Chlorine partitioning between granitic melt and H2O-CO2-NaCl fluids in the Earth’s upper crust and implications for magmatic-hydrothermal ore genesis

Abstract

Carbon dioxide is one of the most abundant volatile components in magmas after H2O along with S and Cl, which are of great importance to the extraction of trace metals into magmatic-hydrothermal fluids and ore minerals. Yet the effect of CO2 on the partition coefficients of chlorine between water-rich fluid and melt (i.e. the ratio of mass Cl concentrations in the corresponding phase; [DClfluid/melt]) is still poorly constrained. We conducted a set of experiments to constrain the effect of CO2 on DClfluid/melt by equilibrating felsic silicate melts with aqueous NaCl-bearing fluids while varying the concentration of CO2 at pressures between 120 and 300 MPa and temperatures of 850 and 1000 °C. The starting melt was synthetized as a glass in the ternary albite-quartz-Al2O3 system to ensure the only significant metal chloride species in the equilibrium fluid phase was NaCl. The results demonstrate that DClfluid/melt values increase with the concentration of Cl in the fluid phase and the silicate melt. The addition of CO2 into aqueous metal chloride-bearing fluids induces a pronounced drop in DClfluid/melt, the extent of which is only weakly affected by pressure, temperature and fluid salinity, at least at relatively low Cl concentrations (<12 wt% NaCl equivalent). The values of DClfluid/melt drop by approximately a factor of 3 in response to the addition of 20–25 mol% CO2 to the fluid phase due to the decreased ability of the fluid to hydrate NaCl ion pairs. A new empirical equation describing wt%-based DClfluid/melt is derived: