FORMATION AND WEATHERING OF ACIDIC HYDROTHERMAL SULFUR-RICH DEPOSITS IN ICELAND: IMPLICATIONS FOR SULFATE FORMATION ON MARS

The sulfate (SO₄) enrichment on the Martian surface has been linked to hydrothermal and sulfuric acid weathering processes in some locations. In terrestrial settings, reduced sulfur (S) minerals (e.g., elemental S, sulfides) are usually dominant in acidic hot spring sediments, yet SO₄ is the main S-species remotely detected in proposed hydrothermal locations on Mars (e.g. Gusev Crater). Therefore, this study characterizes S-rich minerals in Icelandic hot springs and mud pots to test the validity of hydrothermal SO₄ on Mars. Iceland is a useful geochemical analog because of high H₂S emissions and active basaltic volcanism with acidic hydrothermal features and sulfuric acid weathering of basaltic bedrock. In Aug 2016, sediment samples were collected from Icelandic hot springs and mud pots to evaluate the contents and oxidation states of the main S-bearing minerals. The sulfur sequential extraction method was used to determine the quantities and types of S-rich minerals. Results indicate that the most dominate S minerals are elemental S (<0.50 to 7.75 wt % S) and chromium-reducible sulfides (<0.90 to 5.45 wt % S) compared to lower concentrations of acid-soluble sulfates (<0.05 to 1.45 wt % S) and acid-volatile sulfides (<0.05 wt % S). There was no clear relationship between the contents of various S minerals and temperature, pH, or redox. However, the highest wt % S (>5-8) was associated with low pH (<4) waters and a temperature range of ~40 to 70°C. This supports the notion that low pH leads to formation and elevated contents of various S minerals in hydrothermal springs. The observed variability in the contents and types of S minerals may be due to localized and fluctuating H₂S gas emission. Similar to Iceland, we expect that elemental S and sulfide minerals would form concomitant with SO₄ on Mars. The lack of detection of reduced S minerals indicates either i) hydrothermal processes were not responsible for SO₄ enrichment on Mars, ii) subsequent oxidative weathering has oxidized reduced S, and/or iii) elemental S and sulfide minerals are abundant yet undetected with current instrumentation. While spectroscopic detection of elemental S and sulfide is difficult, our results show that an increased effort of detecting these minerals is crucial to better constrain the origin of SO₄ on Mars.