Analyses of High-Iron Sedimentary Bedrock and Diagenetic Features Observed With ChemCam at Vera Rubin Ridge, Gale Crater, Mars: Calibration and Characterization

Curiosity investigated a topographic rise named Vera Rubin Ridge (VRR) in Gale Crater, for which a distinct hematite-like signature was observed from orbit. However, the Chemistry and Camera (ChemCam) and Alpha Particle x-ray Spectrometer (APXS) instruments on board the rover did not record any significant iron enrichment in the bulk of the ridge compared to previous terrains. For this study, we have reverified ChemCam iron calibration at moderate abundances and developed more accurate calibrations at high-iron abundances using iron-oxide mixtures in a basaltic matrix in order to complete the ChemCam calibration database. The high-iron calibration was first applied to the analysis of dark-toned diagenetic features encountered at several locations on VRR, which showed that their chemical compositions are close to pure anhydrous iron oxides. Then, we tracked iron abundances in the VRR bedrock and demonstrated that although there is no overall iron enrichment in the bulk of the ridge (21.2 +/- 1.8-wt.% FeOT) compared to underlying terrains, the iron content is more variable in its upper section with areas of enhanced iron abundances in the bedrock (up to 26.6 +/- 0.85-wt.% FeOT). Since the observed variability in iron abundances does not conform to the stratigraphy, the involvement of diagenetic fluid circulation was likely. An in-depth chemical study of these Fe-rich rocks reveals that spatial gradients in redox potential (Eh) may have driven iron mobility and reactions that precipitated and accumulated iron oxides. We hypothesize that slightly reducing fluids were probably involved in transporting ferrous iron. Mobile Fe2+ could have precipitated as iron oxides in more oxidizing conditions.