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 (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 CO2mol only. The concentrations of CO2mol 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 CO2mol 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 CO2mol to be VCO mol melt 2 = 24.9 ± 2.0 cm3/mol and enthalpy of dissolution to be DslnH = -22.2 ± 6.3 kJ/mol. Changes in volume and enthalpy upon the formation reaction of CO3 2- were calculated to be DrV = -8.6 ± 0.9 cm3/ mol and DrH = +1.1 ± 4.4 kJ/mol, respectively.
ASJC Scopus subject areas
- Geochemistry and Petrology