Aqueous alteration of pyroxene in sulfate, chloride, and perchlorate brines: Implications for post-Noachian aqueous alteration on Mars

Abstract

Abstract Both high and low calcium pyroxene minerals have been detected over large portions of the martian surface in addition to widespread salts in martian soils and dust. Calcium pyroxenes in martian meteorites are associated with secondary evaporite phases, including sulfates, chlorides, and perchlorates, suggesting the pyroxene may have been altered in saline solutions. Therefore, understanding pyroxene mineral weathering in high salinity brines may provide insight into past aqueous alteration on Mars. This study examines both solute-based dissolution rates and qualitative assessments of weathering textures developed during pyroxene-brine alteration experiments to link dissolution rates and textures and aid in interpreting weathering features observed in Mars meteorites and future pyroxene samples returned from Mars. Batch reactor dissolution experiments were conducted at 298 K to compare diopside (a high Ca-pyroxene) dissolution rates in water (18 MΩ cm−1 ultrapure water (UPW); activity of water (ɑH2O) = 1.0), 0.35 mol kg−1 NaCl (ɑH2O = 0.99), 0.35 mol kg−1 Na2SO4 (ɑH2O = 0.98), 2 mol kg−1 NaClO4 (ɑH2O = 0.90), 2.5 mol kg−1 Na2SO4 (ɑH2O = 0.95), 5.7 mol kg−1 NaCl (ɑH2O = 0.75), and 9 mol kg−1 CaCl2 (ɑH2O = 0.35) brines at pH 5–6.6 to determine how changing solution chemistry and activity of water influence pyroxene dissolution. Aqueous Si release rates and qualitative textural analyses indicate diopside dissolution rates are influenced by both solution chemistry and activity of water, with diopside weathering increasing along a trend from: 9 mol kg−1 CaCl2