GEOCHEMICAL AND ENVIRONMENTAL CONTROLS ON SULFUR MINERAL FORMATION AND PRESERVATION IN HYDROTHERMAL SEDIMENTS: IMPLICATIONS FOR THE VOLCANIC AND AQUEOUS HISTORY OF GUSEV CRATER, MARS

Hydrothermal and fumarolic surface deposits within the Columbia Hills of Gusev crater on Mars were found to have elevated concentrations of Fe-Mg-Ca-sulfate (SO₄^2-) minerals. However, this is inconsistent with analogous terrestrial hydrothermal settings that are usually enriched in elemental sulfur (S). Consequently, this raises questions about the hydrothermal origin of the Gusev sediments. To address this discrepancy, we analyzed quantities and S isotope compositions of S-bearing minerals in hydrothermal sediment samples from acidic hot springs, mud pots, and fumaroles in Iceland, Valles Caldera, Lassen, and Yellowstone. Our results indicate that the typical concentrations (e.g., inter-quartile range) of elemental S (0.3-10.5 wt.% S, but as high as ~75 wt.%) and sulfide minerals (0.1-1.7 wt.% S, as high as ~10 wt.%) are significantly higher compared to sulfate (0.1-1.1 wt.% S, as high as ~4.5 wt.%) in the hydrothermal deposits. In most cases, the concentrations of elemental S, sulfides, and sulfates in the sediments decreased with increasing hydrological connectivity and wetter climate. Similar δ³⁴S values between sulfate (-0.1 to +1.4‰) and elemental S (-0.4 to +1.6‰) compared to lower δ³⁴S of sulfide (-2.4 to +0.4‰) suggest that more sulfate is likely derived from the subsequent oxidation of elemental S than sulfide. Conversely, minor amounts of sulfate are formed via direct oxidation of H₂S with higher δ³⁴S (+1.1 to +5.9‰). Despite the prevalence of sulfates rather than elemental S/sulfides in the hydrothermal Gusev deposits, the total S concentrations (2.9-9.3 wt.% S) are highly comparable to those hydrothermal sediments found in the colder and moderately wet climates of the coastal lowlands in Iceland (1.8-10.7 wt.% S). This is in contrast to sediments in the high-altitude and drier climate in Valles Caldera (9.9-37.6 wt.% S), or the wetter climates in Yellowstone (4.1-17.3 wt.% S) and Lassen (0.5-3.5 wt.% S). Because water is needed to further oxidize elemental S and sulfide to sulfate, we infer that the aqueous conditions must have persisted in Gusev crater after the main hydrothermal activity ceased. Later, under conditions with little H₂S emission, complete oxidation of the Gusev hydrothermal deposits likely took place and led to the formation of the Fe-Mg-Ca sulfate minerals currently present.