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排序方式: 共有102条查询结果,搜索用时 15 毫秒
91.
92.
Tomoki Nakamura Aiko Nakato Hatsumi Ishida Shigeru Wakita Takaaki Noguchi Michael E. Zolensky Masahiko Tanaka Makoto Kimura Akira Tshuchiyama Toshihiro Ogami Takahito Hashimoto Mitsuru Konno Masayuki Uesugi Toru Yada Kei Shirai Akio Fujimura Ryuji Okazaki Scott. A. Sandford Yukihiro Ishibashi Masanao Abe Tatsuaki Okada Munetaka Ueno Junichiro Kawaguchi 《Meteoritics & planetary science》2014,49(2):215-227
The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES‐C Regio were characterized by synchrotron‐radiation X‐ray diffraction and field‐emission electron microprobe analysis. Olivine and low‐ and high‐Ca pyroxene, plagioclase, and merrillite compositions of the first‐touchdown particles are similar to those of the second‐touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES‐C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first‐ and second‐touchdown samples, indicate that dust particles of MUSES‐C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES‐C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K‐feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact‐induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na‐K distributions of plagioclase have been disturbed. 相似文献
93.
Takaaki Noguchi Makoto Kimura Takahito Hashimoto Mitsuru Konno Tomoki Nakamura Michael E. Zolensky Ryuji Okazaki Masahiko Tanaka Akira Tsuchiyama Aiko Nakato Toshinori Ogami Hatsumi Ishida Ryosuke Sagae Shinichi Tsujimoto Toru Matsumoto Junya Matsuno Akio Fujimura Masanao Abe Toru Yada Toshifumi Mukai Munetaka Ueno Tatsuaki Okada Kei Shirai Yukihiro Ishibashi 《Meteoritics & planetary science》2014,49(2):188-214
On the basis of observations using Cs‐corrected STEM, we identified three types of surface modification probably formed by space weathering on the surfaces of Itokawa particles. They are (1) redeposition rims (2–3 nm), (2) composite rims (30–60 nm), and (3) composite vesicular rims (60–80 nm). These rims are characterized by a combination of three zones. Zone I occupies the outermost part of the surface modification, which contains elements that are not included in the unchanged substrate minerals, suggesting that this zone is composed of sputter deposits and/or impact vapor deposits originating from the surrounding minerals. Redeposition rims are composed only of Zone I and directly attaches to the unchanged minerals (Zone III). Zone I of composite and composite vesicular rims often contains nanophase (Fe,Mg)S. The composite rims and the composite vesicular rims have a two‐layered structure: a combination of Zone I and Zone II, below which Zone III exists. Zone II is the partially amorphized zone. Zone II of ferromagnesian silicates contains abundant nanophase Fe. Radiation‐induced segregation and in situ reduction are the most plausible mechanisms to form nanophase Fe in Zone II. Their lattice fringes indicate that they contain metallic iron, which probably causes the reddening of the reflectance spectra of Itokawa. Zone II of the composite vesicular rims contains vesicles. The vesicles in Zone II were probably formed by segregation of solar wind He implanted in this zone. The textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on Itokawa. 相似文献
94.
Michael Zolensky Tomoki Nakamura James Martinez Yuma Enokido 《Meteoritics & planetary science》2021,56(1):56-60
We describe a fragmented cryptocrystalline chondrule consisting solely of forsterite (Fo98) in the Murchison CM2 chondrite, with a peculiar porous texture of enigmatic origin. 相似文献
95.
Shinjiro Mizuno Kazuo Kawatate Tomoki Nagahama Tsunehiro Miita 《Journal of Oceanography》1989,45(6):375-384
The Tushima Current in the east channel was measured in a site of the strong current between Tushima and Iki Islands for 43 days of the winter in 1986. The primary results are:First, it was highly barotropic and flowed northeastward with a mean speed of about 20 cm sec–1. Second, it was decelerated by northeast winds rather than by northwest winds in a frequency range of 0.3 and 0.4 cpd. Third, the scale analysis showed that the Tushima Current is approximately in geostrophic balance. The best empirical relationship between the measured Tushima Current and cross-channel adjusted sea level difference is given by
相似文献
96.
Nagisa Yamamoto Akihisa Kitamura Tomohisa Irino Tomoki Kase Syu-ichi Ohashi 《Global and Planetary Change》2008,62(1-2):97-106
We measured the δ18O values of the whole shells of the cavernicolous micro-bivalvia Carditella iejimensis obtained from sediments within a submarine cave (31 m water depth) at Ie Island (Okinawa Island, Japan) in the subtropical Northwest Pacific. Our results show no significant millennial-scale trend in the δ18O record, implying that both springtime temperature and the δ18O of sea water at 30 m depth around the Okinawa Islands have been stable for the past 3000 years at values similar to those of today. Moreover, we found one exceptionally light δ18O value from specimens spanning the past 250 years. The δ18O-derived temperature represents a departure of 2.1 °C from the average value for the past 250 years, being equal to the departure recorded during unusually high temperatures in the spring of 1998. This finding may imply that such high springtime sea surface temperature has been a rare event over the past 3000 years. 相似文献
97.
Tomoki Nakamura Kazushige Tomeoka Toshimori Sekine Hiroshi Takeda 《Meteoritics & planetary science》1995,30(3):344-347
Abstract— We have carried out shock-recovery experiments on the Allende CV3 carbonaceous chondrite using a single-stage propellant gun and succeeded in reproducing oriented, flattened chondrules like those observed in some natural CV3 chondrites. The Allende samples were shocked at equilibrium pressures of 11 and 21 GPa, which are close to the highest values in shock stages S2 and S3, respectively (Stöffler et al., 1991). Chondrules are flattened nearly perpendicular to the compaction axis with mean aspect ratios of 1.34 and 1.62 at pressures of 11 and 21 GPa, respectively; thus, the degree of chondrule flattening is proportional to the shock intensity. The chondrule flattening and foliation are mainly due to collapse of pores in the matrix under shock pressure. High matrix abundance of CV3 chondrites could result in much apparent chondrule flattening relative to ordinary chondrites. Optical and electron microscope observations show that textural and mineralogical characteristics of chondrules and matrix in the shock-loaded samples are very similar to those observed in naturally shocked CV3 chondrites. Our results provide strong support for the interpretation that the chondrule flattening and foliation in CV3 chondrites were caused by shock-induced pressure due to hypervelocity impacts on the meteorite parent bodies. 相似文献
98.
Kaori JOGO Kazuhide NAGASHIMA Ian D. HUTCHEON Alexander N. KROT Tomoki NAKAMURA 《Meteoritics & planetary science》2012,47(12):2251-2268
Abstract– Metamorphosed clasts in the CV carbonaceous chondrite breccias Mokoia and Yamato‐86009 (Y‐86009) are coarse‐grained, granular, polymineralic rocks composed of Ca‐bearing (up to 0.6 wt% CaO) ferroan olivine (Fa34–39), ferroan Al‐diopside (Fs9–13Wo47–50, approximately 2–7 wt% Al2O3), plagioclase (An37–84Ab63–17), Cr‐spinel (Cr/(Cr + Al) = 0.19–0.45, Fe/(Fe + Mg) = 0.60–0.79), nepheline, pyrrhotite, pentlandite, Ca‐phosphate, and rare grains of Ni‐rich taenite; low‐Ca pyroxene is absent. Most clasts have triple junctions between silicate grains, indicative of prolonged thermal annealing. Based on the olivine‐spinel and pyroxene thermometry, the estimated metamorphic temperature recorded by the clasts is approximately 1100 K. Few clasts experienced thermal metamorphism to a lower degree and preserved chondrule‐like textures. The Mokoia and Y‐86009 clasts are mineralogically unique and different from metamorphosed chondrites of known groups (H, L, LL, R, EH, EL, CO, CK) and primitive achondrites (acapulcoites, brachinites, lodranites). On a three‐isotope oxygen diagram, compositions of olivine in the clasts plot along carbonaceous chondrite anhydrous mineral line and the Allende mass‐fractionation line, and overlap with those of the CV chondrule olivines; the Δ17O values of the clasts range from about ?4.3‰ to ?3.0‰. We suggest that the clasts represent fragments of the CV‐like material that experienced metasomatic alteration, high‐temperature metamorphism, and possibly melting in the interior of the CV parent asteroid. The lack of low‐Ca pyroxene in the clasts could be due to its replacement by ferroan olivine during iron‐alkali metasomatic alteration or by high‐Ca ferroan pyroxene during melting under oxidizing conditions. 相似文献
99.
Chaoxia Yuan Tomoki Tozuka Willem A. Landman Toshio Yamagata 《Climate Dynamics》2014,42(11-12):3357-3374
Prediction skill for southern African (16°–33°E, 22°–35°S) summer precipitation in the Scale Interaction Experiment-Frontier coupled model is assessed for the period of 1982–2008. Using three different observation datasets, deterministic forecasts are evaluated by anomaly correlation coefficients, whereas scores of relative operating characteristic and relative operating level are used to evaluate probabilistic forecasts. We have found that these scores for December–February precipitation forecasts initialized on October 1st are significant at 95 % confidence level. On a local scale, the level of prediction skill in the northwestern and central parts of southern Africa is higher than that in northeastern South Africa. El Niño/Southern Oscillation (ENSO) provides the major source of predictability, but the relationship with ENSO is too strong in the model. The Benguela Niño, the basin mode in the tropical Indian Ocean, the subtropical dipole modes in the South Atlantic and the southern Indian Oceans and ENSO Modoki may provide additional sources of predictability. Within the wet season from October to the following April, the precipitation anomalies in December-February are the most predictable. This study presents promising results for seasonal prediction of precipitation anomaly in the extratropics, where seasonal prediction has been considered a difficult task. 相似文献
100.
Lin Li Takaya Nagai Tomoki Ishido Satoko Motai Kiyoshi Fujino Shoichi Itoh 《Physics and Chemistry of Minerals》2014,41(6):431-437
Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO3–MnSiO3 perovskite system at high pressure and high temperature. The (Mg, Mn)SiO3 perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO3 components incorporated seems to obey Vegard’s rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a/b and \( \sqrt{2}\,a/c, \) monotonically decreases from MgSiO3 to MnSiO3 perovskite at about 50 GPa. The orthorhombic distortion in (Mg0.5, Mn0.5)SiO3 perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg0.9, Mn0.1)SiO3 perovskite increases with pressure. (Mg, Mn)SiO3 perovskite incorporating less than 10 mol% of MnSiO3 component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg0.9, Mn0.1)SiO3. MnSiO3 is the first chemical component confirmed to form a complete solid solution with MgSiO3 perovskite at the P–T conditions present in the lower mantle. 相似文献
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