SULFUR ISOTOPIC SYSTEMATICS IN ALTERATION ASSEMBLAGES IN MARTIAN METEORITE ALLAN-HILLS-84001

ALH84001 is a coarse-grained, elastic orthopyroxenite meteorite relate d to the SNC meteorite group (shergottites, nakhlites, Chassigny). Sup erimposed upon the orthopyroxene-dominant igneous mineral assemblage i s hydrothermal signature. This hydrothermal overprint consists of carb onate assemblages occurring in spheroidal aggregates and fine-grained carbonate-sulfide vug-filling. The sulfide in this assemblage has been identified as pyrite, an unusual sulfide in meteorites. Previously, B urgess et al. (1989) reported a bulk delta(34)S for a SNC group meteor ite (Shergotty) of -0.5 +/- 1.5 parts per thousand. Here, we report th e first martian delta(34)S values from individual sulfide grains. Usin g newly developed ion microprobe techniques, we were able to determine delta(34)S of the pyrite in ALH84001 with a 1 alpha precision of bett er than +/-0.5 parts per thousand. The delta(34)S values for the pyrit e range from +4.8 to +7.8 parts per thousand. Within the stated uncert ainties, the pyrite from ALH84001 exhibits a real variability in delta (34)S in this alteration assemblage. In addition, these sulfides are d emonstrably enriched in S-34 relative to Canon Diablo troilite and sul fides from most other meteorites. This signature implies that the plan etary body represented by ALH84001 experienced processes capable of fr actionating sulphur isotopes and that hydrothermal conditions changed during pyrite precipitation (T, pH, fluid composition, etc.). The frac tionated signature of the sulphur in the pyrite is most likely attribu ted to either conditions of pyrite precipitation (low temperature, red uced (low f(o2)) and moderately alkaline (pH > 8) environment) or enri chment of fluids in S-34 by surface processes (weathering or impact pr ocesses) prior to precipitation. These new data are not consistent wit h the pyrite recording either biogenic activity or atmospheric fractio nation of sulphur through nonthermal escape mechanisms or oxidation pr ocesses. This study also demonstrates the usefulness of ion microprobe measurements of sulphur isotopes in constraining conditions on other planetary bodies.