Isotopic composition of carbonates in the SNC meteorites, Allan Hills 84001 and Zagami

Abstract

We have measured the 13C/12C and 14C/12C ratios in CO2 released by acid etching of the meteorites Zagami and Allan Hills 84001. We use the 14C as a label to identify extraterrestrial carbonate phases, as they will have a low 14C/12C ratio (<- 4% modern terrestrial ratio) compared to recent terrestrial material. The new studies on Allan Hills 84001 confirm previous conclusions that the Fe, Mg-rich carbonate grains in this meteorite contain carbon with δ13C as high as +45‰. In contrast, the carbon released from Zagami is depleted in 13C with δ13C as low as -20‰. We conclude that the isotopic composition of the carbon as carbonate released from acid etching of Zagami is different from the carbonates observed in both Allan Hills 84001 and Nakhla. With the assumption that all of these meteorites sample the surface of Mars, we propose that the Zagami carbonate samples a different carbon reservoir on this planet, such as a magmatic source. With this interpretation, the high δ13C values of carbonate observed in Allan Hills 84001 and Nakhla can be ascribed to a fractionated source compared with the originally light carbon. A likely origin for this 13C-enriched component is an isotopically heavy Martian atmosphere; however, given the possibility of biological activity involving Allan Hills carbonates, we cannot exclude this as a source of the isotopic fractionation. Copyright 1997 by the American Geophysical Union.