Mixing relationships and the effects of secondary alteration in the Wishstone and Watchtower Classes of Husband Hill, Gusev Crater, Mars

The Wishstone and Watchtower Class rocks on Husband Hill preserve evidence for a geochemical relationship consistent with two-component mixing between a high Al2O3, TiO2, CaO, Na2O, P2O5 end-member and a second end-member enriched in the elements MgO, Zn, S, Br, and Cl. The first end-member appears to be reasonably well represented by rocks of the Wishstone Class, while the second end-member is consistent with a chemical component, not represented by any lithology encountered by Spirit. The Watchtower Class appears to be an intermediate in the mixture. The concentration of the redox sensitive elements Fe and Mn display no systematic variation between rock classes, and the Fe-redox state of the Watchtower Class is elevated relative to the Wishstone Class. Combined with evidence for two-component mixing, these observations suggest that the mixture was generated by interaction between an initially Wishstone-like deposit and a fluid phase enriched in the elements MgO, Zn, S, Br, and Cl. The fluid phase may have been weakly acidic (pH > 4) to basic (pH >= 9), resulting in oxidation and immobilization of Fe and Mn. A number of secondary rock surface alteration processes which obscure the mixing relationship are also identified. These include (1) removal of CaO and P2O5 from rock surfaces by dissolution of Ca-phosphate minerals under acidic, low water-to-rock ratio conditions, (2) addition of SO3 to rock surfaces via aeolian deposition of high SO3 soil and/or precipitation of sulfate salts from solution, and (3) homogenization of rock surface chemistry by soil contamination.