Sulfur isotopic composition of Fe-Ni sulfide grains in CI and CM carbonaceous chondrites

In situ secondary ion mass spectrometry analyses of 32S, 33S, and 34S in iron-nickel sulfide grains in two CI1 chondrites and six CM chondrites were performed. The results show a wider range of both enrichment and depletion in delta 34S relative to troilite from the Canyon Diablo meteorite (CDT) than has been observed in previous studies. All data points lie within error of a single mass dependent fractionation line. Sulfides from CI1 chondrites show delta 34S(CDT) from -0.7 to 6.8 parts per thousand, while sulfide grains in the CM1 chondrite are generally depleted in heavy sulfur relative to CDT (delta 34S from -2.9 to 1.8 parts per thousand). CM2 chondrites contain sulfide grains that show enrichment and depletion in 34S (delta 34S(CDT) from -7.0 to 6.8 parts per thousand). Sulfates forming from sulfide grains during aqueous alteration on the chondrite parent body are suggested to concentrate light sulfur, leaving the remaining sulfide grains enriched in the heavy isotopes of sulfur. The average degree of enrichment in 34S in CM chondrite sulfides is broadly consistent with previously suggested alteration sequences.