Transformation of Precursor Iron(III) Minerals in Diagenetic Fluids: Potential Origin of Gray Hematite at Vera Rubin Ridge

Abstract

AbstractCoarse‐grained (>3–5 μm) gray hematite particles occur at Vera Rubin ridge (VRR) within Gale crater, Mars. VRR has likely undergone multiple episodes of diagenesis, at least one of which resulted in the formation of gray hematite. The precursor mineralogy and nature of the diagenetic fluids that produced coarse‐grained hematite remain unknown. Analog laboratory experiments were performed on a variety of iron(III) minerals to assess the potential fluid conditions and precursor mineralogy that form coarse‐grained hematite. Gray hematite formed from the transformation of jarosite after 20 days at 200°C. Conversion was complete in chloride‐rich fluids; however, modeling indicates that at lower jarosite‐to‐water ratios, conversion is complete even in chloride‐free, sulfate‐rich conditions. No transformations of jarosite occurred when aged at 98°C. All other precursor minerals (akaganeite, ferrihydrite, goethite, and schwertmannite) did not transform or produced red, fine‐grained hematite under all conditions assessed. Additionally, seeding precursor iron(III) phases with red hematite and coarsening pre‐existing red hematite failed to produce gray hematite. These results suggest that jarosite could have been the precursor of gray hematite at VRR and that transformation is possible in both sulfate‐bearing and chloride‐bearing fluids. Jarosite produces gray hematite because the acidic conditions it generates yield both a low degree of hematite supersaturation, producing few nuclei, and high dissolved iron concentrations, enabling rapid hematite growth. Gray hematite readily forms under oxic conditions and its occurrence at VRR is not a marker for a redox interface. The associated diagenetic event was thus unlikely to have generated substantial new chemical energy for life.