CARBONATES IN FRACTURES OF MARTIAN METEORITE ALLAN-HILLS-84001 - PETROLOGIC EVIDENCE FOR IMPACT ORIGIN

Carbonates in Martian meteorite Allan Hills 84001 occur as grains on p yroxene grain boundaries, in crushed zones, and as disks, veins, and i rregularly shaped,grains in healed pyroxene fractures. Some carbonate disks have tapered Mg-rich edges and are accompanied by smaller, thinn er and relatively homogeneous, magnesite microdisks. Except for the mi crodisks, all types of carbonate grains show the same unique chemical zoning pattern on MgCO3-FeCO3-CaCO3 plots. This chemical characteristi c and the close spatial association of diverse carbonate types show th at all carbonates formed by a similar process. The heterogeneous distr ibution of carbonates in fractures, tapered shapes of some disks, and the localized occurrence of Mg-rich microdisks appear to be incompatib le with growth from an externally derived CO2-rich fluid that changed in composition over time. These features suggest instead that the frac tures were closed as carbonates grew from an internally derived fluid and that the microdisks formed from a residual Mg-rich fluid that was squeezed along fractures. Carbonate in pyroxene fractures is most abun dant near grains of plagioclase glass that are located on pyroxene gra in boundaries and commonly contain major or minor amounts of carbonate . We infer that carbonates in fractures formed from grain boundary car bonates associated with plagioclase that were melted by impact and dis persed into the surrounding fractured pyroxene. Carbonates in fracture s, which include those studied by McKay et al. (1996), could not have formed at low temperatures and preserved mineralogical evidence for Ma rtian organisms.