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51.
Seiichi Miura Kiyoshi Suyehiro Masanao Shinohara Narumi Takahashi Eiichiro Araki Asahiko Taira 《Tectonophysics》2004,389(3-4):191
A seismic refraction–reflection experiment using ocean bottom seismometers and a tuned airgun array was conducted around the Solomon Island Arc to investigate the fate of an oceanic plateau adjacent to a subduction zone. Here, the Ontong Java Plateau is converging from north with the Solomon Island Arc as part of the Pacific Plate. According to our two-dimensional P-wave velocity structure modeling, the thickness of the Ontong Java Plateau is about 33 km including a thick (15 km) high-velocity layer (7.2 km/s). The thick crust of the Ontong Java Plateau still persists below the Malaita Accreted Province. We interpreted that the shallow part of the Ontong Java Plateau is accreted in front of the Solomon Island Arc as the Malaita Accreted Province and the North Solomon Trench are not a subduction zone but a deformation front of accreted materials. The subduction of the India–Australia Plate from the south at the San Cristobal Trench is confirmed to a depth of about 20 km below sea level. Seismicity around our survey area shows shallow (about 50 km) hypocenters from the San Cristobal Trench and deep (about 200 km) hypocenters from the other side of the Solomon Island Arc. No earthquakes occurred around the North Solomon Trench. The deep seismicity and our velocity model suggest that the lower part of the Ontong Java Plateau is subducting. After the oceanic plateau closes in on the arc, the upper part of the oceanic plateau is accreted with the arc and the lower part is subducted below the arc. The estimation of crustal bulk composition from the velocity model indicates that the upper portion and the total of the Solomon Island Arc are SiO2 58% and 53%, respectively, which is almost same as that of the Izu–Bonin Arc. This means that the Solomon Island Arc can be a contributor to growing continental crust. The bulk composition of the Ontong Java Plateau is SiO2 49–50%, which is meaningfully lower than those of continents. The accreted province in front of the arc is growing with the convergence of the two plates, and this accretion of the upper part of the oceanic plateau may be another process of crustal growth, although the proportion of such contribution is not clear. 相似文献
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Masateru Ishiguro Takahiro Hiroi David J. Tholen Sho Sasaki Yuji Ueda Tokuhiro Nimura Masanao Abe Beth E. Clark Aya Yamamoto Fumi Yoshida Ryosuke Nakamura Naru Hirata Hideaki Miyamoto Yasuhiro Yokota Tatsuaki Hashimoto Takashi Kubota Akiko M. Nakamura Robert W. Gaskell Jun Saito 《Meteoritics & planetary science》2007,42(10):1791-1800
Abstract— We obtained color images of near‐Earth asteroid 25143 Itokawa by the Hayabusa multiband imaging camera to characterize the regional color properties. Images were obtained for the whole disk from the gate position (GP) and home position (HP) at a spatial resolution of 0.8–3.7 m/pixel. Whole‐disk spectra are adjusted to the telescopic data obtained by the University of Hawai'i's 88‐inch telescope using the Eight Color Asteroid Survey (ECAS) system. The disk‐resolved measurements show large variations in the three visible channels. We present a map of an index related to the degree of space weathering, which has been newly developed based on laboratory measurements. We find large variations in the degree of space weathering on Itokawa. Fresh materials are observed in regions of steep slopes and craters, whereas mature materials are ubiquitously distributed. This result suggests that pristine ordinary chondrite‐like materials have been exposed through weathered layers by excavation. By also examining close‐up images obtained during touchdown rehearsal, we find that most rocks in Itokawa's rough terrains are weathered. Instead of a regolith blanket, the surface of this small asteroid is covered with weathered rocks and gravels. 相似文献
54.
Genji Saito Kozo Uto Kohei Kazahaya Hiroshi Shinohara Yoshihisa Kawanabe Hisao Satoh 《Bulletin of Volcanology》2005,67(3):268-280
Among the series of eruptions at Miyakejima volcano in 2000, the largest summit explosion occurred on 18 August 2000. During this explosion, vesiculated bombs and lapilli having cauliflower-like shapes were ejected as essential products. Petrological observation and chemical analyses of the essential ejecta and melt inclusions were carried out in order to investigate magma ascent and eruption processes. SEM images indicate that the essential bombs and lapilli have similar textures, which have many tiny bubbles, crystal-rich and glass-poor groundmass and microphenocrysts of plagioclase, augite and olivine. Black ash particles, which compose 40% of the air-fall ash from the explosion, also have similar textures to the essential bombs. Whole-rock analyses show that the chemical composition of all essential ejecta is basaltic (SiO2=51–52 wt%). Chemical analyses of melt inclusions in plagioclase and olivine phenocrysts indicate that melt in the magma had 0.9–1.9 wt% H2O, <0.011 wt% CO2, 0.04–0.17 wt% S and 0.06–0.1 wt% Cl. The variation in volatile content suggests degassing of the magma during ascent up to a depth of about 1 km. The ratio of H2O and S content of melt inclusions is similar to that of volcanic gas, which has been intensely and continuously emitted from the summit since the end of August 2000, indicating that the 18 August magma is the source of the gas emission. Based on the volatile content of the melt inclusions and the volcanic gas composition, the initial bulk volatile content of the magma was estimated to be 1.6–1.9 wt% H2O, 0.08–0.1 wt% CO2, 0.11–0.17 wt% S and 0.06–0.07 wt% Cl. The basaltic magma ascended from a deeper chamber (10 km) due to decrease in magma density caused by volatile exsolution with pressure decrease. The highly vesiculated magma, which had at least 30 vol% bubbles, may have come into contact with ground water at sea level causing the large explosion of 18 August 2000.Editorial responsibility: S. Nakada, T. DuittAn erratum to this article can be found at 相似文献
55.
Mitrofanov I. G. Litvak M. L. Kozyrev A. S. Sanin A. B. Tret'yakov V. I. Grin'kov V. Yu. Boynton W. V. Shinohara C. Hamara D. Saunders R. S. 《Solar System Research》2004,38(4):253-257
We present the results of 20 months of observations of Mars by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. We show that there are two extended subpolar regions with a soil water content of several tens of percent in the northern and southern hemispheres of Mars. The southern subpolar region is well described by a two-layer model, according to which a soil with a water content of up to 55% by mass lies under a relatively dry soil with a water mass fraction of 2% and a thickness of 15–20 g/cm2. The distribution of water in Martian regolith northern subpolar region is in good agreement with the homogeneous model and does not require invoking the more complex two-layer soil model. The water-ice content in the subsurface layer of the northern subpolar region reaches 53 % by mass. We show that there are two regions with a relatively high water content near the Martian equator. These are Arabia Terra and the Medusae Fossae formation region southwest of Olympus Mons. In these regions, a lower layer with 9–10% of water by mass may underlie the upper layer of relatively dry material 30 g/cm2 in thickness. The moistest spot near the equator is at about 30° E and 10° N. Its lower-layer soil may contain more than 16% of water by mass. 相似文献
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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
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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