Nebular history of amoeboid olivine aggregates     

N. SUGIURA  M. I. PETAEV  M. KIMURA  A. MIYAZAKI  H. HIYAGON 《Meteoritics & planetary science》2009,44(4):559-572

Abstract— Minor element (Ca, Cr, and Mn) concentrations in amoeboid olivine aggregates (AOAs) from primitive chondrites were measured and compared with those predicted by equilibrium condensation in the solar nebula. CaO concentrations in forsterite are low, particularly in porous aggregates. A plausible explanation appears that an equilibrium Ca activity was not maintained during the olivine condensation. CaO and MnO in forsterite are negatively correlated, with CaO being higher in compact aggregates. This suggests that the compact aggregates formed either by a prolonged reheating of the porous aggregates or by condensation and aggregation of forsterite during a very slow cooling in the nebula.  相似文献   

Laboratory measurements of ‘porosity‐free’ intrinsic Vp and Vs in an olivine gabbro of the Oman ophiolite: Implication for interpretation of the seismic structure of lower oceanic crust          下载免费PDF全文

Satoshi Saito  Masahiro Ishikawa  Makoto Arima  Yoshiyuki Tatsumi 《Island Arc》2015,24(2):131-144

In order to determine ‘porosity‐free’ intrinsic ultrasonic compressional (Vp) and shear wave (Vs) velocities and Vp/Vs of an olivine gabbro from the Oman ophiolite, we developed a new experimental system using a piston‐cylinder type high‐pressure apparatus. The new system allowed us to measure velocities at pressures ranging from 0.20 to 1.00 GPa and at temperatures up to 300°C for Vp and 400°C for Vs. At room temperature, the Vp and Vp/Vs increase rapidly with pressure up to 0.40 GPa, while between 0.45 and 1.00 GPa the increase is more gradual. The change in increasing rate is attributed to closure of porosity at pressures above 0.45 GPa. Based on the linear regression of data obtained at higher pressures (0.45–1.00 GPa) and extrapolation to the lower pressures, combined with temperature derivatives of velocities of the sample measured at 1.00 GPa, we determined the intrinsic Vp and Vs of the sample as a function of pressure (P, in GPa) and temperature (T, in °C). The intrinsic velocities can be expressed as Vp (km/s) = 7.004 + 0.096 × P ? 0.00015 × T, and Vs (km/s) = 3.827 + 0.007 × P ? 0.00008 × T. We evaluated the intrinsic Vp and Vs of the olivine gabbro at oceanic crustal conditions and compared them with a velocity depth‐profile of the borehole seismic observatory WP‐2 area in the northwestern Pacific Basin. Although the intrinsic Vp (～7.0 km/s) and Vs (～3.8 km/s) for the olivine gabbro studied are comparable to those of seismic layer 3 in the WP‐2 area, the estimated vertical gradients of intrinsic velocities are significantly smaller than those reported from layer 3. These results suggest that velocity profiles of layer 3 in the WP‐2 area may reflect the presence of a minor porosity in lower oceanic crust, which closes with increasing depth and/or continuous changes in mineralogy of layer 3 rocks.  相似文献   

Asteroids approaching the Earth from directions around the Sun     

Syuzo Isobe  Makoto Yoshikawa 《Earth, Moon, and Planets》1996,72(1-3):263-266

We have estimated close asteroid encounters with the Earth by numerical integrations of a system with the Sun, 9 planets, and 188 near-earth-asteroids during the period 1994–4600. Asteroids approach the Earth from directions within 30? around the Sun in more than 20% of encounters with the closest distance less than 0.01 AU. Since ground-based observations cannot detect these objects, we should develop space-borne and/or lunar observatories in a short time to allow enough warning time before a catastrophic collision.  相似文献   

Geochemical characteristics of pelagic chert sequences across the Permian-Triassic boundary in southwest Japan   总被引：1，自引：0，他引：1  

Hroaki  Ishiga  Kotaro  Ishida  Kaori  Dozen Makoto  Musashino 《Island Arc》1996,5(2):180-180

Abstract Geochemical characteristics, mainly of major and trace elements and REE (rare earth elements) of bedded chert and shale/mudstone sequences, across the Permian/Triassic boundary in southwest Japan are examined. The boundary is characterized by the disappearance of bedded cherts, and the interval between the Upper Permian cherts and Lower Triassic (probably Smithian) cherts comprises siliceous shales and organic black mudstones. Bedded cherts are characterized by a gradual depletion of chemical elements from Middle to Upper Permian. However, overlying siliceous shales exhibit a clear enrichment in some elements, especially alkaline metals (such as K, Rb and Cs) and Ti, Th, Y, P₂O₅, and REE in comparison with elements of the PAAS (post Archean Australian shales). This indicates that average components of the upper continental crust were transported in the boundary interval, possibly caused by volcanic activity. Ce-negative shifting in NASC (North American Shales Composite)-normalized REE patterns is characteristic of this interval, and could be related to the deposition of siliceous rocks in Ce-depleted seawater. This was probably caused by an invasion of water mass with a Ce-negative anomaly into the previously existing water mass of the Paleo-Tethys. Weak negative Eu-anomalies in this interval are suggestive of plagioclase fractionation caused by acid volcanisms and the LREE/HREE ratios in the interval show a slightly light-REE enrichment. Organic black mudstones are characteristically intercalated in the interval. These rocks are usually regarded as a product of oceanic deterioration, but in pelagic conditions, organic materials were formed by high primary production that resulted from the active upwelling of ocean floor water currents with rich nutrients. This may have been caused by the inferred mixing of water masses of the Paleo-Tethys and of the Panthalassa in Early Triassic time which was regarded as an event synchronous with an increase in volcanic activity on highly matured island arcs and/or continents.  相似文献   

A deep section of accretionary complex: Susunai Complex in Sakhalin Island, Northwest Pacific Margin     

Gaku  Kimura Masayuki  Sakakibara Hisashi  Ofuka Hideo  Ishizuka Sumio  Miyashita Makoto  Okamura Olex A.  Melinikov Vilera  Lushchenko 《Island Arc》1992,1(1):166-175

Abstract A deep section of accretionary complex, the metamorphosed Susunai Complex, is observed on Sakhalin Is., Russia. High pressure part of pumpellyite-actinolite facies metavolcanics, metacherts and metapelites are well exposed and constitute a tectonic pile preserving primary structures related to underplating of the oceanic crust. Three stages of deformation, D₁ through D₃, suggest successive deformation during subduction, underplating and exhumation of the complex. Oceanic material in the complex is more abundant than other well documented ancient accretionary complexes (e.g. the Shimanto Belt in southwest Japan and the Ghost Rocks Formation in Alaska), which were shallowly underplated. At Susunai, deep down-stepping of a décollément has scraped off the upper part of the oceanic crust, primarily the pillowed basalt horizon. This down-stepping results from crustal weakening as overpressured water is released from the fractured oceanic crust during metamorphism.  相似文献   

D/H fractionation factors between serpentine and water at 100° to 500°C and 2000 bar water pressure,and the D/H ratios of natural serpentines     

Hitoshi Sakai  Makoto Tsutsumi 《Earth and Planetary Science Letters》1978,40(2):231-242

D/H fractionation factors between serpentine (clinochrysotile) and water were experimentally determined to be: 1000 In α_ser-w = 2.75 × 10 ⁷/T² ? 7.69 × 10⁴/T + 40.8 in the temperature range from 100 to 500°C. The present results do not support the semi-empirical fractionation factors employed by Wenner and Taylor [1] for the interpretation of δD values of natural serpentines. About 100 serpentines from the Japanese Islands have δD values from ?110 to ?40‰ SMOW, with antigorite being from ?40 to ?60‰. The results are in accord with the two conclusions by Wenner and Taylor [1,2], that is, the presence of a latitude ?δD correlation and the more uniform and higher δD values of antigorite than chrysotile and lizardite.According to the present fractionation factors, almost none of the continental lizardite-chrysotile serpentines could have formed at a temperature below 500°C under equilibrium with fluids of δD values similar to the present-day local meteoric waters. The fluid responsible for oceanic serpentinization could be either a mixture of oceanic and magmatic water or oceanic water alone. However, full interpretation of the δD values of natural serpentines should wait until kinetic behaviors of hydrogen isotopes in serpentinization are better understood.  相似文献   

The solubility of alumina in enstatite and the phase equilibria in the join MgSiO3-MgAl2SiO6 at 10–25 kbar     

Makoto Arima  Kosuke Onuma 《Contributions to Mineralogy and Petrology》1977,61(3):251-265

The solubility of alumina in enstatite was determined in the range of 1100–1500° C and 10–25 kbar. The alumina content in enstatite coexisting with sapphirine and quartz increases with increasing temperature and pressure, while that in enstatite coexisting with sapphirine and sillimanite or with pyrope decreases with increasing pressure and decreasing temperature. Two univariant lines, pyrope = enstatite_ss + sillimanite + sapphirine_ss and enstatite_ss + sillimanite =sapphirine_ss + quartz were confirmed. The invariant point involving these phases is metastable. The alumina content of orthopyroxene can not be used either as a pressure indicator or as a temperature indicator without taking the mineral assemblage into account.  相似文献   

Long-term stability of flow-path structure in crystalline rocks distributed in an orogenic belt, Japan     

Hidekazu Yoshida  Makoto Takeuchi  Richard Metcalfe 《Engineering Geology》2005,78(3-4):275-284

Most evaluations of the contaminant retardation processes likely to be important in geological disposal (e.g. for high level radioactive waste (HLW)) consider only the present characteristics of fractures and associated mineral infills. Relatively little attention has been given to possible long-term changes in these features, and their influence on groundwater flow. The work reported here seeks to provide analogous evidence that such changes are not likely to be important and hence to improve confidence in the presently adopted evaluation methodology and its long-term applicability.

In the orogenic belt that is formed by the Japanese islands, there are wide areas of crystalline rock. The rocks in each area have a distinctive age sequence which is partly reflected in the characteristics of the fracture systems and associated mineral fillings that occur. These characteristics generally imply that groundwater and solutes can be conducted through fracture networks, except in the cases of fault zones or crushed zones. The structural and mineralogical features of these networks readily illustrate how certain contaminants might react and be retarded by the fracture fillings and open pore geometry, due to chemical sorption and/or physical retardation.

Here, we describe the fracture systems developed in crystalline rocks with different ages that are intruded into the Japanese orogenic belt. The aim is to build a model for the long-term fracturing process and hence to evaluate fracture ‘stability’. In particular, the comparisons are made between the fracture geometries and the frequencies observed in the 1.9–0.8 Ma Takidani Granodiorite (the youngest exposed pluton in the world), the ca. 67 Ma Toki Granite and the ca. 117 Ma Kurihashi Granodiorite located in central to northwest Japan. The observations show that all these crystalline rocks have similar fracture frequencies, with 1 to 2 fractures per meter in the massive part of rock bodies. Mineralogical studies and dating analyses of fracture fillings also suggest that fractures are relatively physically stable. Major new fractures tend not to be created in the massive part of rock bodies even when a pluton has been subjected to the regional stresses of plate movements with a duration of about 100 Ma. The results show the unique characteristics of the fracture forming process and the relatively stable geometries of fracture network systems in crystalline rocks distributed within the orogenic belt. This analogue also enables us to provide a model to build confidence in a technical approach applicable for modeling of hydrogeology and geology over long time scales under the orogenic stress field present in Japan. The model may also be useful for other stable tectonic settings as well as for a characterizing sites in crystalline rocks for the possible geological disposal of HLW and other toxic wastes.  相似文献   

Interpretation of two-dimensional magnetotelluric profile data with three-dimensional inversion: synthetic examples   总被引：4，自引：1，他引：4  

Weerachai Siripunvaraporn  Gary Egbert  Makoto Uyeshima 《Geophysical Journal International》2005,160(3):804-814

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On the sensitivity characteristics of Lacoste & Romberg gravimeter (model G)     

Ichiro Nakagawa  Sinzi Nakai  Ryūichi Shichi  Hirokazu Tajima  Sadakatu Izutuya  Yoshiteru Kono  Toshihiro Higashi  Hiromi Fujimoto  Makoto Murakami  Kazuo Tajima  Minoru Funaki 《Journal of Geodesy》1985,59(1):55-67

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