The composition and evolution of primordial solutions on Mars, with application to other planetary bodies

We examine a model for Mars involving bulk primordial solutions (oceans and lakes) that were relatively Mg-rich and SO4/(SO4 + Cl)-rich. Such solutions could be produced when (1) volatiles leached a planet (or portions of a planet) with an ultramafic-mafic composition in a process called planetary leaching; and/or by (2) impactor leaching where meteoritic and/or cometary impactor fragments were leached. When Mg-SO4/(SO4 + Cl)-rich solutions are concentrated, we predict that the following sequence of salts precipitates: phosphates; carbonates; gypsum; epsomite: bloedite halite; hexahydrite; and, finally bischofite. This sequence is modified slightly if appreciable Fe-, Mg- or Na-carbonates, Fe-sulfates, Mg-phosphate, or other halide salts crystallized before the Mg-Na-sulfate salts, or if HCO3+CO3 concentrations vary due to other effects (e.q., atmosphere CO2 levels change). On Mars, the initial primordial solutions would have been relatively salt-rich and water-poor; therefore, the surface solutions formed Mg-Na-SO4-Cl salts (cements, veneers, and dust) and subsurface solutions or ice (solid H2O). This model is supported by the compositions of cements in the regolith on Mars (high Mg, Na, S, and Cl) and geochemical and petrographic evidence that the salts precipitated in the predicted sequence. We suggest that the partial pressure of oxygen was above the heniatite-inagnetite buffer where Fe3'-(hydrous)oxides are stable and SO42- or HSO4- are solutes in any solution. Such a partial pressure of oxygen may have been attained via H-2-Ioss. In contrast, on the Galilean satellites (Europa, Ganyinede, and Callisto) surface solutions were relatively Nvater-rich and formed ice, Mg-SO4-rich salts, and solutions, thereby producing surface features dependent on the initial water content and the crystallization path. Unlike the Na-Cl-rich oceans on Earth, the solutions of these planetary bodies likely did not change greatly from their bulk primordial Mg-rich, SO,/(SO, + Cl)-rich compositions- fience they did not attain compositions similar to modern seawater. Copyright (C) 2004 Elsei:ier Ltd