¹M.H. Steiner, ¹E.M. Hausrath, ²M.E. Elwood Madden, ¹O. Tschauner, ³B.L. Ehlmann, ⁴A.A. Olsen, ¹S.R. Gainey, ⁵J.S. Smith
Geochimica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2016.08.035]
¹Department of Geoscience, University of Nevada, Las Vegas 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010
²School of Geology and Geophysics, University of Oklahoma, 100 E Boyd, Suite 710, Norman, OK 73019
³Division of Planetary Science, California Institute of Technology, 1200 East California Boulevard Pasadena, CA 91125
⁴Department of Earth Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, ME 04469
⁵HPCAT, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439
Copyright Elsevier

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 (aH2O) of 1.00 (0.01 M CaCl2 or NaCl), 0.75 (saturated NaCl or 3.00 mol kg-1 CaCl2), and 0.50 (5.00 mol kg-1 CaCl2). Experiments at aH2O = 1 (0.01 M CaCl2) 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 CaCl2-containing solutions decrease with decreasing activity of water as follows: 1.18×10-12 ± 9 x 10-14 moles mineral m-2 s-1(aH2O = 1)> 2.36 x 10-13 ± 3.1 x 10-14 moles mineral m-2 s-1(aH2O = 0.75)> 2.05 x 10-14 ± 2.9 x 10-15 moles mineral m-2 s-1 (aH2O = 0.50). Similar results were observed at 25.0 °C in NaCl-containing solutions : 1.89 x 10-12 ± 1 x 10-13 moles mineral m-2 s-1 (aH2O = 1)> 1.98 x 10-13 ± 2.3 x 10-14 moles mineral m-2 s-1(aH2O = 0.75). This decrease in apparent dissolution rate constants with decreasing activity of water follows a relationship of the form: log kdiss = 3.70 ± 0.20 x aH2O – 15.49, where kdiss is the apparent dissolution rate constant, and aH2O 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 x aH2O – 11.00) ( Olsen et al., 2015)and jarosite (log R = 3.85 ± 0.43 x aH2O – 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 x 10-12 ± 1 x 10-13 moles mineral m-2 s-1) are slightly greater than those in the dilute CaCl2 solutions (1.18 x 10-12 ± 9 x 10-14 moles 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 CaCl2- containing solutions at temperatures of 4.0 °C, 25.0 °C, and 45.0 °C: 2.33×10-13 ± 1.3 x 10-14 moles mineral m-2 s-1(4.0 °C), 1.18 x 10-12 ± 9 x 10-14 moles mineral m-2 s-1(25.0 °C), and 4.98 x 10-12 ± 3.8 x 10-13 moles 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.