Water solubility in haplogranitic melts coexisting with H 2 O-H 2 fluids

Abstract

The water solubility in haplogranitic melts (normative composition Ab39Or32Qz29) coexisting with H2O-H2 fluids at 800 and 950 °C and 1, 2 and 3 kbar vapour pressure has been determined using IR spectroscopy. The experiments were performed in internally heated pressure vessels and the hydrogen fugacity (fH2) was controlled using the double capsule technique and oxygen buffer assemblages (WM and IW). Due to the limited lifetimes of these oxygen buffers the water solubility was determined from diffusion profiles (concentration-distance profiles) measured with IR spectroscopy in the quenched glasses. The reliability of the experimental strategy was demonstrated by comparing the results of short- and long-duration experiments performed with pure H2O fluids. The water solubility in Ab39Or32Qz29 melts equilibrated with H2O-H2 fluids decreases progressively with decreasing fH2O, as fH2 (or XH2) increases in the fluid phase. The effect of H2 on the evolution of the water solubility is similar to that of CO2 or another volatile with a low solubility in the melt and can be calculated in a first approximation with the Burnham water solubility model. Recalculation of high temperature water speciation for AOQ melts coexisting with H2O-H2 fluids at 800 °C, 2 kbar suggests that the concentrations of molecular H2O are proportional to fH2O (calculated using available mixing models), indicating Henrian behaviour for the solubility of molecular H2O in haplogranitic melts.