Petrology of Yamato-791493, lodranite: Melting, crystallization, cooling history, and relationship to other meteorites
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概要
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Yamato-791493 is a unique meteorite which is similar to Lodran, an associated "stony-iron" meteorite, but it differs from Lodran in that it contains plagioclase. Y-791493 consists of small spinel, large euhedral to subhedral olivine and orthopyroxene, anhedral clinopyroxene, metallic iron-nickel, troilite, plagioclase, and merrillite. Textural and petrofabric studies show that Y-791493 was not formed by an accumulation of crystals on the stagnant floor of a reservoir but was formed in the presence of strong magmatic laminar flow. Y-791493 cooled continuously from high temperature (above 1200℃) to low temperature (below 500℃) without any thermal interruption. The estimated cooling rate from 800 to 600℃ is (10)^3℃/Ma. Orthopyroxene and clinopyroxene show compositional zoning formed during cooling from about 1100 to 800℃. The composition of spinel shows a strong size dependence due to differences in crystallization (Al, Cr, and Ti) and in subsolidus equilibration (Mg and Fe). Similarity in major element abundances to H-chondrites suggests that the precursor material of Y-791493 was similar to the average H-chondrite but was more enriched in Mn and P. Y-791493 was formed by partial melting of the precursor material, followed by crystallization of major portion with minor loss of partial melt under more reduced conditions than the average H-chondrite. From the bulk major chemical and mineralogical similarities, it is suggested that Y-791493 has an intimate genetical relationship to the Lodran, Acapulco, and ALH-77081 meteorites, and probably to "winonaites", Brachina, and silicates in IAB and IIICD irons. These meteorites were formed from a common precursor material in the same parental body, but by different degree of melting. Lodran is a product of the largest degree of melting followed by Y-791493 and Acapulco or ALH-77081.
- 国立極地研究所の論文
著者
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Nagahara Hiroko
Geological Institute Faculty Of Science University Of Tokyo
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Ozawa Kazuhito
Geological Institute University Of Tokyo
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