ACAPULCO-LIKE AND LODRAN-LIKE ACHONDRITES - PETROLOGY, GEOCHEMISTRY, CHRONOLOGY, AND ORIGIN

We have performed petrologic and geochemical studies of five primitive achondrites: ALHA81187 and ALHA81261 (Acapulco-like),EET 84302 (trans itional, but Acapulco-like), and LEW 88280 and MAC 88177 (Lodran-like) . We have also performed Ar-39-Ar-40 chronology on ALHA81187, ALHA8126 1, and EET 84302. LEW 88280 and MAC 88177 contain more ferroan olivine s, 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 silic ates; EET 84302, with the most calcic plagioclase, contains olivine an d pyroxene with an mg# intermediate between ALHA81187 and ALHA81261. S imilarly, spinels in EET 84302 have the same mg# as those in ALHA81187 , while pyroxenes in the former are more ferroan than in the latter. A capulco-like achondrites have Sm/Sc ratios between 0.8-1.5 times H cho ndrites, and Na/Sc ratios are between similar to 0.8-1.0 times H chond rites, 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 temper ature Fe-Ni-S melt. In contrast, the Lodran-like achondrites are deple ted in the highly incompatible lithophile elements relative to more co mpatible elements: Sm/Sc ratios are similar to 0.2-0.7 times and Na/Sc ratios are 0.04-0.18 times H chondrites. These data are consistent wi th a model for formation of the Lodran-like achondrites as partial mel ting residues. However, the low Ir/Ni and high Se/Co ratios in two Lod ran-like achondrites suggest the metal + troilite in these rocks is do minated by the low melting fraction in the Fe-Ni-S system. This metal + troilite was added to the silicates after partial melting depleted t hem in a basaltic fraction. The Ar-Ar release spectra yield good plate au ages of 4.507 +/- 0.024 Ga (ALHA81187), 4.511 +/- 0.007 Ga (ALHA812 61), and 4.519 +/- 0.017 Ga (EET 84302), in good agreement with previo usly determined Ar-Ar ages for other Acapulco-like achondrites. Togeth er, the Ar-Ar ages demonstrate that metamorphism on the Acapulco-Lodra n parent body occurred 4.51 +/- 0.02 Ga ago, roughly 50 Ma after its f ormation. The geochemical and petrologic evidence on the Acapulco- and Lodran-like achondrites suggest that heating on the parent body was l ocalized and heterogeneous in space and time. The parent body was prob ably not subjected to parent body-wide magmatism, in contrast to the c ase for 4 Vesta and the HED meteorites.