Sulfur in Martian magmas from sulfur concentration at sulfide saturation applied to regional chemical maps

The sulfur cycle on Mars plays a critical role in shaping its surface and atmospheric chemistry. Mars' near-surface sulfur inventory largely originated from mantle-derived magmas that erupted during the Noachian, Hesperian, and Amazonian eons. Satellites permit measurements of bulk sulfur in Martian regolith, and rover and meteorite measurements capture snapshots of sulfur in specific samples. However, the concentration of sulfur in Martian magmas prior to degassing, which governs the transfer of interior sulfur to the near-surface, remains uncertain. Because Mars' mantle may be 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 bulk surface chemistry to calculate the sulfur concentration at sulfide saturation (SCSS) for late Noachian through Amazonian Martian magmas. We further consider a range in mantle source sulfur and constraints on degree of melting to account for mantle sulfide exhaustion, in order to estimate sulfur concentrations in primitive melts. We find that the concentration of sulfur in Martian magmas ranged between ∼1330 and 4550 ppm S. These results underscore that the GRS sulfur concentration data, from ∼15,000 and 29,000 ppm globally, do not represent the sulfur content of primitive basalts, but rather reflect myriad processes that cycled sulfur within the critical zone of exchange between the atmosphere and crust. We define a new metric, the Sulfur Enrichment Index (SEI), that tracks the enrichment in present-day regolith sulfur relative to the original magmatic sulfur concentration in volcanic regions. We show that sulfur release is inefficient for magmas emplaced at >1–2 km depth. Accounting for the total extruded volume of magma from the late Noachian through the Amazonian, our estimates of primary magmatic sulfur concentrations lead to a cumulative yield of ∼2–68 × 10 19 g of sulfur to the Martian atmosphere from ∼3.8 Ga to the present. For comparison, only ∼10 18 –10 19 g of sulfur is now hosted within the upper decimeters of the Martian crust at mid-latitudes. We therefore infer that volcanogenic sulfur, like water, has likely been sequestered within Mars' crust. • Sulfur content in Martian magmas remained relatively steady through time. • Gamma ray spectroscopy provides an alternative basis for estimating sulfur in Martian magmas. • Estimates of sulfur yield to the atmosphere exceed sulfur observed on the surface. • The southern highlands of Mars show a greater accumulation of secondary sulfur than the northern lowlands.