Fe oxidation processes at Meridiani Planum and implications for secondary Fe mineralogy on Mars

Fe oxidation processes may have occurred during groundwater-mediated diagenesis in Meridiani Planum sediments. To address this question, melanterite oxidation experiments were conducted at epsomite saturation as a function of pH. Results show that schwertmannite is initially formed from acidic Fe oxidation and that its formation and aging to mixtures of jarosite and nanocrystalline goethite is strongly controlled by pH over the range similar to 2.0-4.0. The pH is controlled in turn by Fe oxidation and Fe3+ hydrolysis. In one 77-d oxidation experiment, nanocrystalline hematite was tentatively identified by Mossbauer spectroscopy. Accordingly, aging experiments with synthetic nanocrystalline goethite were conducted (1) to further resolve the formation mechanisms of Fe-phases identified from oxidation experiments and (2) to test whether low water activity (a(w)) controls the thermodynamically favored goethite to hematite transition at low temperature. Mossbauer spectroscopy and total X-ray scattering show no observable changes after 4 months of aging, and instead, these results point to a jarosite precursor for the tentatively identified hematite. On the basis of these results, we suggest that the oxidation and maturation of initially formed Fe2+-bearing saline minerals may account in large part for the distribution of secondary Fe minerals at the Martian surface, contributing to the association of Fe oxides and Mg/Ca sulfates observed from orbital surface analyses. We hypothesize that oxidation of Fe2+ sulfates at low temperature could account for sustained diagenetic acidity in addition to much of the observed Fe mineralogy in Meridiani Planum outcrop rocks. The origin of the gray crystalline hematite at Meridiani, however, is deserving of further experimental work to test this mechanism.