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

1996; Elsevier BV; Volume: 60; Issue: 5 Linguagem: Inglês

10.1016/0016-7037(95)00423-8

1872-9533

David W. Mittlefehldt, Marilyn M. Lindstrom, D. D. Bogard, D. H. Garrison, S. W. Field,

earthquake and tectonic studies

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.