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

Coarse‐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. Plain Language Summary: Hematite is often formed from other pre‐existing iron‐bearing minerals in water‐rich environments. Depending on its grain size, hematite can be either red (fine‐grained) or gray (coarse‐grained) and these grain sizes likely indicate distinct past conditions. Gray hematite has been observed at Vera Rubin ridge (VRR) in Gale crater, Mars, but how it formed is currently unknown. Laboratory experiments in this study demonstrate that jarosite, an iron‐ and sulfur‐bearing mineral, converts to gray hematite in fluids at elevated temperatures. Increased chloride in the fluid promotes more complete transformation but complete transformation will also occur in systems with sufficiently low jarosite‐to‐water ratios. Other iron‐bearing minerals assessed (akaganeite, ferrihydrite, goethite, schwertmannite, and fine‐grained hematite) did not transform or coarsen to gray hematite under any experimental conditions studied. The gray hematite at VRR likely formed by conversion from jarosite during alteration after the sediments in Gale crater had already formed. The conditions associated with this event did not provide any new chemical energy for life. Key Points: Jarosite transforms to gray hematite at 200°C in both sulfate‐ and chloride‐bearing fluidsGoethite, akaganeite, ferrihydrite, schwertmannite, and red hematite do not form gray hematite after 20 days in the conditions assessedJarosite may be a viable precursor mineral to the gray hematite at Vera Rubin ridge due to the acidity it generates during dissolution [ABSTRACT FROM AUTHOR]