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231.
Shogo Aoki Chiho Kabashima Yasuhiro Kato Takafumi Hirata Tsuyoshi Komiya 《地学前缘(英文版)》2018,9(4):1049-1072
Banded Iron Formations(BIFs) are chemical sediments, ubiquitously distributed in the Precambrian supracrustal belts; thus their trace element compositions are helpful for deciphering geochemical evolution on the Earth through time. However, it is necessary to elucidate factors controlling the whole-rock compositions in order to decode the ancient seawater compositions because their compositions are highly variable. We analyzed major and trace element contents of the BIFs in the 3.8-3.7 Ga Isua supracrustal belt(ISB), southern West Greenland. The BIFs are petrographically classified into four types:Black-,Gray-, Green-and White-types, respectively. The Green-type BIFs contain more amphiboles, and are significantly enriched in Co, Ni, Cu, Zn, Y, heavy rare earth element(HREE) and U contents. However,their bulk compositions are not suitable for estimate of seawater composition because the enrichment was caused by secondary mobility of metamorphic Mg, Ca and Si-rich fluid, involvement of carbonate minerals and silicate minerals of olivine and pyroxene and/or later silicification or contamination of volcanic and clastic materials. The White-type BIFs are predominant in quartz, and have lower transition element and REE contents. The Gray-type BIFs contain both quartz and magnetite. The Black-type BIFs are dominated by magnetite, and contain moderate to high transition element and REE contents. But,positive correlations of V, Ni, Zn and U contents with Zr contents suggest that involvement of detrital,volcanic and exhalative materials influences on their contents. The evidence for significant influence of the materials on the transition element contents such as Ni in the BIFs indicates the transition element contents in the Archean ocean were much lower than previously estimated. We reconstructed secular variations of V,Co, Zn and U contents of BIFs through time, which show Ni and Co contents decreased whereas V, Zn and U contents increased through time. Especially, the Ni and Co contents drastically decreased in the Mesoarchean rather than around the Great Oxidation Event. On the other hand, the V,Zn and U contents progressively increased from the Mesoarchean to the Proterozoic. Stratigraphical trends of the BIFs show increase in Y/Ho ratios and decrease in positive Eu anomaly upwards, respectively. The stratigraphic changes indicate that a ratio of hydrothermal fluid to seawater component gradually decrease through the deposition, and support the Eoarchean plate tectonics, analogous to the their stratigraphic variations of seafloor metalliferous sediments at present and in the Mesoarchean. 相似文献
232.
Numerical simulations of complex earthquake cycles are conducted using a two-degree-of-freedom spring-block model with a rate- and state-friction law, which has been supported by laboratory experiments. The model consisted of two blocks coupled to each other and connected by elastic springs to a constant-velocity, moving driver. By widely and systematically varying the model parameters, various slip patterns were obtained, including the periodic recurrence of seismic and aseismic slip events, and several types of chaotic behaviour. The transition in the slip pattern from periodic to chaotic is examined using bifurcation diagrams. The model system exhibits typical period-doubling sequences for some parameter ranges, and attains chaotic motion. Simple relationships are found in iteration maps of the recurrence intervals of simulated earthquakes, suggesting that the simulated slip behaviour is deterministic chaos. Time evolutions of the cumulative slip distance in chaotic slip patterns are well approximated by a time-predictable model. In some cases, both seismic and aseismic slip events occur at a block, and aseismic slip events complicate the earthquake recurrence patterns. 相似文献
233.
E. P. Pavlenko T. Kato O. I. Antonyuk A. Imada R. Ishioka H. Maehara 《Astrophysics》2011,54(4):483-495
Results are presented from photometric studies of the dwarf nova V1108 Her conducted at the primary focus of the 2.6-m G.
A. Shajn Telescope at the Crimean Astrophysical Observatory during June-July 2008, 4 years after the 2004 outburst. An orbital
period of 0.05672(4) days is found for the system. An analysis of observations made earlier during the 2004 outburst reveals
an orbital signal which indicates that V1108 Her is an eclipsing system. The mass ratio of the secondary component to the
white dwarf is estimated to be q = 0.068, which makes it highly likely that the secondary component of this system is a brown dwarf. The orbital light curves
indicate a complex structure for the accretion disk whose radius has reached a 2:1 resonance. An explanation is suggested
for a quasi-periodic modulation in the brightness at 1/4 of the orbital period observed in V1108 Her and other WZ Sge systems. 相似文献