Acapulco- and Lodran-like achondrites: Petrology, geochemistry, chronology, and origin

Abstract

We have performed petrologic and geochemical studies of five primitive achondrites: ALHA81187 and ALHA81261 (Acapulco-like), EET 84302 (transitional, but Acapulco-like), and LEW 88280 and MAC 88177 (Lodran-like). We have also performed 39Ar- 40Ar chronology on ALHA81187, ALHA81261, and EET 84302. LEW 88280 and MAC 88177 contain more ferroan olivines, orthopyroxenes, clinopyroxenes, and chromites than do the Acapulco-like achondrites. Plagioclase is present in only ALHA81187, ALHA81261, and EET 84302. Its composition does not track that of the mafic silicates; EET 84302, with the most calcic plagioclase, contains olivine and pyroxene with an mg# intermediate between ALHA81187 and ALHA81261. Similarly, spinels in EET 84302 have the same mg# as those in ALHA81187, while pyroxenes in the former are more ferroan than in the latter. Acapulco-like achondrites have Sm/Sc ratios between 0.8–1.5 times H chondrites, and Na/Sc ratios are between 0.81.0 times H chondrites, indicating that silicate partial melts have not been lost from these rocks. Siderophile and chalcophile elements are fractionated and show a trend of increasing Ir/Ni ratios with decreasing Se/Co ratios. This variation is consistent with fractionation by partial melting in the Fe-Ni-S system. EET 84302 is very depleted in Se and troilite is a accessory phase, indicating essentially total loss of the low temperature Fe-Ni-S melt. In contrast, the Lodran-like achondrites are depleted in the highly incompatible lithophile elements relative to more compatible elements: Sm/Sc ratios are 0.2−0.7 times and Na/Sc ratios are 0.04–0.18 times H chondrites. These data are consistent with a model for formation of the Lodran-like achondrites as partial melting residues. However, the low Ir/Ni and high Se/Co ratios in two Lodran-like achondrites suggest the metal + troilite in these rocks is dominated by the low melting fraction in the Fe-Ni-S system. This metal + troilite was added to the silicates after partial melting depleted them in a basaltic fraction. The Ar-Ar release spectra yield good plateau ages of 4.507 ± 0.024 Ga (ALHA81187), 4.511 ± 0.007 Ga (ALHA81261), and 4.519 ± 0.017 Ga (EET 84302), in good agreement with previously determined Ar-Ar ages for other Acapulco-like achondrites. Together, the Ar-Ar ages demonstrate that metamorphism on the Acapulco-Lodran parent body occurred 4.51 ± 0.02 Ga ago, roughly 50 Ma after its formation. The geochemical and petrologic evidence on the Acapulco- and Lodran-like achondrites suggest that heating on the parent body was localized and heterogeneous in space and time. The parent body was probably not subjected to parent body-wide magmatism, in contrast to the case for 4 Vesta and the HED meteorites.