Sulfide isotopic compositions in shergottites and ALH84001, and possible implications for life on Mars

Abstract

The shergottite and ALH84001 meteorites hold keys for understanding geologic and possibly biologic processes on Mars. Recently, it has been proposed that carbonates in ALH84001, and the Fe-sulfides they contain, are products of extraterrestrial biogenic activity (McKay et al., 1996). Here we report ion microprobe analyses of sulfides in shergottites and ALH84001. The sulfur isotope ratios of igneous pyrrhotites in shergottites (mean δ34SCDT: Shergotty = −0.4%, Zagami = +2.7%, EETA79001A = −1.9%, EETA79001B = −1.7%, LEW88516 = −1.9%, QUE94201 = +0.8%) are similar to those of terrestrial ocean-floor basalts, suggesting that the sulfur isotopic composition of the Martian mantle may be similar to that of the mantle of the Earth. The sulfur isotopic systematics of ALH84001 sulfides are distinct from the shergottites. Measured sulfur isotope ratios of eight pyrite grains (δ34SCDT = +2.0 to +7.3%) in crushed zones confirm previously reported analyses of isotopically heavy sulfides (Shearer et al., 1996) and are indistinguishable from an Fe-sulfide zone within a carbonate globule (δ34SCDT = +6.0%). Analyses of synthesized, fine-grained mixtures of sulfide, carbonate, and magnetite indicate that the measured sulfur isotope ratio is independent of the presence of carbonate and magnetite in the sputtered volume, confirming the accuracy of the analysis of the fine-grained sulfide in the carbonate globule. Terrestrial biogenic sulfate reduction typically results in light isotopic enrichments. The similarity of δ34S values of the sulfides in ALH84001 imply that the Fe-sulfide zones within ALH84001 carbonates are probably not the result of bacterial reduction of sulfate.