 CARBONATE DISSOLUTION RATES IN HIGH SALINITY BRINES. S.P. Parnell1 and C. M. PhillipsLander1, L.E. McGraw1, and M.E. Elwood Madden1, 1Department of Geology and Geophysics, University of Oklahoma 100 East Boyd Street, Sarkeys Energy Center, Norman, OK. 73019. sheriee@ou.edu, charity.m.lander@ou.edu.    Introduction: Carbonates can provide promising insights into the past aqueous environments on Mars. Calcite and magnesite have been observed in SNC meteorite ALH84001 [1], the Phoenix TEGA instrument produced results consistent with carbonates [2,3], rover spectra consistent with carbonate-rich rocks have been observed at Home Plate in Gusev Crater [4] , and spectra indicative of carbonates have been collected by orbiters from other locations on the surface of Mars [56].   However, global scale carbonate deposits similar to those observed on Earth are not evident on Mars, suggesting that conditions on Mars either prevented widespread carbonate precipitation or acid weathering subsequently dissolved carbonates at the surface [7-9]. Abundant salt deposits suggest that brines have been active throughout Mars' history, and may continue to be active today [10]. Recent experiments show that mineral dissolution rates, as well as reaction products, may vary significantly in briny systems [11-15]. Here we investigate carbonate mineral (calcite and magnesite) dissolution rates in high salinity chloride and sulfate brines to determine the effects of high salinity brines on carbonate mineral preservation and reaction products.  Methods: Optical calcite (CaCO3) obtained from OU's teaching minerals collection and magnesite (MgCO3) obtained from Bramado, Brazil via Excalibur Minerals were crushed, sonicated, washed, and dried. Samples were characterized using BET to determine surface area, and electron microprobe and X-ray diffraction was used to determine mineral composition. Batch dissolution experiments were conducted in which calcite or magnesite was added at a ratio of 1 g mineral: 1 L-1 solution. Solutions utilized included 18 MΩ ultra-pure (UPW) with an activity of water (aw)=1 and four brine solutions: 194 g kg-1 MgSO4 (aw=0.97), 174 g kg-1 Na2SO4 (aw=0.96), 333 g kg-1 NaCl (aw=0.75), and 1000 g kg-1 CaCl2 (aw=0.35). Calcite dissolution was not measured in CaCl2 brine. Magnesite dissolution was not measured in MgSO4.Triplicate experiments were conducted over 52 days at room temperature, with samples of the mineral-solution slurry collected at regular intervals, filtered, and preserved for further analysis using Atomic Absoprtion Spectroscopy to determine Ca and Mg concentrations released to solution over time. We fit the solute curves and took the first derivative of the fit to determine the initial dissolution rate in each solution. We then divided the initial rate by the BET-derived surface area values, to calculate surface area normalized dissolution rates (Figure 1). Carbonate mineral lifetimes were calculated using a shrinking sphere model [11,16]:   where Δt is the lifetime of the particle (sec), d is the diameter of the particle (m; assumed 0.001 m), Vm is the molar volume of the mineral (m3 mol−1), and r is the rate of dissolution (mol m−2 s−1). Calcite and Magnesite Dissolution Rates: In our experiments, calcite dissolved ~10 times faster than magnesite in UPW. However, calcite and magnesite showed similar dissolutions rates in NaCl and Na2SO4 brine experiments. As activity of water decreases with higher salinity, dissolution rates also decrease. This suggests that water exchange reactions likely play a significant role in determining carbonate dissolution rates [17]. Both Cl- and SO42- may also influence carbonate dissolution rates by forming aqueous complexes with Ca2+ and Mg2+[18], thus partially countering the decrease in rate due to lower activity of water. Future experiments at lower ionic strength will further test this hypothesis.  Implications for Mars: Since calcite dissolves faster than magnesite in UPW (Figure 1 and [19]), magnesite particles are expected to persist longer than calcite particles of equal size in dilute systems (Figure 1). Therefore, if Mars experienced periods of brief water activity, magnesite might remain preserved on the surface for >80 years, while calcite would have been relatively quickly dissolved (1 mm particle lifetime <20 years). However, similar dissolution rates in high salinity brines result in similar particle lifetimes for both mineral phases. For example, 1mm diameter particles of both calcite and magnesite are expected to fully dissolve after ~40 years in saturated Na2SO4 brine and ~135 years of alteration in saturated NaCl brines. Magnesite particles may be preserved for up to 400 years in CaCl2 brine. Therefore, while acidic solutions would rapidly dissolve carbonate deposits on the surface, neutral or alkaline brines may have slowed carbonate dissolution, preserving some deposits for hundreds to thousands of years. Carbonates sporadically distributed across Mars' surface can link climate and aqueous conditions,  providing evidence of near-surface chemical weathering processes.     Figure 1. Upper graph shows calcite and magnesite initial dissolution rates in brines with different water activities (aw). Calcite dissolves ~10 times faster in dilute solutions, but similar dissolution rates are observed in the high salinity brines. This leads to similar participle lifetimes (lower graph) of 30-500 years.   Acknowledgements: This work was supported by our funders, NASA MFRP grant #NNX13AG75G and the University of Oklahoma School of Geology and Geophysics.  References: [1] Bridges (2001) Space Science Reviews. [2] Boynton, et al. (2009) Science, 325, 61-64.   [3] K.M Cannon, et al. (2012) LPSC Abstract #20120000902. [4] Morris, R. et al. (2010). Science, 421-424. [5] Ehlmann, B. L. et al. (2008). Science, , 1828-1832. [6] Michalski, J. R., & Niles, P. B. (2010). Nature Geoscience, 751-755. [7] Fairén, A. et al. (2004). Nature, 423-426. [8] Halevy, I (2007). Science, 1903-1907. [9] M. A. Bullock, J. M. Moore, (2007). Geophys. Res. Lett. 34, L19201. [10] Ojha L. et al. (2015) Nat. Geosci. 8, 82-83. [11] Pritchett et al. (2012) EPSL 357-358, 327-336. [12] Dixon et al. (2015) JGR DOI: 10.1002/2014JE004779 [13] Hausrath and Brantley (2010) JGR Planets, 115, E12001 [14] Olsen et al. (2015) JGR Planets, 120, 388-400. [15] Steiner et al. (2015)LPSC 2350. 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