Iron sulfide weathering by oxyhalogen species: Implications for iron sulfate and (oxyhydr)oxides formation on Mars

Abstract

Sulfate salts are abundant, widespread, and temporally distributed on the surface of Mars. Several processes, including sulfide weathering, have been proposed to explain the formation and accumulation of oxidized iron(III) hydroxysulfate (e.g., jarosite) in Martian sediments. Oxidative weathering of iron sulfide minerals (like pyrite and pyrrhotite) could explain the presence of sulfate as well as the relative absence of sulfide minerals in Martian sediments. However, the effectiveness of oxyhalogen compounds, plausible oxidants present on Mars, to weather iron sulfides remain unknown. Here we investigate the oxidative weathering of iron sulfide minerals, pyrite and pyrrhotite, by chlorate and bromate in Mars-relevant fluids. Our results demonstrate that both oxyhalogen species readily oxidize iron sulfides and produce an alteration assemblage comprised of elemental sulfur, Fe(III) (oxyhydr)oxide (magnetite, goethite, and lepidocrocite), and Fe(III) hydroxysulfates (jarosite and schwertmannite). The mineral products depend strongly on the type of sulfide, oxidant, and initial solution pH. Owing to their abundance and highly reactive nature, oxyhalogen brines could be important Fe(III) hydroxysulfate-forming reagents on early and modern Mars and substantially impact the survivability of sulfide minerals at the Martian surface. Additionally, sulfide bearing units might serve as indicators of minimal post-depositional alteration. Oxyhalogens may be responsible for the loss of magmatic sulfides in surface materials given the prevalence of oxyhalogen brines and the reactivity of the sulfides.