A mineralogical study of the proposed paired eucrites Y-792769 and Y-793164 with reference to cratering events on their parent body

Abstract

A comparison of the mineralogy of Yamato (Y-) 792769 and Y-793164 has been performed to confirm the proposed pairing of these two specimens and to correlate their common shock-sintered textures with the resetting of their isotopic ages. The matrices of the two meteorites share this common texture as described for Y-792769,but their clast types are different. Y-792769 contains pyroxene fragments with Mg-rich compositions, which are not present in the matrix. A large basaltic clast present in Y-793164 includes acicular plagioclase crystals replacing the pyroxene grains. Pyroxene clasts in the matrix of the two meteorites showed a small compositional range between 30 and 45 in Mg×100/(Mg+Fe) mol% and are similar to the more comminuted matrix pyroxenes, but lithic clast pyroxenes are uniform in bulk chemistry within a single clast type. The event which produced these monomict eucrites with minor compositional differences may have taken place earlier than the reset ^Ar-^Ar age of 3.99 Ga previously found for Y-792769. Some pyroxenes are more Fe-rich than those of common monomict eucrites such as Juvinas. The common pyroxene components and the shock-sintered textures support the proposed pairing of Y-792769 and Y-793164. Rb and Sr abundances and isotopic data do not strongly support the proposed pairing of the meteorites, but could be consistent with observed compositional heterogeneities within and between them. Sm and Nd abundances and isotopic data are similar for matrix samples of the meteorites and are consistent with their pairing