Dissolution of nontronite in chloride brines and implications for the aqueous history of Mars

Increasing evidence suggests the presence of recent liquid water, including brines, on Mars. Brines have therefore likely impacted clay minerals such as the Fe-rich mineral nontronite found in martian ancient terrains. To interpret these interactions, we conducted batch experiments to measure the apparent dissolution rate constant of nontronite at 25.0 °C at activities of water (aH_2O) of 1.00 (0.01 M CaCl_2 or NaCl), 0.75 (saturated NaCl or 3.00 mol kg^(−1) CaCl_2), and 0.50 (5.00 mol kg^(−1) CaCl_2). Experiments at aH_2O = 1.00 (0.01 M CaCl_2) were also conducted at 4.0 °C, 25.0 °C, and 45.0 °C to measure an apparent activation energy for the dissolution of nontronite. Apparent dissolution rate constants at 25.0 °C in CaCl_2-containing solutions decrease with decreasing activity of water as follows: 1.18 × 10^(−12) ± 9 × 10^(−14) mol mineral m^(−2) s^(−1) (aH_2O = 1.00) > 2.36 × 10^(−13) ± 3.1 × 10^(−14) mol mineral m^(−2) s^(−1) (aH_2O = 0.75) > 2.05 × 10^(−14) ± 2.9 × 10^(−15) mol mineral m^(−2) s^(−1) (aH_2O = 0.50). Similar results were observed at 25.0 °C in NaCl-containing solutions: 1.89 × 10^(−12) ± 1 × 10^(−13) mol mineral m^(−2) s^(−1) (aH_2O = 1.00) > 1.98 × 10^(−13) ± 2.3 × 10^(−14) mol mineral m^(−2) s^(−1) (aH_2O = 0.75). This decrease in apparent dissolution rate constants with decreasing activity of water follows a relationship of the form: log k_(diss) = 3.70 ± 0.20 × aH_2O − 15.49, where k^(diss) is the apparent dissolution rate constant, and aH_2O is the activity of water. The slope of this relationship (3.70 ± 0.20) is within uncertainty of that of other minerals where the relationship between dissolution rates and activity of water has been tested, including forsteritic olivine (log R = 3.27 ± 0.91 × aH_2O − 11.00) (Olsen et al., 2015) and jarosite (log R = 3.85 ± 0.43 × aH_2O − 12.84) (Dixon et al., 2015), where R is the mineral dissolution rate. This result allows prediction of mineral dissolution as a function of activity of water and suggests that with decreasing activity of water, mineral dissolution will decrease due to the role of water as a ligand in the reaction. Apparent dissolution rate constants in the dilute NaCl solution (1.89 × 10^(−12) ± 1 × 10^(−13) mol mineral m^(−2) s^(−1)) are slightly greater than those in the dilute CaCl_2 solutions (1.18 × 10^(−12) ± 9 × 10^(−14) mol mineral m^(−2) s^(−1)). We attribute this effect to the exchange of Na with Ca in the nontronite interlayer. An apparent activation energy of 54.6 ± 1.0 kJ/mol was calculated from apparent dissolution rate constants in dilute CaCl_2-containing solutions at temperatures of 4.0 °C, 25.0 °C, and 45.0 °C: 2.33 × 10^(−13) ± 1.3 × 10^(−14) mol mineral m^(−2) s^(−1) (4.0 °C), 1.18 × 10^(−12) ± 9 × 10^(−14) mol mineral m^(−2) s^(−1) (25.0 °C), and 4.98 × 10^(−12) ± 3.8 × 10^(−13) mol mineral m^(−2) s^(−1) (45.0 °C). The greatly decreased dissolution of nontronite in brines and at low temperatures suggests that any martian nontronite found to be perceptibly weathered may have experienced very long periods of water–rock interaction with brines at the low temperatures prevalent on Mars, with important implications for the paleoclimate and long-term potential habitability of Mars.