SULFIDE ISOTOPIC COMPOSITIONS IN SHERGOTTITES AND ALH84001, AND POSSIBLE IMPLICATIONS FOR LIFE ON MARS

The shergottite and ALH84001 meteorites hold keys for understanding ge ologic 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., 19 96). Here we report ion microprobe analyses of sulfides in shergottite s and ALH84001. The sulfur isotope ratios of igneous pyrrhotites in sh ergottites (mean delta(34)S(CDT):Shergotty = -0.4 parts per thousand, Zagami = +2.7 parts per thousand, EETA79001A = -1.9 parts per thousand , EETA79001B = -1.7 parts per thousand, LEW88516 = -1.9 parts per thou sand, QUE94201 = +0.8 parts per thousand) are similar to those of terr estrial ocean-floor basalts, suggesting that the sulfur isotopic compo sition of the Martian mantle may be similar to that of the mantle of t he Earth. The sulfur isotopic systematics of ALH84001 sulfides are dis tinct from the shergottites. Measured sulfur isotope ratios of eight p yrite grains (delta(34)S(CDT) = +2.0 to +7.3 parts per thousand) in cr ushed 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 (delta(34)S(CDT) = +6.0 parts per thousand). Analyses of synthesized, fine-grained mixtures of sulfi de, carbonate, and magnetite indicate that the measured sulfur isotope ratio is independent of the presence of carbonate and magnetite in th e sputtered volume, confirming the accuracy of the analysis of the fin e-grained sulfide in the carbonate globule. Terrestrial biogenic sulfa te reduction typically results in light isotopic enrichments. The simi larity of delta(34)S values of the sulfides in ALH84001 imply that the Fe-sulfide zones within ALH84001 carbonates are probably not the resu lt of bacterial reduction of sulfate. Copyright (C) 1997 Elsevier Scie nce Ltd.