Quantitative assessment of aqueous sulfate contributions from Fe3+- and O2-driven oxidation pathways of hydrothermal sulfur using oxygen isotopes

Abstract

Sulfate (SO42−) has been found in elevated concentrations in modern hydrothermal settings on Earth and linked to coupled ferric iron (Fe3+)- and molecular oxygen (O2)-driven oxidations of hydrothermal H2S and secondary sulfur (S) deposits enriched in elemental S and sulfide minerals. However, the relative proportions of SO42− from these two oxidation mechanisms in surface acidic hydrothermal systems are unclear. To address this uncertainty, we determined the aqueous SO42− contributions via Fe3+- and O2-driven oxidation pathways in acidic hot springs and mudpots of Iceland (Námafjall, Krýsuvík) and the United States (Valles Caldera, Lassen, Yellowstone). Approximately 40 water and 11 fumarole sediment samples were collected for oxygen isotope analyses (δ18O). The oxidation processes appear to be accompanied by large variation of the δ18O of SO42− (−8.8 to +5.5 ‰) similar to the δ18O of hot spring and mudpot water (−15.5 to +6.3 ‰), which is greatly controlled by inflow of meteoric water (e.g., runoff, groundwater) and surface evaporation. In the studied sites, the Fe3+-driven oxidation of hydrothermal elemental S and sulfide minerals appears to be an important source of SO42−, contributing ∼55 to 100% of SO42− when compared to contributions from O2-driven oxidation. Additionally, the distinctive high δ18O values of SO42− (up to +13.7 ‰) accompanied direct oxidation of H2S and/or hydrothermal S minerals by atmospheric O2 in dry (fumarolic) environments. This, in turn, is in agreement with previously determined O isotope fractionations (εsulfate-atm.oxygen ∼ 9.8 ‰) for low-temperature sulfide oxidation in the presence of O2. Our new quantitative measures of oxidation pathways signify the importance of Fe3+-driven oxidation of hydrothermal S to SO42− in the surface volcanic settings, which have significant implications for understanding widespread occurrences of SO42− found on the Fe-rich volcanic terrains of Mars.