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 spectrom etry. Hf isotopic compositions indicate the presence of both geochemically enriched and depleted components, reflecting early segregation of Martian c rust and mantle. Comparison with Sm-Nd and U-Th-Pb isotopic systems suggest s 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 de pleted component could correspond to large-degree melts from mantle already depleted by the extraction of enriched Liquids. The general lack of correl ation between major element composition and isotopic properties of SNC mete orites 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 m eteorites. Recent (0.18-0.33 Ga old) fractionation of Lu/Hf and Sm/Nd in sh ergottites is inconsistent with purely magmatic partitioning of these eleme nts and appears more Likely to be a product of metasomatic processes involv ing the circulation of P2O5-rich fluids, triggered either by distant magmat ic activity or, more probably, by impacts. (C) 1999 Elsevier Science B.V. A ll rights reserved.