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