Pore structure of sheared coals and related coalbed methane     

Huoyin Li  Yujiro Ogawa 《Environmental Geology》2001,40(11-12):1455-1461

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Mineralogical characterization of a unique material having heavy oxygen isotope anomaly in matrix of the primitive carbonaceous chondrite Acfer 094     

Yusuke Seto  Naoya Sakamoto  Takashi Kaito  Hisayoshi Yurimoto 《Geochimica et cosmochimica acta》2008,72(11):2723-2734

We report the mineral compositions and micro-texture of the isotopically anomalous (δ^17,18O_SMOW ∼ +180‰) Fe-S-Ni-O material recently discovered in matrix of the primitive carbonaceous chondrite Acfer 094 [Sakamoto N., Seto Y., Itoh S., Kuramoto K., Fujino K., Nagashima K., Krot A. N. and Yurimoto H. (2007) Oxygen isotope evidence for remnants of the early solar system primordial water. Science317, 231-233]. Synchrotron radiation X-ray diffraction and transmission electron microscopy studies indicate that this material consists of the symplectitically intergrown magnetite (Fe₃O₄) and pentlandite (Fe_5.7Ni_3.3S₈) with magnetite/pentlandite volume ratio of ∼2.3. Magnetite forms column-shaped grains (10-30 nm in diameter and 100-200 nm in length); pentlandite occurs as worm-shaped grains or aggregates of grains 100-300 nm in size between magnetite crystals. Although both the X-ray diffraction and electron energy loss spectra support identification of iron oxide as magnetite, the electron diffraction patterns show that magnetite has a weak 3-fold superstructure, possibly due to ordering of vacancies. We infer that the isotopically anomalous symplectite formed by sulfurization and oxidization of metal grains either in the solar nebula or on an icy planetesimal. The intersite cation distribution of pentlandite suggests that timescale of oxidation was no longer than 1000 years.  相似文献   

Cycladophora davisiana (Radiolaria) in the Okhotsk Sea: A key for reconstructing glacial ocean conditions     

Yusuke Okazaki  Osamu Seki  Takeshi Nakatsuka  Tatsuhiko Sakamoto  Minoru Ikehara  Kozo Takahashi 《Journal of Oceanography》2006,62(5):639-648

Cycladophora davisiana, a radiolarian species dwelling at mesopelagic depths, is known as a representative glacial fauna due to its unique distribution during glacial periods. In the present ocean, abundant production of C. davisiana is only observed in the Okhotsk Sea, indicating an adaptation of C. davisiana for seasonal sea-ice covered conditions. We found pronounced abundant production of C. davisiana during the early to middle Holocene in the Okhotsk Sea, suggesting more favorable conditions for C. davisiana than the present Okhotsk Sea. In order to clarify the reason, oceanographic conditions during the Holocene were reconstructed based on biomarkers, lithogenic grains including ice-rafted debris (IRD), biogenic opal, and total organic carbon (TOC) in two sediment cores from the Okhotsk Sea. These indicators suggest that the pronounced C. davisiana production may be attributed to: 1) a supply to mesopelagic depths under intensified stratification of fine organic particles derived from coccolithophorids, bacteria, and detrital materials; and 2) cold, well-ventilated intermediate water formation.  相似文献   

Fluid–Sediment Interaction in a Marine Shallow-Water Hydrothermal System in the Wakamiko Submarine Crater, South Kyushu, Japan     

Miwako Nakaseama    Jun-Ichiro Ishibashi    Keita Ogawa    Hiroshi Hamasaki    Keiko Fujino  Toshiro Yamanaka 《Resource Geology》2008,58(3):289-300

The Wakamiko submarine crater is a small depression located in Kagoshima Bay, southwest Japan. Marine shallow‐water hydrothermal activity associated with fumarolic gas emissions at the crater sea floor (water depth 200 m) is considered to be related with magmatic activity of the Aira Caldera. During the NT05‐13 dive expedition conducted in August 2005 using remotely operated vehicle Hyper‐Dolphine (Japan Agency for Marine‐Earth Science and Technology), an active shimmering site was discovered (tentatively named the North site) at approximately 1 km from the previously known site (tentatively named the South site). Surface sediment (up to 30 cm) was cored from six localities including these active sites, and the alteration minerals and pore fluid chemistry were studied. The pore fluids of these sites showed a drastic change in chemical profile from that of seawater, even at 30 cm below the surface, which is attributed to mixing of the ascending hydrothermal component and seawater. The hydrothermal component of the North site is estimated to be derived from a hydrothermal aquifer at 230°C based on the hydrothermal end‐member composition. Occurrence of illite/smectite interstratified minerals in the North site sediment is attributed to in situ fluid–sediment interaction at a temperature around 150°C, which is in accordance with the pore fluid chemistry. In contrast, montmorillonite was identified as the dominant alteration mineral in the South site sediment. Together with the significant low potassium concentration of the hydrothermal end‐member, the abundant occurrence of low‐temperature alteration mineral suggests that the hydrothermal aquifer in the South site is not as high as 200°C. Moreover, the montmorillonite is likely to be unstable with the present pore fluid chemistry at the measured temperature (117°C). This disagreement implies unstable hydrothermal activity at the South site, in contrast to the equilibrium between the pore fluid and alteration minerals in the North site sediment. This difference may reflect the thermal and/or hydrological structure of the Wakamiko Crater hydrothermal system.  相似文献   

类石材料--灰泥的冻融过程     

Takashi Ono 《冰川冻土》2004,26(Z1):64-69

Serious failure on the slope of rock ground can be caused by a cyclic action of freezing and thawing in the cold regions. The frost susceptibility and the effect of freezing and thawing onthe rock material, however, have not been well investigated. In order to find out the freezing effect on the rock materials, mortar specimens are frozen as a pseudo-rock material under the constant rate of freezing by means of controlling the temperature of both ends of specimen. The freezing process is given one-dimensionally to the cylindrical samples in the laboratory to simulate the in-situ freezing phenomena in the natural ground. Formation of ice lens, frost heave and water intake during freezing process are observed on the mortar specimen under constant freezing rate, which probably causes cracks or large deformation in the real rock ground. The values of the velocity of elastic wave propagation are compared before and after freezing process to estimate the degree of weathering due to freezing and thawing.  相似文献   

Seafloor spreading, obduction and triple junction tectonics of the Mineoka Ophiolite, Central Japan     

Yujiro Ogawa  Akiko Takahashi 《Tectonophysics》2004,392(1-4):131

The Tertiary Mineoka ophiolite occurs in a fault zone at the intersection of the Honshu and Izu forearcs in central Japan and displays structural evidence for three major phases of deformation: normal and oblique-slip faults and hydrothermal veins formed during the seafloor spreading evolution of the ophiolite at a ridge-transform fault intersection. These structures may represent repeated changes in differential stress and pore-fluid pressures during their formation. The second series of deformation is characterized by oblique thrust faults with Riedel shears and no significant mineral veining, and is interpreted to have resulted from transpressional dextral faulting during the obduction of the ophiolite through oblique convergence and tectonic accretion. This deformation occurred at the NW corner of a TTT-type (trench–trench–trench) triple junction in the NW Pacific rim before the middle Miocene. The third series of deformation of the ophiolite is marked by contractional and oblique shear zones, Riedel shears, and thrust faults that crosscut and offset earlier structures, and that give the Mineoka fault zone its lenticular (phacoidal) fabric at all scales. This deformation phase was associated with the establishment and the southward migration of the TTT Boso triple junction and with the kinematics of oblique subduction and forearc sliver fault development. The composite Mineoka ophiolite hence displays rocks and structures that evolved during its complex geodynamic history involving seafloor spreading, tectonic accretion, and triple junction evolution in the NW Pacific Rim.  相似文献   

Origin of layering in cumulate gabbros in the Oman ophiolite: Insights from magnetic susceptibility measurements in the Wadi Sadm area     

Kiichiro  Kawamura  Takahiro  Hosono  Huda Mohamed  Allawati  Yujiro  Ogawa  Hidetsugu  Taniguchi 《Island Arc》2005,14(4):564-570

Abstract   Magnetic susceptibility and the anisotropy of magnetic susceptibility were measured on an 800-cm-thick succession of cumulate gabbro in the Sadm area of the Oman ophiolite. The section contained three distinct cumulate units. The susceptibility tends to decrease upward in each from a melanocratic layer (several tens of centimeters thick) to a leucocratic layer (a few meters thick). The susceptibility decreases in accordance with the decreasing number of magnetite grains, which are the alteration product mainly of olivine minerals. This suggests the cyclic downward accumulation of olivine in the cumulate gabbro. The apparent strain deduced from the patterns of magnetic and grain fabrics was the result mostly of simple shear, so that the layering of gabbro is understood to be formed primarily by a crystal cumulus process followed by simple shear deformation.  相似文献   

Transpressional tectonics of the Mineoka Ophiolite Belt in a trench–trench–trench-type triple junction, Boso Peninsula, Japan     

Ryota  Mori  Yujiro  Ogawa 《Island Arc》2005,14(4):571-581

Abstract   Structures developed in metamorphic and plutonic blocks that occur as knockers in the Mineoka Ophiolite Belt in the Boso Peninsula, central Japan, were analyzed. The aim was to understand the incorporation processes of blocks of metamorphic and plutonic rocks with an arc signature into the serpentinite mélange of the Mineoka Ophiolite Belt in relation to changes in metamorphic conditions during emplacement. Several stages of deformation during retrogressive metamorphism were identified: the first faulting stage had two substage shearing events (mylonitization) under ductile conditions inside the crystalline blocks in relatively deeper levels; and the second stage had brittle faulting and brecciation along the boundaries between the host serpentinite bodies in relatively shallower levels (zeolite facies). The first deformation occurred during uplift before emplacement. The blocks were intensively sheared by the first deformation event, and developed numerous shear planes with spacing of a few centimeters. The displacement and width of each shear plane were a few centimeters and a few millimeters, respectively, at most. In contrast, the fault zone of the second shearing stage reached a few meters in width and developed during emplacement of the Mineoka Ophiolite. Both stages occurred under a right-lateral transpressional regime, in which thrust-faulting was associated with strike-slip faulting. Such displacement on an outcrop scale is consistent with the present tectonics of the Mineoka Belt. This implies that the same tectonic stress has been operating in the Boso trench–trench–trench-type triple junction area in the northwest corner of the Pacific since the emplacement of the Mineoka Ophiolite. The Mineoka Ophiolite Belt must have worked as a forearc sliver fault during the formation of a Neogene accretionary prism further south.  相似文献   

An experimental study on felsic rock–artificial seawater interaction: implications for hydrothermal alteration and sulfate formation in the Kuroko mining area of Japan     

Yasumasa Ogawa  Naotatsu Shikazono  Daizo Ishiyama  Hinako Sato  Toshio Mizuta 《Mineralium Deposita》2005,39(8):813-821

Experimental studies on the interactions between artificial seawater (ASW) and fresh rhyolite, perlite and weakly altered dacitic tuff containing a small amount of smectite suggest changing cation transfer during smectite-forming processes. Initially, dissolution of K from the rocks accompanies incorporation of Mg and Ca from ASW during both earlier (devitrification stage) and later smectite formation, whereas Ca incorporated with early smectite formation redissolves with progressive reaction. Barium mobility increases toward the later smectite-forming reactions. Therefore, the large amounts of barite, anhydrite and gypsum in Kuroko ore deposits are considered to have precipitated from hydrothermal solutions derived from the interaction with previously altered felsic rocks during late smectite formation, rather than by the reaction with fresh felsic rocks.Editorial handling: D. Lentz  相似文献   

Theoretical and experimental evidence for the post-cotunnite phase transition in zirconia at high pressure     

Daisuke Nishio-Hamane  Haruhiko Dekura  Yusuke Seto  Takehiko Yagi 《Physics and Chemistry of Minerals》2015,42(5):385-392

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