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

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 < UPW (−9.82 ± 0.03 log mol m−2 s−1) ≈ 2 mol kg−1 NaClO4 ≈ 0.35 mol kg−1 Na2SO4 (−9.80 ± 0.07) ≈ 5.7 mol kg−1 NaCl (−9.69 ± 0.04)  < 0.35 mol kg−1 NaCl (−9.45 ± 0.34)  < 2.5 mol kg−1 Na2SO4 (−8.99 ± 0.09). Dissolution rates increase in sodium sulfate brines with increasing salinity. In contrast, Si-based dissolution rates in 0.35 mol kg−1 NaCl are faster than those measured in 5.7 mol kg−1 NaCl and UPW. However, all of the Si-based rates measured in the chloride and sulfate salt solutions are likely affected by precipitation of Si-rich secondary clay minerals, which removed Si from solution. Qualitative textural analyses indicate similar degrees of dissolution occurred in UPW and 2 M NaClO4; however, no aqueous rate determinations could be made in perchlorate brines due to explosion hazards. Aqueous Si was below detection limits in the 9 mol kg−1 CaCl2 experiments, but textural analysis suggests limited diopside dissolution occurred. Therefore, despite low water activity, diopside dissolution proceeds in both dilute to high salinity brines, readily forming clay minerals under a wide range of conditions. This suggests that outcrops on Mars containing pyroxene preserved with sulfate, chloride, perchlorate, and/or clay minerals likely record relatively short periods (<1 million years) of aqueous alteration. Si-rich spherules similar to those observed in SNC meteorites were also observed in the 5.7 mol kg−1 NaCl brine experiments, indicating that silicate mineral alteration in chloride brines may lead to Si-rich alteration products and coatings.