Understanding the Mechanisms of Sulfate Formation in Acidic Hydrothermal Systems Using Field Analogs and Laboratory Experiments
Date of Award
Doctor of Philosophy
Harry McSween Jr, Linda Kah, John Schwartz
SO42- [sulfate] has been found in elevated concentrations on the surface of Mars and linked to oxidation of volcanic sulfur. However, the oxidation mechanisms are unclear due to the lack of molecular O2 [oxygen] in the Martian atmosphere. To address this uncertainty, I investigated the mechanisms of H2S [hydrogen sulfide] oxidation to SO42- in O2-depleted acidic hydrothermal systems of Iceland and the United States (Valles Caldera, Lassen, and Yellowstone) as geochemical analogs. Water and sediment samples were collected for chemical and δ18O [oxygen isotopes] analyses. The measured DO [dissolved oxygen] concentrations were low (0.01 to 1.03 mg/L [milligrams per liter]) and could not account for high concentrations of SO42- (100 to 24,461 mg/L). However, high concentrations of leachable Fe [iron] were present in the hydrothermal water and sediment (average of 158 mg/L and 71,302 mg/kg [milligrams per kilogram], respectively), implying that Fe-driven oxidation of H2S via Fe3+ [ferric Fe] reduction to Fe2+ [ferrous Fe] is likely involved in SO42- formation. Oxygen isotope results show large variation in the δ18O of SO42- (-8.8 to +5.5 ‰ [per mil]) similar to δ18O of water (-15.5 to +6.3 ‰) in the field samples which might be a result of evaporation and oxygen isotope exchange between SO42-, SO32- [sulfite] and water altering the δ18O of SO42-. Therefore, laboratory experiments were conducted to measure the effect of isotopic exchange on the δ18O of SO42- in Mars analog sites. Results suggest that although this oxygen isotope exchange may occur under hydrothermal conditions, it cannot be distinguished from previously established fractionations for both O2- and Fe-driven oxidation of sulfide. Additionally, experiments were conducted to better understand oxidation mechanism of H2S gas by Fe and O2. Results show that O2 appears to be a slow oxidant, resulting in small amounts of SO42-. During Fe-driven oxidation even less SO42- was formed because of rapid precipitation of insoluble sulfur intermediates. Overall, SO42- formation in surface hydrothermal environments is complex, requiring constant supply of oxidants (e.g., Fe, O2) and changes of hydrological conditions. Nevertheless, this process provides an invaluable alternative explanation for the formation of sulfates in O2-depleted Martian environments.
Ende, Jessica, "Understanding the Mechanisms of Sulfate Formation in Acidic Hydrothermal Systems Using Field Analogs and Laboratory Experiments. " PhD diss., University of Tennessee, 2020.