Hydrogen-alkali exchange between silicate melts and two-phase aqueous mixtures: an experimental investigation

Experiments were performed in the three-phase system high-silica rhyolite melt + low-salinity aqueous vapor + hydrosaline brine, to investigate the exchange equilibria for hydrogen, potassium, and sodium in magmatic-hydrothermal systems at 800 degrees C and 100 MPa, and 850 degrees C and 50 MPa. The K-H,K-Na (aqm/melt) and K-H,K-K (aqm/melt) for hydrogen-sodium exchange between a vapor pro + brine mixture and a silicate melt are inversely proportional to the total chloride concentration (Sigma Cl) in the vapor + brine mixture indicating that HCl/NaCl and HCl/KCl are higher in the low-salinity aqueous vapor relative to high-salinity brine. The equilibrium constants for vapor/melt and brine/melt exchange were extracted from regressions of K-H,K-Na (aqm/melt) and K-H,K-K (aqm/melt) versus the proportion of aqueous vapor relative to brine in the aqueous mixture (F-aqv) at P and T, expressed as a function of Sigma Cl. No significant pressure effect on the empirically determined exchange constants was observed for the range of pressures investigated. Model equilibrium constants are: K-H,K-Na (aqv/melt) (vapor/melt) = 26(+/-1.3) at 100 MPa (800 degrees C), and 19(+/-7.0) at 50 MPa (850 degrees C); K-H,K-K (aqv/melt) = 14(+/-1.1) at 100 MPa (800 degrees C), and 24(+/-12) at 50 MPa (850 degrees C); K-H,K-b (aqb/melt) (brine/melt) = 1.6(+/-0.7) at 100 MPa (800 degrees C), and 3.9(+/-2.3) at 50 MPa (850 degrees C); and K-H,K-K (aqb/melt) = 2.7(+/-1.2) at 100 MPa (800 degrees C) and 3.8(+/-2.3) at 50 MPa (850 degrees C). Values for K-H,K-K (aqv/melt) and K-H,K-K (aqb/melt) were used to calculate KCl/HCl in the aqueous vapor and brine as a function of melt aluminum saturation index (ASI: molar Al2O3/(K2O + Na2O + CaO)) and pressure. The model log KCl/HCl values show that a change in melt ASI from peraluminous (ASI = 1.04) to moderately metaluminous (ASI = 1.01) shifts the cooling pathway (in temperature-log KCl/HCl space) of the aqueous vapor toward the andalusite + muscovite + K-feldspar reaction point.