Sulfur isotopic compositions of individual sulfides in Martian meteorites ALH84001 and Nakhla: implications for crust-regolith exchange on Mars

Atmospheric chemical reactions on Mars have been invoked to explain non-mas s-dependent Delta S-33 anomalies (Delta S-33 = delta S-33-0.516 delta S-34) reported from bulk analyses of Martian meteorites. To explore this signatu re in detail, a new ion microprobe multi-collector technique was developed to obtain precise in situ S-32, S-33 and S-34 measurements of individual su lfide grains from Martian meteorites ALH84001 ( > 4.0 Ga) and Nakhla (1.3 G a). This technique permits high-precision simultaneous measurement of multi ple isotopes to uniquely evaluate Delta S-33 at the grain scale (< 30 <mu>m ). Our data reveal resolvable non-mass-dependent Delta S-33 anomalies in tw o separate ALH84001 pyrite grains (Delta S-33= -0.74 +/- 0.39 parts per tho usand and -0.51 +/- 0.38 parts per thousand, 20); none were detectable in N akhla pyrrhotite (total range in Delta S-33 = -0.4 +/- 0.5 parts per thousa nd to -0.07 +/- 0.5 parts per thousand, 2 sigma), Our results might reflect a difference in how these meteorites exchanged sulfur with the Martian reg olith and/or differences in their sources (atmospheric versus meteoritic) o f anomalous sulfur, Nebular heterogeneities in sulfur isotope composition a re indicated by Delta S-33 anomalies preserved in, for example, the ureilit es. The Delta S-33 anomalies in ALH84001 pyrite could suggest that early (p re-4 Ga) additions of a meteoritic component carried isotopically anomalous sulfur to the Martian regolith, and was stored there as seen in the detect ion of Delta S-33 anomalies from bulk measurements of Nakhla. Therefore, me teoritic contributions should also be considered in addition to atmospheric effects when explaining the large non-mass-dependent anomalies seen in Mar tian meteorites. These studies provide insight into how hydrothermal system s have facilitated exchange between volatile reservoirs on Mars, a planet t hat lacks efficient crustal recycling mechanisms and preserves ancient (and anomalous) Delta S-33 signatures. (C) 2000 Elsevier Science B.V. All right s reserved.