of exhumation tectonics between the two oceanic core complexes.

DOI： 10.1029/2008GC002207

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

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 similar to 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 I-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 degrees 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) m(2). 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. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2008.08.033

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publisher：Tokyo Geographical Society  

&emsp;Gabbroic rocks recovered from deep holes in the oceanic crust significantly vary in the abundance and assemblage of alteration minerals, showing a close association with the original lithology and distribution of dikes and veins. The mineralogical variation is considered to reflect the durability of primary minerals, accessibility and composition of alteration fluids, and alteration temperature. Textural relationships of alteration minerals suggest a common cooling history of oceanic gabbros from granulite or pyroxene hornfels facies to zeolite facies conditions. It is considered that regardless of spreading rate, the static formation of upper greenschist- to lower amphibolite-facies minerals is the dominant alteration process at the lower crust near oceanic ridges, whereas subgreenschist-facies alteration represents the exhumation histories of gabbroic masses from depth. High-temperature plastic shear zones with almost anhydrous recrystallization of primary minerals develop locally at slow-spreading ridges, and possibly provide pathways for later hydrothermal fluids.<br>&emsp;In contrast to the gabbroic rocks, oceanic peridotites have a monotonous mineralogy formed during low-temperature serpentinization processes, making it difficult for us to depict their cooling histories or in-situ alteration processes at the upper mantle.<br>&emsp;The hypothesis that oceanic Moho represents a serpentinization front in peridotites is suitable for the uniformity of crustal thickness inferred from seismological observations, but lacks a rationale for supplying a constant amount of water to the upper mantle or for the cessation of serpentinization at a constant degree. Alternatively, preferential alteration of pyroxene at relatively high-temperature conditions might form the oceanic crust of uniform thickness.

DOI： 10.5026/jgeography.117.253

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GEOLOGICAL SOC AMERICA, INC  

Oceanic core complexes expose gabbroic rocks on the sealloor 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 peridotite: 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-slow-spreading ridges.

DOI： 10.1130/G23531A.1

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Serpentinite mylonites from the Happo ultramafic complex show evidence of two stages of mylonitization at different temperature conditions. Peridotite mylonites exhibit two types of olivine - porphyroclasts and neoblasts - produced at the earlier stage. The olivine neoblasts have a stretching lineation with a fabric suggesting plastic deformation along (0 1 0) [0 0 1]. In addition to the olivine fabric, the stable association of olivine, orthopyroxene and tremolite in the peridotites that survived later serpentinization, and the Si and Na contents of tremolite, suggest that the earlier mylonitization took place at temperatures between 700 and 800 degrees C. Later mylonitization was associated with high-temperature serpentinization to form serpentinite mylonites. In contrast to a common type of serpentinite in orogenic belts, the serpentinite mylonites are cohesively foliated, rich in olivine and diopside, and poor in antigorite. The diopside has low Al, Cr and Na contents typical of a retrograde origin, and the olivine has a homogeneous composition except in areas subjected to contact metamorphism at a later stage. Modal composition and mineral chemistry suggest that the serpentinite mylonites were formed by a hydration reaction of tremolite and olivine to produce diopside and antigorite under stable conditions of olivine, at temperatures between 400 and 600 degrees C. Later-stage mylonitization has preferentially been superimposed on the earlier-stage mylonite zone with a common direction of foliation. The difference in temperature between the two mylonitization stages suggests that the shear zone was episodically active during the emplacement of the Happo complex. Conditions of relatively high temperature for serpentinization at a convergent plate boundary and high permeability caused by the early mylonitization favoured the formation of the serpentinite mylonites.

DOI： 10.1111/j.1525-1314.2005.00605.x

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Compositionally heterogeneous crystals of olivine occur in thermally metamorphosed serpentinites from Southwest Japan. They have a variation in forsterite (Fo) content up to 10 mol% within a specimen. The chemical heterogeneity is the result of two causes: the intermingling of metamorphic neoblasts with relict crystals of primary olivine, and the compositional variation within the neoblasts themselves. The metamorphic olivine in each specimen shows a bimodal distribution of Fo content with a highly magnesian group (Fo(93-98), varying among specimens) and a relatively ferroan one (Fo(85-94)). Textural relationships and the variation of NiO and FeO contents between the two groups of olivine suggest that the heterogeneity results from the local involvement of magnetite and awaruite in dehydration reactions of serpentine at the initial stages of metamorphism.

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Petrographic observations. and mineralogical and geochemical analyses, have revealed that the Hegenshan ophiolite is of mid-ocean ridge origin and has been subjected to dynamothermal metamorphism at medium P/T conditions. The metamorphism is characterized by a prograde change in paragenesis from the greenschist to epidote-amphibolite facies, with peak temperature conditions of 570-640degreesC at pressures of 4-10 kbar. The amphiboles formed by this metamorphism show K-Ar ages of 110-130 Ma. The metamorphic conditions and K-Ar ages suggest that the Hegenshan ophiolite is located at the suture between the Siberian and North China continental blocks, where the continental collision in this area took place in middle Mesozoic time. Given the temporal and spatial distribution of the igneous activity around the Da Hinggan Ling Mountains, it is suggested that the extensive Yanshanian magmatism in this region resulted predominantly from a southward subduction of an oceanic plate prior to collision. Alternatively, it may possibly have resulted from the collision itself, at the final stage. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0012-821X(02)00774-4

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Tohoku University  

The Mizoguchi area, Southwest Japan, has been interpreted as a western continuation of the Hida terrane, based on the similarity of the mineralogy and radiometric age of metamorphic rocks. However, new geochemical data show metapelite and metabasite from the Mizoguchi area differ from those of the Hida terrane in the following points, i) Metapelite is relatively rich in Sc compared to La and Th, and showgently inclined chondrite-normalized REE patterns, ii) In discrimination diagrams for basaltic rocks, metabasite plots in the fields of arc tholeiites or N-MORBs
however, they are relatively depleted in light REEs but much richer in LIL elements than N-MORBs. From these geochemical features the Mizoguchi metamorphic rocks are interpreted to have formed in the fore-arc region of an oceanic island arc. This is in contrast to generally accepted continental setting for the Hida area and suggests the two areas are geologically distinct.

DOI： 10.2465/jmps.97.227

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

Blueschist tectonic blocks occur in serpentinites at Mochimaru , Hiroshima prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with ou without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high PIT metamorphism around the boundary between the blueschist and pumpellyite-actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole Pock compositions. The P-T conditions are estimated to be within 200-350 degrees C and 5-7kbar. These are higher PIT conditions than those of the regional metamorphism of Southwest Japan. The difference in the P-T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic Pocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramafic rocks. The reducing effect of serpentinization is common throughout the high PIT metamorphic terranes of Southwest Japan.

DOI： 10.1046/j.1440-1738.1999.00228.x

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Blackwell Publishing  

Basic and ultrabasic xenoliths included in Cenozoic alkali basalts from the Kibi and Sera plateaus, Southwest Japan, can be classified into five groups on the basis of mineral association and texture. Their equilibration P-T conditions estimated from paragenesis and mineral chemistry indicate that the dominant rock type from the lower crust to upper mantle changes with increasing depth as follows: (i) pyroxene granulite (Group V) and metasediments
(ii) garnet gabbro (Group III) and corundum anorthosite (Group IV)
(iii) spinel pyroxenite (Group II)
and (iv) spinel peridotite and pyroxenite (Group I). Groups II and III show a lower degree of recrystallization than Groups I and V, and have similarities in composition and mineral chemistry to host basalts. Based on these facts along with the P-T conditions of equilibration, Groups II and III are interpreted as formed from basaltic magma that intruded beneath the crust-mantle boundary at an early stage of the magmatism of the alkali basalts, where the lower crust and uppermost mantle had consisted of Group V and metasediments, and Group I, respectively. It follows that the crust has grown downward due to underplating of basaltic magma beneath the bottom of pre-existing crust. Group IV has commonly the same mineral assemblage, corundum + calcic plagioclase + aluminous spinel, and shows locally, nearby kyanite crystals, almost the same texture as fine-grained aggregates in a quartzite xenolith. The aggregates appear to have been formed by reaction between kyanite and host basalt, and accordingly Group IV is interpreted as formed by reaction between metasediments and basaltic magma at the time of the underplating. The Kibi, Sera and Tsuyama areas are distinguished from the areas nearby the Sea of Japan by the occurrence of the garnet gabbro and corundum anorthosite xenoliths, by the absence of the association of olivine + plagioclase in basic and ultrabasic xenoliths, 'and by the lower temperature of equilibration of basic xenoliths. From these facts it is stressed that in general the crust becomes thinner and geothermal gradient becomes higher towards the back-arc side. Such a regional variation in crustal structure must reflect the tectonic situation of Southwest Japan at the time of the magmatism of the alkali basalts, namely rifting and shallow-level magmatism at the back-arc side.

DOI： 10.1111/j.1440-1738.1997.tb00050.x

Scopus

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1180/minmag.1990.054.377.07

CiNii Article

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Authorship：Lead author,　Corresponding author  

DOI： 10.2465/ganko.85.531

CiNii Article

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Oceanic core complex formation, Atlantis Massif, Mid-Atlantic Ridge: drilling into the footwall and hanging wall of a tectonic exposure of deep, young oceanic lithosphere to study deformation, alteration, and melt generation. Integrated Ocean Drilling Program Expedition 305 Preliminary Report

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Language：English  

DOI： 10.2204/iodp.sd.3.01.2006

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J-GLOBAL

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J-GLOBAL

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Language：English  

CiNii Article

CiNii Books

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Publisher：Tokyo Geographical Society  

DOI： 10.5026/jgeography.99.4_382

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Language：Japanese   Presentation type：Poster presentation  

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Language：Japanese   Presentation type：Oral presentation (general)  

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Language：Japanese   Presentation type：Oral presentation (general)  

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Language：English   Presentation type：Poster presentation  

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Language：Japanese   Presentation type：Oral presentation (general)  

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