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

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 SO4 2-contributions via Fe3+- and O2-driven oxidation pathways in acidic hot springs and mudpots of Iceland (N & PRIME;amafjall, KrATIN SMALL LETTER Y WITH ACUTEsuvik) and the United States (Valles Caldera, Lassen, Yellowstone). Approximately 40 water and 11 fumarole sediment samples were collected for oxygen isotope analyses (& delta;18O). The oxidation processes appear to be accompanied by large variation of the & delta;18O of SO42- (-8.8 to +5.5 %o) similar to the & delta;18O of hot spring and mudpot water (-15.5 to +6.3 %o), 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 SO4 2-, contributing -55 to 100% of SO42- when compared to contributions from O2-driven oxidation. Additionally, the distinctive high & delta;18O values of SO42- (up to +13.7 %o) 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 (& epsilon;sulfate-atm.oxygen - 9.8 %o) 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.