Early diagenesis at and below Vera Rubin ridge, Gale crater, Mars

Data returned by NASA’s Mars Science Laboratory Curiosity rover showed evidence for abundant secondary materials, including Fe-oxides, phyllosilicates, and an amorphous component on and below Vera Rubin ridge in the Murray formation. We used equilibrium thermochemical modeling to test the hypothesis that altered sediments were deposited as detrital igneous grains and subsequently underwent diagenesis. Chemical compositions of the Murray formations’ altered components were calculated using data returned by the chemistry and mineralogy X-ray diffraction instrument and the alpha particle X-ray spectrometer on board Curiosity. Reaction of these alteration compositions with a CO2-poor and oxidizing dilute aqueous solution was modeled at 25–100 °C, with 10–50% Fe3+/Fetot of the host rock. The modeled alteration assemblages included abundant phyllosilicates and Fe-oxides at water-to-rock ratios >100. Modeled alteration abundances were directly comparable to observed abundances of hematite and clay minerals at a water-to-rock ratio of 10,000, for system temperatures of 50–100 °C with fluid pH ranging from 7.9 to 9.3. Modeling results suggest that the hematite–clay mineral assemblage is primarily the result of enhanced groundwater flow compared to the Sheepbed mudstone observed at Yellowknife Bay, and underwent further, localized alteration to produce the mineralogy observed by Curiosity.