Albite dissolution rates in brines: Implications for late-stage weathering on Mars

Plagioclase minerals have been detected over large portions of the Mars surface. Average plagioclase mineral compositions are approximately An(60) for dust free areas on Mars; however, these compositions likely reflect a mixture of more and less sodic plagioclase minerals at the surface. Plagioclase minerals have also been observed in association with chloride and sulfate salts on the Mars surface and in meteorites. Understanding plagioclase dissolution in brines provides insight into post-Noachian weathering on Mars. Batch reactor dissolution experiments were conducted at 298 K to compare albite dissolution rates in water (18 M Omega cm (-1)ultrapure water (UPW) adjusted to pH 2 with H2SO4; activity of water (alpha H2O) = 1.0), 2.7 mol kg(-1) MgSO4 (aH(2)O = 0.92), 1.24 mol kg(-1) MgCl2 (aH(2)O = 0.92), 2.9 mol kg(-1) MgCl2 (aH(2)O = 0.75), and 5.8 mol kg(-1) MgCl2 (aH(2)O = 0.33) brines at pH 2 to determine how changing solution chemistry and activity of water influence albite dissolution. Aqueous Si-based dissolution rates indicate albite dissolution rates decreased from -8.80 +/- 0.02 to 9.49 +/- 0.40 log mol m(-2) s(-1) as the activity of water decreased from 1 to 0.33. Na-based dissolution rates followed the same trend, decreasing from -8.58 +/- 0.04 in UPW to -9.45 +/- 0.15 log mol m(-2) s(-1) in 2.9 mol kg(-1) MgCl2. Anion chemistry did not appear to effect albite dissolution in high salinity brines at acid pH. Estimated 1 mm albite grain lifetimes increase from similar to 60-100 to similar to 587 of years as activity of water decreases, suggesting that post-Noachian weathering on Mars was limited in duration and/or extent.