Controls on S mineral formation and preservation in hydrothermal sediments: Implications for the volcanic, aqueous, and climatic history of Gusev crater, Mars

Abstract

Acidic hydrothermal and fumarolic surface deposits within the Columbia Hills in Gusev crater on Mars were found to have elevated concentrations of Fe-Mg-Ca-sulfate minerals. However, this is inconsistent with analogous terrestrial hydrothermal settings that are usually enriched in elemental sulfur (S). Consequently, this raises questions about the origin and hydrothermal history 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 with elevated H2S emissions in Iceland and the United States (e.g., Valles Caldera, Lassen, and Yellowstone). Our results indicate that the typical concentrations (e.g., inter-quartile range) of elemental S and sulfide minerals (0.3 to 10.5 wt% S, but as high as ~75 wt% S; and 0.1 to 1.7 wt% S, but as high as ~10 wt% S) are significantly higher compared to sulfate (0.1 to 1.1 wt% S, but as high as ~4.5 wt% S) in the surface hydrothermal deposits. In most cases, the concentrations of elemental S, sulfides, and sulfates in the sediments decreased with increasing hydrological connectivity and in wetter climates. 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 which had higher δ34S values (+1.1 to +5.9‰). Our laboratory experiments carried over a wide range of temperatures (25, 65, and 85 °C) and low pH (~2) indicate that elemental S and pyrite undergo subsequent oxidation to sulfate via both ferric iron (Fe3+) and O2. While the amount of sulfate increased with increasing temperature in the presence of both Fe3+ and O2, Fe3+ appears to be a more efficient oxidizer than O2. For example, pyrite oxidation by only Fe3+ resulted in ~1.5× more sulfate (~80 to 180 mg/L SO42−) than by only O2 (~40 to 140 mg/L SO42−). In contrast, considerably less sulfate was formed during the oxidation of elemental S, although in the presence of O2 ~10× more sulfate (~0.1 to 45 mg/L SO42−) was formed than when Fe3+ was present (~0.3 to 7.5 mg/L SO42−).Despite the prevalence of sulfate minerals rather than elemental S and sulfides in the hydrothermal Gusev deposits on Mars, the total S concentrations measured by the Spirit rover (2.9 to 9.3 wt% S) are highly comparable to the total S in hydrothermal sediments formed in colder and moderately wet climates such as coastal Iceland (1.8 to 10.7 wt% S). This contrasts with sediments formed in the high-altitude and drier climate of Valles Caldera (9.9 to 37.6 wt% S), or the wetter climates of Yellowstone (4.1 to 17.3 wt% S) and Lassen (0.5 to 3.5 wt% S). Because water is needed to further oxidize the hydrothermal elemental S and sulfide to sulfate, we infer that the aqueous conditions must have persisted in Gusev crater for a period of time after the main hydrothermal activity ceased. Later, under low water-to-rock conditions with little (or no) H2S emission, complete oxidation of the Gusev hydrothermal deposits likely took place and led to the formation of the sulfate minerals that were identified by the Spirit rover.