ABSTRACT

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 (CO_2mol) and carbonate anions (CO₃^2–) are dissolved in the quenched rhyolitic glasses based on infrared spectroscopy. This result contrasts with the general understanding that high-SiO₂ rhyolitic melt dissolves CO_2mol only. The concentrations of CO_2mol and CO₃^2– 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 CO_2mol and CO₃^2–, respectively. The water content ranged from 2.6 to 6.1 wt%. Using the thermodynamic analysis, we calculated the partial molar volume of CO_2mol to be = 24.9 ± 2.0 cm³/mol and enthalpy of dissolution to be Δ_slnH = –22.2 ± 6.3 kJ/mol. Changes in volume and enthalpy upon the formation reaction of CO₃^2– were calculated to be Δ_rV = –8.6 ± 0.9 cm³/mol and Δ_rH = +1.1 ± 4.4 kJ/mol, respectively.