Implications from sulfur isotopes of the Nakhla meteorite for the origin of sulfate on Mars

The isotope geochemistry of sulfur in the SNC (Shergottite, Nakhlite, Chassignite) meteorites may carry important implications for the origin and chemistry of sulfates and sulfides in the Martian surface and subsurface. We report measurements of the sulfur isotope composition for three sulfur-bearing components of the Nakhla meteorite. These results yield a negative Δ33S signal of −1.25‰ and a near zero Δ36S signal for total sulfate as well as a small nonzero Δ33S of −0.09‰ for sulfide phases. The Δ33S of total sulfate measured here is more negative than the Δ33S reported previously for water soluble sulfate and points to sulfur isotopic heterogeneity in the sulfate of Nakhla.Current understanding of mass-independent isotope effects associated with sulfur oxidation reactions rules out hydrothermal oxidation mechanisms for producing nonzero Δ33S and points to a significant role for photochemical effects (including hyperfine chemistry), likely located in the Martian atmosphere. The near zero Δ36S signal allows for the possibility of hyperfine chemistry. Because of the relationship between SNC sulfate and the sulfate deposits seen on Mars, these results place constraints on the generation of sulfur(VI) at the Martian surface and provide a case for future direct measurements of Δ33S (and Δ36S) of Martian sulfate.