The Lu-Hf isotope geochemistry of shergottites and the evolution of the Martian mantle-crust system

We report the first Lu-Hf isotopic data from SNC (Martian) meteorites with high-precision analyses of five shergottites by plasma sector mass spectrometry. Hf isotopic compositions indicate the presence of both geochemically enriched and depleted components, reflecting early segregation of Martian crust and mantle. Comparison with Sm-Nd and U-Th-Pb isotopic systems suggests that the enriched component reflects the extraction-of very small-degree melts at depths in excess of 200 km in the presence of garnet, while the depleted component could correspond to large-degree melts from mantle already depleted by the extraction of enriched Liquids. The general lack of correlation between major element composition and isotopic properties of SNC meteorites argues against the segregation of the lithophile elements into Earth-like continents or Moon-like highlands on Mars, although early magma ocean fractionation may explain many of the primary isotopic properties of SNC meteorites. Recent (0.18-0.33 Ga old) fractionation of Lu/Hf and Sm/Nd in shergottites is inconsistent with purely magmatic partitioning of these elements and appears more Likely to be a product of metasomatic processes involving the circulation of P2O5-rich fluids, triggered either by distant magmatic activity or, more probably, by impacts. (C) 1999 Elsevier Science B.V. All rights reserved.