CO2 solubility in dacitic melts equilibrated with H2O-CO2 fluids:: Implications for modeling the solubility of CO2 in silicic melts

The solubility of CO2 in dacitic melts equilibrated with H2O-CO2 fluids was experimentally investigated at 1250 C and 100 to 500 MPa. CO, is dissolved in dacitic glasses as molecular CO2 and carbonate. The quantification of total CO2 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 CO2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO2 species concentrations from the MIR spectra we have measured total CO2 contents of dacitic glasses by secondary ion mass spectrometry (SIMS). At all pressures, the dependence of CO2 solubility in dacitic melts on x(CO2,total)(fluid) shows a strong positive deviation from linearity with almost constant CO2 solubility at x(CO2)(fluid) > 0.8 (maximum CO2 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 CO2. A similar nonlinear variation of CO2 solubility with x(CO2)(fluid) has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO2 (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 CO2 molecules. A thermodynamic model describing the dependence of the CO2 solubility in hydrous rhyolitic and dacitic melts on T P f(CO2) and the mol fraction of water in the melt (x(water)) has been developed. An exponential variation of the equilibrium constant K, with x(water) is proposed to account for the nonlinear dependence of x(CO2,total)(melt) on x(CO2)(fluid). The model reproduces the CO2 solubility data for dacitic melts within 14% relative and the data data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low x(CO2)(fluid)). Data obtained for rhyolitic melts at 75 MPa and 850degreesC show a stronger deviation from the model, suggesting a change in the solubility behavior of CO2 at low pressures (a Henrian behavior of the CO2 solubility is observed at low pressure and low H2O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the x(CO2)(fluid) range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO2 is much lower in rhyolitic melts (31.7 cm(3)/mol) than in dacitic melts (46.6 cm3/mol). The dissolution enthalpy for CO2 in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO2 solubility in silicic melts. Copyright (C) 2004 Elsevier Ltd.