Modified sulfur isotopic compositions of sulfides in the nakhlites and Chassigny

Variable sulfur isotopic ratios of sulfide minerals in the nakhlites and Ch assigny have been measured by ion microprobe. The ranges and means of delta (34)S values of pyrrhotite and pyrite in nakhlites become more negative in the sequence Nakhla (delta(34)S: -1.7 to +4.9 parts per thousand; mean delt a(34)S = +1.5 +/- 2.0 parts per thousand) > Governador Valadares (delta(34) S: -2.4 to +3.8 parts per thousand; mean delta(34)S = +0.7 +/- 2.4 parts pe r thousand) > Lafayette (-6.1 to +0.1 parts per thousand; mean delta(34)S = -3.2 +/- 2.1 parts per thousand). This is also the sequence of increasing degrees of subsolidus re-equilibration, suggesting that S-32 enrichment may be related to the subsolidus thermal history. A chalcopyrite vein cross cu tting a pyrrhotite in Nakhla, coupled with chalcopyrite having slightly lig hter delta(34)S values, suggests that subsolidus fluids may have become iso topically lighter (with respect to sulfur) in Nakhla with time. Pyrite has replaced pyrrhotite in Lafayette, suggesting that fO(2) and/or fS(2) increa sed after pyrrhotite crystallization. A model involving subsolidus hydrothe rmal modification of igneous sulfide minerals (with delta(34)S similar to 0 parts per thousand) due to late-stage oxidation of fluids provides a reaso nable explanation for the sulfur isotopic systematics of the nakhlites and Chassigny. Sulfur isotopic alteration is believed to have occurred during t he waning stages of nakhlite magmatism, rather than during a much later low -temperature (<100 degrees C) iddingsite formation event, based on the inef fectiveness of abiogenic sulfur isotopic fractionation below 200 degrees C. Variable mixing of two isotopically different fluids also could have produ ced the observed fractionations, although an isotopically light reservoir o f sulfur is problematic. Other possible mechanisms evaluated to explain the sulfur isotopic values of the sulfide minerals include martian mantle hete rogeneity, possible influence of martian biological processes, and magmatic degassing of SO2. Copyright (C) 2000 Elsevier Science Ltd.