Experimental constraints on the evaporation of partially oxidized acid-sulfate waters at the martian surface

Isothermal evaporation experiments were carried out on an acidic (pH 2), partially oxidized (Fe2+/FeT ∼0.5) brine with a cation composition consistent with derivation from the chemical weathering of martian basalt. During evaporation, the brine composition evolved to a highly acidic (un-scaled pH −1.3) Mg–Fe–SO4–Cl brine depleted in Ca, Al and K. Evaporite minerals identified throughout the course of the experiment include (in order of crystallization): gypsum, Mg-rich voltaite, (Mg0.7, Fe2+0.3)SO4·7H2O and rhomboclase. The solid solution compositions of voltaite and (Mg0.7, Fe2+0.3)SO4·7H2O, although uncommon in analogous environments on Earth, result from the distinct chemistry of evaporating martian surface fluids. Analysis of brine compositions with available thermodynamic models indicates that, although gypsum and rhomboclase precipitate at equilibrium saturation, kinetic controls on the precipitation of copiapite-group minerals affect the subsequent sulfate mineralogy and evolving chemistry of the entire system. In addition, geochemical simulations of the experimental evaporation process suggest that the appearance of voltaite and rhomboclase indicate a “metastable” evaporation pathway for martian fluids whereby bilinite and copiapite-group minerals did not form despite thermodynamic saturation. Comparison of the experimentally-produced assemblage to available observations of saline minerals at the martian surface represents a step toward systematically characterizing evaporite mineralogy as a function of Fe-oxidation in the initially dilute fluid. Deconvolving the complexity of Fe-sulfate formation in martian environments ultimately will help to exploit the sensitivity of these minerals to pH and redox conditions present at the ancient martian surface.