Geochemical and climatic controls on the sulfur cycle in volcanic settings: Implications for the origin of sulfur-rich deposits investigated by the Spirit and Opportunity rovers on Mars

On Earth, volcanic activity with elevated sulfur (S) degassing in the presence of water leads to the formation of hydrothermal deposits enriched in S-bearing minerals. Similar processes may have been an important source of S on Mars. The landing sites of Gusev crater and Meridiani Planum investigated by the Spirit and Opportunity rovers, respectively, showed elevated SO₄^2- [sulfate] concentrations, suggesting high- and low-temperature aqueous processes. However, the SO₄^2- contribution from subsequent aqueous weathering of hydrothermal S deposits has been poorly constrained, thus its importance to regional S cycling in the landing sites is unclear. In this study, geochemical and hydrological approaches were used to determine sediment and aqueous S sources in terrestrial volcanic terrains at local and regional scales. The results of local scale studies in Iceland and the U.S. showed that hydrothermal sediments are typically enriched in elemental S (~0.3-22 wt.% S) with minor sulfide (~0.1-3.9 wt.% S) and sulfate (~0.1-2.7 wt.% S), and their concentrations decrease in wet climates. Sulfur isotopes suggested that subsequent oxidation of elemental S and sulfide to SO₄^2- is controlled by water-rock interactions under varied temperatures and oxidant availability (O₂ [oxygen]/Fe³⁺ [ferric iron]). Despite the prevalence of sulfate minerals rather than elemental S/sulfides in the hydrothermal deposits of Gusev crater, the total S concentration is comparable to those formed in coastal Iceland. It is inferred that complete oxidation of Gusev hydrothermal deposits to SO₄^2- took place later under low water-to-rock conditions with little S emission. The results of regional scale studies in Hawaii and Iceland showed that significantly higher SO₄^2- loads (~20-260 tons/yr/km²) occur in aqueous systems on younger, wetter volcanic terrains with active hydrothermal S degassing and mineralization. Conversely, smaller SO₄^2- loads (~2.1-10 tons/yr/km²) occur in older, drier and non-active terrains. Comparison of terrestrial SO₄^2- loads to the amount of SO₄^2- present in Meridiani Planum suggests timescales of water activity spanning ~30-60 million years. Overall, complex interactions of high- and low-temperature water with hydrothermal S minerals and gases under transitioning climatic conditions sufficiently explain the formation of sedimentary deposits on Mars enriched in secondary sulfates such as in Gusev crater and Meridiani Planum..