Mechanisms of sulfate formation in acidic hydrothermal sites of Iceland, Lassen, Valles Caldera, and Yellowstone: Implications for possible oxidation pathways in martian volcanic settings

Sulfate (SO42-) has been found in elevated concentrations on the surface of Mars and linked to oxidation of volcanically derived sulfur (S) in the past, with the best examples of hydrothermal S-rich deposits in Gusev crater. However, the oxidation mechanisms of volcanic S are unclear due to the lack of abundant molecular oxygen (O-2) in the Martian atmosphere. To address this uncertainty, we investigated the mechanisms of hydrogen sulfide (H2S) oxidation to SO42- in O-2-depleted acidic hydrothermal systems of Iceland and the United States (Valles Caldera, Lassen, and Yellowstone) as geochemical analogs. Approximately 50 water and sediment samples were collected for chemical and stable isotope analyses (delta S-34, delta O-18, delta H-2). At the time of sampling, the measured dissolved O-2 (DO) concentrations were low (0.01 to 1.03 mg/L) because of elevated temperatures (60 to 90 degrees C) and did not account for high concentrations of SO42- (up to 24,461 mg/L) in the acidic hot springs and mud pots. However, high concentrations of leachable iron (Fe) were present in the hydrothermal water and sediments (average of 158 mg/L and 71,302 mg/kg, respectively), implying that in addition to atmospheric O-2, the Fe-driven oxidation of H2S via ferric Fe reduction to ferrous Fe is likely involved. Our laboratory experiments studying H2S oxidation carried out in a wide temperature range (25 degrees C, 50 degrees C, 90 degrees C) and low pH of 2 suggest that under acidic hydrothermal conditions, ferric Fe reduction involves stepwise oxidation of H2S with preferential formation of insoluble elemental S, which is later oxidized to SO42-. In contrast, the Gusev hydrothermal sediments on Mars appear to be enriched in Fe3+-sulfates with minor amounts of Ca-Mg-sulfates and negligible elemental S. This implies that the Gusev sediments must have experienced subsequent oxidation, most likely after the main episode of hydrothermal activity.