OS43A-2013
Anhydrite Solubility and Ca Isotope Fractionation in the Vapor-Liquid Field of the NaCl-H2O System: Implications for Hydrothermal Vent Fluids at Mid-ocean Ridges
Thursday, 17 December 2015
Poster Hall (Moscone South)
Peter Scheuermann1, Drew D Syverson2, John A Higgins3 and William E Seyfried Jr2, (1)University of Minnesota Twin Cities, Earth Sciences, Minneapolis, MN, United States, (2)University of Minnesota Twin Cities, Minneapolis, MN, United States, (3)Princeton University, Princeton, NJ, United States
Abstract:
Hydrothermal experiments were performed at 410, 420 and 450°C between 180-450 bar to investigate anhydrite (CaSO4) solubility and Ca isotope fractionation in the liquid-vapor stability field of the NaCl-H2O system. Experiments were conducted in flexible gold reaction cells and a fixed volume Ti reactor to reach all pressures between the critical curve and three-phase boundary. During isothermal decompression at 410°C, anhydrite solubility in the liquid phase increases (1 to 9 mmol/kg Ca), whereas the solubility decreases in the vapor phase (130 to < 10 umol/kg Ca). At 410°C and 290-270 bar, the partition coefficient, log Km = log (mv / ml), for Ca decreases from -1.35 to -2.46, and that of SO4 decreases from -1.76 to -2.82. At 420°C the Ca:SO4 ratio of the starting solution was 2:1, and the pH25°C decreases in the liquid and increases in the vapor upon decompression. Ca hydrolysis in the liquid and complex interactions between undetermined aqueous species in the vapor could explain this pattern. At 410 and 450°C, the experiments started with a Ca:SO4 ratio of 1:1. Along the 410°C isotherm, pH25°C initially increases in both the liquid and vapor, potentially caused by precipitation of an H+ bearing salt, such as NaHSO4. 30-40 bar below the critical curve there is a sudden decrease in pH25°C as the putative salt phase may become unstable and dissolve. At 450°C, pH25°C decreases in the vapor and increases in the liquid, as HCl and H2SO4 partition into the vapor. Ca isotope data at 420°C between 375-300 bar indicate that the vapor is isotopically light relative to the liquid. At lower pressures both phases approach the isotopic composition of the coexisting anhydrite, suggesting that dissolved Ca speciation becomes more structurally similar to anhydrite. This study furthers our understanding of elemental partitioning and isotopic fractionation in mineral-fluid systems with implications for mass transfer reactions at/near the magma-hydrothermal boundary at mid-ocean ridges.
