2017 Volume 51 Issue 3 Pages 251-262
We carried out equilibrium experiments of the CO2-H2O-rhyolite system at 0.1–1.5 GPa and 850 and 1200°C to examine the solubility and speciation of CO2 in high-SiO2 rhyolite (SiO2 > 76 wt%). We observed that both CO2 molecules (CO2mol) and carbonate anions (CO32–) are dissolved in the quenched rhyolitic glasses based on infrared spectroscopy. This result contrasts with the general understanding that high-SiO2 rhyolitic melt dissolves CO2mol only. The concentrations of CO2mol and CO32– were 199–9200 ppm and 58–2100 ppm, respectively, as quantified based on the Beer-Lambert’s law and newly determined extinction coefficients of 1192 ± 130 L·cm–1·mol–1 and 91 ± 28 L·cm–1·mol–1 for CO2mol and CO32–, respectively. The water content ranged from 2.6 to 6.1 wt%. Using the thermodynamic analysis, we calculated the partial molar volume of CO2mol to be = 24.9 ± 2.0 cm3/mol and enthalpy of dissolution to be ΔslnH = –22.2 ± 6.3 kJ/mol. Changes in volume and enthalpy upon the formation reaction of CO32– were calculated to be ΔrV = –8.6 ± 0.9 cm3/mol and ΔrH = +1.1 ± 4.4 kJ/mol, respectively.