PETROGRAPHY AND BULK CHEMISTRY OF MARTIAN ORTHOPYROXENITE ALH84001 - IMPLICATIONS FOR THE ORIGIN OF SECONDARY CARBONATES

New petrologic and bulk geochemical data for the SNC-related (Martian) meteorite ALH84001 suggest a relatively simple igneous history overpr inted by complex shock and hydrothermal processes. ALH84001 is an igne ous orthopyroxene cumulate containing penetrative shock deformation te xtures and a few percent secondary extraterrestrial carbonates. Rare e arth element (REE) patterns for several splits of the meteorite reveal substantial heterogeneity in REE abundances and significant fractiona tion of the REEs between crushed and uncrushed domains within the mete orite. Complex zoning in carbonates indicates nonequilibrium processes were involved in their formation, suggesting that CO2 rich fluids of variable composition infiltrated the rock while on Mars. We interpret petrographic textures to be consistent with an inorganic origin for th e carbonate involving dissolution-replacement reactions between CO2-ch arged fluids and feldspathic glass in the meteorite. Carbonate formati on clearly post dated processes that last redistributed the REE in the meteorite. (C) 1997 Elsevier Science Ltd.