Origin of olivine megacrysts and the groundmass crystallization of the Dar al Gani 476 shergottite
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概要
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The DaG 476 martian meteorite shows a porphyritic texture with megacrysts of olivine and orthopyroxene set in a groundmass of pyroxene and maskelynite. Previous studies on major and trace elements and isotopes of this meteorite implied a relationship to other martian meteorites. However, the origin of the olivine and orthopyroxene megacrysts is still under dispute, and therefore the formation of DaG 476 is unclear, although this sample is one of the most important martian meteorites. We performed crystallization experiments, a MELTS calculation and a cooling rate calculation to investigate the formation of DaG 476. The experimental and calculated results suggest that the parent melt of the DaG groundmass was more Fe- and Al-rich than the actual groundmass bulk composition, suggesting that the groundmass of DaG 476 contains a mafic cumulus component, alternatively fractionated liquid has escaped at the last crystallization stage. We evaluated three models for the origin of the olivine megacrysts (1) phenocryst origin, (2) xenocryst origin: homogeneous olivine was modified by exchange with the host magma and diffusion, and (3) xenocryst origin: chemical zoning of olivine was produced by the fractional crystallization. The mineralogy of DaG 476 and calculation results showed that all models were theoretically possible. However, models (1) and (2) need complex processes to produce observed natures of DaG 476. Hence, model (3) seems the most plausible, although this model also leaves some open questions. The fragment-like texture of olivine and the results of cooling rate calculation suggest that the formation of the DaG shergottites occurred in a rapid cooling condition in any of the formation models (1-3). Therefore, DaG seems to have crystallized near the martian surface.
- 国立極地研究所の論文
著者
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MIKOUCHI TAKASHI
Department of Earth and Planetary Science, The University of Tokyo
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Miyamoto Masamichi
Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo
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Monkawa Akira
Department Of Earth And Planetary Science Graduate School Of Science University Of Tokyo
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Koizumi Eisuke
Department Of Earth And Planetary Science Graduate School Of Science University Of Tokyo
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Koizumi Eisuke
Department Of Computer And Mathematical Sciences The Graduate School Of Information Sciences Tohoku
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Mikouchi Takashi
Department Of Earth And Planetary Science University Of Tokyo
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Mikouchi Takashi
Department Of Earth And Planetary Science Graduate School Of Science The University Of Tokyo
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Miyamoto Masamichi
Department Of Earth And Planetary Science University Of Tokyo
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Miyamoto Masamichi
Department Of Earth And Planetary Science Graduate School Of Science University Of Tokyo
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