CO 2 solubility in dacitic melts equilibrated with H 2O-CO 2 fluids: Implications for modeling the solubility of CO 2 in silicic melts

Abstract

The solubility of CO 2 in dacitic melts equilibrated with H 2O-CO 2 fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO 2 is dissolved in dacitic glasses as molecular CO 2 and carbonate. The quantification of total CO 2 in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm −1 can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO 2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO 2 species concentrations from the MIR spectra we have measured total CO 2 contents of dacitic glasses by secondary ion mass spectrometry (SIMS). At all pressures, the dependence of CO 2 solubility in dacitic melts on x fluid CO 2, total shows a strong positive deviation from linearity with almost constant CO 2 solubility at x CO 2 fluid > 0.8 (maximum CO 2 solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO 2. A similar nonlinear variation of CO 2 solubility with x CO 2 fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO 2 ( Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO 2 molecules. A thermodynamic model describing the dependence of the CO 2 solubility in hydrous rhyolitic and dacitic melts on T, P, f CO 2 and the mol fraction of water in the melt ( x water ) has been developed. An exponential variation of the equilibrium constant K 1 with x water is proposed to account for the nonlinear dependence of x CO 2, total melt on x CO 2 fluid. The model reproduces the CO 2 solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100–500 MPa (except for six outliers at low x CO 2 fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO 2 at low pressures (a Henrian behavior of the CO 2 solubility is observed at low pressure and low H 2O concentrations in the melt). We recommend to use our model only in the pressure range 100–500 MPa and in the x CO 2 fluid range 0.1–0.95. The thermodynamic modeling indicates that the partial molar volume of total CO 2 is much lower in rhyolitic melts (31.7 cm 3/mol) than in dacitic melts (46.6 cm 3/mol). The dissolution enthalpy for CO 2 in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO 2 solubility in silicic melts.