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-mass-dependent Δ33S anomalies (Δ33S=δ33S−0.516δ34S) reported from bulk analyses of Martian meteorites. To explore this signature in detail, a new ion microprobe multi-collector technique was developed to obtain precise in situ 32S, 33S and 34S measurements of individual sulfide grains from Martian meteorites ALH84001 (>4.0 Ga) and Nakhla (1.3 Ga). This technique permits high-precision simultaneous measurement of multiple isotopes to uniquely evaluate Δ33S at the grain scale (<30 μm). Our data reveal resolvable non-mass-dependent Δ33S anomalies in two separate ALH84001 pyrite grains (Δ33S=−0.74±0.39‰ and −0.51±0.38‰, 2σ); none were detectable in Nakhla pyrrhotite (total range in Δ33S=−0.4±0.5‰ to −0.07±0.5‰, 2σ). Our results might reflect a difference in how these meteorites exchanged sulfur with the Martian regolith and/or differences in their sources (atmospheric versus meteoritic) of anomalous sulfur. Nebular heterogeneities in sulfur isotope composition are indicated by Δ33S anomalies preserved in, for example, the ureilites. The Δ33S anomalies in ALH84001 pyrite could suggest that early (pre-4 Ga) additions of a meteoritic component carried isotopically anomalous sulfur to the Martian regolith, and was stored there as seen in the detection of Δ33S anomalies from bulk measurements of Nakhla. Therefore, meteoritic contributions should also be considered in addition to atmospheric effects when explaining the large non-mass-dependent anomalies seen in Martian meteorites. These studies provide insight into how hydrothermal systems have facilitated exchange between volatile reservoirs on Mars, a planet that lacks efficient crustal recycling mechanisms and preserves ancient (and anomalous) Δ33S signatures.