SULFUR IN MARTIAN MAGMAS FROM SULFUR CONTENT AT SULFIDE SATURATION APPLIED TO REGIONAL CHEMICAL MAPS

Mars’ sulfur (S) cycle plays a critical role in shaping its surface and atmospheric chemistry. The budget of Martian surface S largely originated from mantle-derived magmas that generated extensive volcanism during the Noachian, Hesperian, and Amazonian geological periods. Remote sensing measurements regional maps of bulk S in Martian regolith, and rover and meteorite measurements capture snapshots of S in specific samples. However, the concentration of sulfur in Martian magmas prior to degassing, which governs the overall transfer of S to the near-surface, remains uncertain. Because Mars’ mantle is sulfur-rich, most primary mantle melts are expected to be in equilibrium with residual mantle sulfide. In this work, we therefore use Gamma Ray Spectroscopy (GRS) regional maps of Si, Fe, Ca, Al, and K to calculate the S content at sulfide saturation (SCSS) for Middle Noachian through Amazonian Martian magmas. We find that the concentration of S in Martian magmas over this timespan was an order of magnitude lower than the concentration of S measured by GRS in regolith, with variations of several hundred ppm between provinces. Our SCSS results agree with published estimates of SCSS that used other methods including thermochemical analyses and SCSS models applied to Martian meteorites. We infer that the GRS S concentration data, from ~15,000 – 29,000 ppm globally, do not represent the S content of primitive magmas, but rather reflect myriad processes that cycled S on and near the Martian surface. Our refined constraints on the S concentrations within Martian magmas help define the budget of S available to source observed sedimentary sulfates and S present in Mars’ regolith. Combining published estimates of the total extruded volume of magma with our estimates of primary magmatic sulfur concentrations, we also calculate the yield of volcanogenic S to the Martian atmosphere from ~3.8 Ga to the present.