Nontronite dissolution rates and implications for Mars

The Fe-rich smectite nontronite M-1.05(+)[Si(6.9)8Al(1.02)][Al0.29Fe3.68Mg0.04]O-20(OH)(4) has been detected using orbital data at multiple locations in ancient terrains on Mars, including Mawrth Vallis, Nilli Fossae, north of the Syrtis Major volcanic plateau, Terra Meridiani, and the landing site of the Mars Science Laboratory (MSL), Gale Crater. Given the antiquity of these sites (>3.0 Ga), it is likely that nontronite has been exposed to the martian environment for long periods of time and therefore provides an integrated record of processes in near surface environments including pedogenesis and diagenesis. In particular, nontronite detected at Mawrth Vallis is overlain by montmorillonite and kaolinite, and it has been previously suggested that this mineralogical sequence may be the result of surface weathering. In order to better understand clay mineral weathering on Mars, we measured dissolution rates of nontronite in column reactors at solution pH values of 0.9, 1.7, and 3.0, and two flow rates (0.16 ml/h and 0.32 ml/h). Solution chemistry indicates stoichiometric dissolution at pH = 0.9 and non-stoichiometric dissolution at pH = 1.7 and 3.0. Mineral dissolution rates based on elemental release rates at pH = 1.7 and 3.0 of Ca, Si and Fe follow the order interlayer > tetrahedral > octahedral sites, respectively. The behavior of all experiments suggests far from equilibrium conditions, with the exception of the experiment performed at pH 3.0 and flow rate 0.16 ml/h. A pH-dependent dissolution rate law was calculated through Si release from experiments that showed no dependence on saturation (far from equilibrium conditions) under both flow rates and is r = 10(-12.06 (+/-0.123)) . 10(-0.297 (+/-0.058).pH) where r has the units mol mineral m(-2) s(-1). When compared to dissolution rates from the literature, our results indicate that nontronite dissolution is significantly slower than dissolution of the primary phases present in basalt under acidic conditions, suggesting that once nontronite forms it could remain stable at or near the surface of Mars for extended periods of time. Nontronite dissolution rates are faster than dissolution rates of montmorillonite (Rozalen et al., 2008) and kaolinite (Huertas et al., 1999), suggesting that chemical weathering of a mixed clay deposit would enrich the proportions of montmorillonite and kaolinite through the preferential dissolution of nontronite. Vis-NIR analyses of our reacted products and thermodynamic modeling of our experimental conditions both indicate the precipitation of amorphous silica within columns, and amorphous silica has also been observed in association with phyllosilicates on the martian surface (Mustard et al., 2008; Ehlmann et al., 2009; Murchie et al., 2009a). Therefore, chemical weathering of strata containing mixtures of montmorillonite, nontronite and kaolinite provides a potential formation mechanism for the mineralogic stratigraphy observed at Mawrth Vallis and other locations on Mars. (C) 2013 Elsevier Ltd. All rights reserved.