Carbonate dissolution rates in high salinity brines: Implications for post-Noachian chemical weathering on Mars

Abstract A diverse suite of carbonate minerals including calcite (CaCO3) and magnesite (MgCO3) have been observed on the martian surface and in meteorites. Terrestrial carbonates usually form via aqueous processes and often record information about the environment in which they formed, including chemical and textural biosignatures. In addition, terrestrial carbonates are often found in association with evaporite deposits on Earth. Similar high salinity environments and processes were likely active on Mars and some areas may contain active high salinity brines today. In this study, we directly compare calcite and magnesite dissolution in ultrapure water, dilute sulfate and chloride solutions, as well as near-saturated sulfate and chloride brines with known activity of water (ɑH2O) to determine how dissolution rates vary with mineralogy and ɑH2O, as well as aqueous cation and anion chemistry to better understand how high salinity fluids may have altered carbonate deposits on Mars. We measured both calcite and magnesite initial dissolution rates at 298 K and near neutral pH (6–8) in unbuffered solutions containing ultrapure water (18 MΩ cm−1 UPW; ɑH2O = 1), dilute (0.1 mol kg−1; ɑH2O = 1) and near-saturated Na2SO4 (2.5 mol kg−1, ɑH2O = 0.92), dilute (0.1 mol kg−1, ɑH2O = 1) and near-saturated NaCl (5.7 mol kg−1, ɑH2O = 0.75). Calcite dissolution rates were also measured in dilute and near-saturated MgSO4 (0.1 mol kg−1, ɑH2O = 1 and 2.7 mol kg−1, ɑH2O = 0.92, respectively) and MgCl2 (0.1 mol kg−1, ɑH2O = 1 and 3 mol kg−1, ɑH2O = 0.73, respectively), while magnesite dissolution rates were measured in dilute and near-saturated CaCl2 (0.1 mol kg−1, ɑH2O = 1 and 9 mol kg−1, ɑH2O = 0.35). Initial calcite dissolution rates were fastest in near-saturated MgCl2 brine, while magnesite dissolution rates were fastest in dilute (0.1 mol kg−1) NaCl and CaCl2 solutions. Calcite dissolution rates in near-saturated Na2SO4 were similar to those observed in the dilute solutions (−8.00 ± 0.12 log mol m−2 s−1), while dissolution slowed in both NaCl solutions (0.1 mol kg−1; −8.23 ± 0.10 log mol m−2 s−1 and (5.7 mol kg−1; −8.44 ± 0.11 log mol m−2 s−1), as well as near-saturated MgSO4 brine (2.7 mol kg−1; −8.35 ± 0.05 log mol m−2 s−1). The slowest calcite dissolution rates observed in the near-saturated NaCl brine. Magnesite dissolution rates were ∼5 times faster in the dilute salt solutions relative to UPW, but similar to UPW (−8.47 ± 0.06 log mol m−2 s−1) in near-saturated Na2SO4 brines (−8.41 ± 0.18 log mol m−2 s−1). Magnesite dissolution slowed significantly in near-saturated CaCl2 brine (−9.78 ± 0.10 log mol m−2 s−1), likely due to the significantly lower water activity in these experiments. Overall, magnesite dissolution rates are slower than calcite dissolution rates and follow the trend: All dilute salt solutions >2.5 mol kg−1 Na2SO4 ≈ UPW > 5.7 mol kg−1 NaCl >> 9 mol kg−1 CaCl2. Calcite rates follow the trend 3 mol kg−1 MgCl2 > 2.5 mol kg−1 Na2SO4 ≈ UPW ≈ all dilute salt solutions >2.7 mol kg−1 MgSO4 ≈ 5.7 mol kg−1 NaCl. Magnesite dissolution rates in salt solutions generally decrease with decreasing ɑH2O in both chloride and sulfate brines, which indicates water molecules act as ligands and participate in the rate-limiting magnesite dissolution step. However, there is no general trend associated with water activity observed in the calcite dissolution rates. Calcite dissolution accelerates in near-saturated MgCl2, but slows in near-saturated NaCl brine despite both brines having similar water activities (ɑH2O = 0.73 and 0.75, respectively). High Mg calcite was observed as a reaction product in the near-saturated MgCl2, indicating Mg2+ from solution likely substituted for Ca2+ in the initial calcite, releasing additional Ca2+ into solution and increasing the observed calcite dissolution rate. Calcite dissolution rates also increase slightly as Na2SO4 concentration increases, while calcite dissolution rates slow slightly with increasing concentration of MgSO4 and NaCl. However, all of the carbonate rates vary by less than 0.5 log units and are within or near the standard deviation observed for each set of replicate experiments. Carbonate mineral lifetimes in high salinity brines indicate magnesite may be preferentially preserved compared to calcite on Mars. Therefore, Mg-carbonates that have experienced post-depositional aqueous alteration are more likely to preserve paleoenvironmental indicators and potential biosignatures. Rapid weathering of carbonates in circum-neutral pH sulfate brines may provide a potential source of cations for abundant sulfate minerals observed on Mars, Ceres, and other planetary bodies.