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
xfluidCO2,total shows a strong positive deviation from linearity with almost constant CO
2 solubility at
xCO2fluid > 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
xCO2fluid 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,
fCO2 and the mol fraction of water in the melt (
xwater) has been developed. An exponential variation of the equilibrium constant K
1 with
xwater is proposed to account for the nonlinear dependence of
xCO2,totalmelt on
xCO2fluid. 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
xCO2fluid). 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
xCO2fluid 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.
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