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

Steiner, M. H. and Hausrath, E. M. and Madden, M. E. Elwood and Tschauner, O. and Ehlmann, B. L. and Olsen, A. A. and Gainey, S. R. and Smith, J. S. (2016) Dissolution of nontronite in chloride brines and implications for the aqueous history of Mars. Geochimica et Cosmochimica Acta, 195 . pp. 259-276. ISSN 0016-7037.

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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.

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Ehlmann, B. L.0000-0002-2745-3240
Additional Information:© 2016 Elsevier Ltd. Received 26 April 2016; accepted in revised form 27 August 2016; Available online 4 September 2016. We would like to acknowledge the Mars Fundamental Research Program grant NNX12AH96G, the UNLV Faculty Opportunity Award, and the UNLV Graduate and Professional Student Association for travel funding. The authors would like to thank Chris Adcock, Kirellos Sefein, Valerie Tu, Renee Schofield, Courtney Bartlett, and Angela Garcia for insightful conversation and lab assistance, and Minghua Ren and Michael Strange for aid in FE-SEM imaging. Part of the work was conducted at the HPCAT (Sector 16) of the Advanced Photon Source (APS), Argonne National Laboratory and the 12.2.2 beamline of the Advanced Light Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. ALS beamline 12.2.2 is partially supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 11-57758. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. This work was in part supported by the National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Cooperative Agreement #DE-NA0001982. We also appreciate thoughtful reviews by four anonymous reviewers that greatly strengthened the paper.
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University of Nevada, Las VegasUNSPECIFIED
Department of Energy (DOE)DE-AC02-06CH11357
Department of Energy (DOE)DE-AC02-05CH11231
NSFEAR 11-57758
Department of Energy (DOE)DE-NA0001974
Department of Energy (DOE)DE-FG02-99ER45775
Department of Energy (DOE)DE-NA0001982
Subject Keywords:Mars; Nontronite; Clay minerals; Dissolution; Brines; Habitability; Chlorides
Record Number:CaltechAUTHORS:20161104-091802689
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Official Citation:M.H. Steiner, E.M. Hausrath, M.E. Elwood Madden, O. Tschauner, B.L. Ehlmann, A.A. Olsen, S.R. Gainey, J.S. Smith, Dissolution of nontronite in chloride brines and implications for the aqueous history of Mars, Geochimica et Cosmochimica Acta, Volume 195, 15 December 2016, Pages 259-276, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71736
Deposited By: Tony Diaz
Deposited On:04 Nov 2016 16:54
Last Modified:03 Apr 2019 19:09

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