Mineralogy of the Asuka 87 and 88 eucrites and crustal evolution of the HED parent body

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Mineralogical study of three apparently crystalline eucrites, Asuka (A)-87272,A-881388 and A-881394 revealed that their textures are not primary crystallization products from a magma. A-87272 is a monomict breccia, but the finegrained matrix is recrystallized to a granulitic texture with fine, rounded pyroxene crystals set in a plagioclase matrix. Large fragments of pyroxene are inverted to orthopyroxene with coarse exsolution lamellae on (001) and fine ones on (100). A-881388 contains a large, rounded opaque grain with a tail, and with an ilmenite-chromite-troilite-metal assemblage in fine-grained granulitic silicates, suggesting recrystallization. A-881394 is coarser grained than A-881388 and contains more magnesian pyroxene as in cumulate eucrites, but the plagioclase composition is extremely calcic (An 98) and the grains are composed with a few rounded grains. The A-881394 chromite, showing a pokilitic texture with rounded plagioclase and minor pyroxene, suggests a metamorphic texture. Our present interpretation for a common formation process among these eucrites is that despite their crystalline texture, they might have experienced extensive metamorphism after the initial crystallization in the early history of the crustal evolution.
国立極地研究所の論文