Antarctic howardites and their primitive crust

概要

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Lithic clasts and mineral fragments in Yamato-79 howardites, Y-790727,Y-791208 and Y-791492 have been investigated by optical microscopy, electron microprobe and single crystal X-ray diffraction. These howardites are chemically intermediate between ordinary diogenites and eucrites, and distinct from Y-7308. Some lithic clasts are also intermediate in mineralogy between endmember diogenites and eucrites, including a cumulate eucrite clast with a partly inverted pigeonite, an ordinary eucrite-like gabbroic clast intermediate in chemical composition between cumulate eucrites and ordinary eucrities, and an augite rich clast with a silica mineral. A partial melting model and fractional crystallization models have been compared to account for these new howardites, and models involving two-stage process are proposed. These howardites may have been products of excavation by an impact or impacts shallower than that for Y-7308 but deeper than those for polymict eucrites.
国立極地研究所の論文