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排序方式: 共有497条查询结果,搜索用时 15 毫秒
491.
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
492.
We modeled the possible parent bodies of Itokawa, which was heated within by the decay energy of 26Al. Based on mineralogic studies of dust particles derived from Itokawa by the Hayabusa spacecraft, it appeared that they were thermally metamorphosed at a peak temperature of 800 °C, and kept at 700 °C or higher at 7.6 Myr after CAI formation. Our numerical results show that the parent bodies of Itokawa would have been larger than 20 km in radius and accreted at a period between 1.9 and 2.2 Myr after CAI formation, to satisfy mineralogic and isotopic evidence from dust particles. 相似文献
493.
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
494.
Yumi Iwasaki Kimihito Nakamura Haruhiko Horino Shigeto Kawashima 《Hydrogeology Journal》2014,22(8):1841-1855
Assessing factors that influence groundwater levels such as land use and pumping strategy, is essential to adequately manage groundwater resources. A transient numerical model for groundwater flow with infiltration was developed for the Tedori River alluvial fan (140 km2), Japan. The main water input into the groundwater body in this area is irrigation water, which is significantly influenced by land use, namely paddy and upland fields. The proposed model consists of two models, a one-dimensional (1-D) unsaturated-zone water flow model (HYDRUS-1D) for estimating groundwater recharge and a 3-D groundwater flow model (MODFLOW). Numerical simulation of groundwater flow from October 1975 to November 2009 was performed to validate the model. Simulation revealed seasonal groundwater level fluctuations, affected by paddy irrigation management. However, computational accuracy was limited by the spatiotemporal data resolution of the groundwater use. Both annual groundwater levels and recharge during the irrigation periods from 1975 to 2009 showed long-term decreasing trends. With the decline in rice-planted paddy field area, groundwater recharge cumulatively decreased to 61 % of the peak in 1977. A paddy-upland crop-rotation system could decrease groundwater recharge to 73–98 % relative to no crop rotation. 相似文献
495.
Satoru?HaraguchiEmail author Jun-Ichi?Kimura Ryoko?Senda Koichiro?Fujinaga Kentaro?Nakamura Yutaro?Takaya Teruaki?Ishii 《Contributions to Mineralogy and Petrology》2017,172(5):25
An intra-arc rift (IAR) is developed behind the volcanic front in the Izu arc, Japan. Bimodal volcanism, represented by basalt and rhyolite lavas and hydrothermal activity, is active in the IAR. The constituent minerals in the rhyolite lavas are mainly plagioclase and quartz, whereas mafic minerals are rare and are mainly orthopyroxene without any hydrous minerals such as amphibole and biotite. Both the phenocryst and groundmass minerals have felsic affinities with a narrow compositional range. The petrological and bulk chemical characteristics are similar to those of melts from some partial melting experiments that also yield dry rhyolite melts. The hydrous mineral-free narrow mineral compositions and low-Al2O3 affinities of the IAR rhyolites are produced from basaltic middle crust under anhydrous low-temperature melting conditions. The IAR basalt lavas display prominent across-arc variation, with depleted elemental compositions in the volcanic front side and enriched compositions in the rear-arc side. The across-arc variation reflects gradual change in the slab-derived components, as demonstrated by decreasing Ba/Zr and Th/Zr values to the rear-arc side. Rhyolite lavas exhibit different across-arc variations in either the fluid-mobile elements or the immobile elements, such as Nb/Zr, La/Yb, and chondrite-normalized rare earth element patterns, reflecting that the felsic magmas had different source. The preexisting arc crust formed during an earlier stage of arc evolution, most probably during the Oligocene prior to spreading of the Shikoku back-arc basin. The lack of systematic across-arc variation in the IAR rhyolites and their dry/shallow crustal melting origin combines to suggest re-melting of preexisting Oligocene middle crust by heat from the young basaltic magmatism. 相似文献
496.
Keiko?MatsumotoEmail authorView authors OrcID profile Michihiko?Nakamura 《Contributions to Mineralogy and Petrology》2017,172(10):83
Air entrainment in fragmented magmas controls the dynamics of volcanic eruptions. Pyroclast oxidation kinetics may be applied to quantify the degree of magma–air interaction. Pyrrhotite (Po) in volcanic rocks is often oxidized to form magnetite (Mt) and hematite (Hm), and its reaction mechanisms are well constrained. To test utilizing Po oxidation as a marker for magma–air interactions, we compared the occurrence of Po oxidation products from three different eruption styles during the Sakurajima 1914–1915 eruption. Pumices from the Plinian eruption include columnar-type Fe oxides (Mt with subordinate width of Hm) often accompanied by relict Po. This columnar type is also found in clastogenic lava, where it is almost completely oxidized to Hm. The effusive lava contains framboidal aggregates of subhedral to anhedral Mt crystals without Hm. The formation mechanisms of columnar and framboidal Fe oxides were estimated. The columnar type Fe oxides were formed syn-eruptively through gaseous reactions, as opposed to the melt in a magma chamber, as demonstrated by the Ti-free nature of the columnar Mt and its synchronous oxidation to Hm. By contrast, the framboidal type was formed in a melt with decreasing fS2. The calculation of Hm growth in a conductively cooling pumice clast constrains the surface temperature of pumice in the eruption column. The paragenesis and oxidation degree of Po and Fe oxides are consistent with the eruption processes in terms of magma fragmentation, air entrainment, and welding, and can, therefore, be a responsive marker for the magma–air interaction. 相似文献
497.
P. L. Tikhomirov V. Yu. Prokof’ev I. A. Kal’ko A. V. Apletalin Yu. N. Nikolaev K. Kobayashi E. Nakamura 《Geotectonics》2017,51(2):131-151
The paper presents new data on the isotopic age and chemical composition of volcanic rocks from the Tytyl’veem and Mangazeika basins of western Chukotka superposed on Mesozoides of the Verkhoyansk–Chukotka Tectonic Region. The results of SIMS U–Pb zircon dating (121.4 ± 2.8 and 118.0 ± 2.0 Ma) corroborate the Aptian age of the Tytyl’veem Formation. This age, in turn, indicates its formation after closure of the South Anyui ocean (Neocomian), but before origination of the Okhotsk–Chukotka Belt (Albian–Campanian). Post-collisional Aptian igneous rocks are widespread in the northern Verkhoyansk–Chukotka Tectonic Region; the legth of the corresponding igneous province is no less than 1400 km. In geochemical characteristics, the post-collisional volcanic rocks occurring in Western Chukotka are similar with the rocks from Andean-type igneous belts. 相似文献