Terrestrial Alteration of CM2 Chondritic Carbonates in a Suite of Paired Antarctic Meteorites.

Abstract

The &#948;18O, &#948;17O, &#948;13C, and &#948;14C compositions of carbonate grains were measured from paired Antarctic CM2 chondrites (EET96006, EET96016, EET96017, and EET96019). Oxygen isotopic compositions reveal both terrestrial and extraterrestrial carbonate sources. Bulk &#948;13C and &#948;14C measurements suggest at least two terrestrial carbonate components: 1) carbonate derived from equilibration of atmospheric CO2 with silicates during weathering reactions, and 2) carbonate derived from a second undefined source. Oxygen and carbon isotope data suggest that silicate weathering reactions drove oxygen isotopic composition of Antarctic water away from the terrestrial fractionation line (TFL) (D17O < 3.6). Further oxygen isotopic compositional constraints were placed by Secondary Ionization Mass Spectrometry (SIMS) of in-situ carbonates. We suggest three sources for meteoritic carbonate: 1) primary calcite formed in the presence of evolving planetesimal water and 2) secondary calcite derived from alkalinity laden terrestrial water and 3) alkalinity formed as the result of weathering of meteoritic silicates.