Diagenesis and clay mineral formationat Gale Crater, Mars

International audienceThe Mars Science Laboratory rover Curiosity found host rocks of basaltic composition andalteration assemblages containing clay minerals at Yellowknife Bay, Gale Crater. On the basis of theobserved host rock and alteration minerals, we present results of equilibrium thermochemical modeling ofthe Sheepbed mudstones of Yellowknife Bay in order to constrain the formation conditions of its secondarymineral assemblage. Building on conclusions from sedimentary observations by the Mars Science Laboratoryteam, we assume diagenetic, in situ alteration. The modeling shows that the mineral assemblage formedby the reaction of a CO2-poor and oxidizing, dilute aqueous solution (Gale Portage Water) in an open systemwith the Fe-rich basaltic-composition sedimentary rocks at 10–50°C and water/rock ratio (mass of rockreacted with the starting fluid) of 100–1000, pH of ~7.5–12. Model alteration assemblages predominantlycontain phyllosilicates (Fe-smectite, chlorite), the bulk composition of a mixture of which is close to that ofsaponite inferred from Chemistry and Mineralogy data and to that of saponite observed in the nakhliteMartian meteorites and terrestrial analogues. To match the observed clay mineral chemistry, inhomogeneousdissolution dominated by the amorphous phase and olivine is required. We therefore deduce a dissolvingcomposition of approximately 70% amorphous material, with 20% olivine, and 10% whole rock component