Petrogenesis of the EET 92023 achondrite and implications for early impact events

1,2A. Yamaguchi, 3N. Shirai, 3C. Okamoto, 3M. Ebihara
Meteoritics & Planetary Science (in Press) link to Article [DOI: 10.1111/maps.12821]
1National Institute of Polar Research, Tachikawa, Tokyo, Japan
2Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, Japan
1Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo, Japan
Published by arrangement with John Wiley & Sons

We report petrology and geochemistry of an achondrite EET 92023 and compare it with normal and anomalous eucrites. EET 92023 is an unbrecciated achondrite and shows a granular texture mainly composed of low-Ca pyroxene and plagioclase, petrologically similar to normal cumulate eucrites such as Moore County. However, this rock contains a significant amount of kamacite and taenite not common in unbrecciated, crystalline eucrites. EET 92023 contains a significant amount of platinum group elements (PGEs) (ca. 10% of CI), several orders of magnitude higher than those of monomict eucrites. We suggest that the metallic phases carrying PGEs were incorporated by a projectile during or before igneous crystallization and thermal metamorphism. The projectile was likely to be an iron meteorite rather than chondritic materials, as indicated by the lack of olivine and the presence of free silica. Therefore, the oxygen isotopic signature is indigenous, rather than due to contamination of the projectile material with different oxygen isotopic compositions. A significant thermal event involving partial melting and metamorphism after the impact event indicates that EET 92023 records early impact events which took place shortly after the crust formation on a differentiated protoplanet when the crust was still hot.

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