open access

https://hdl.handle.net/2115/76588

Carbonate ions in high-SiO2 rhyolite observed in fluid-melt equilibrium experiments

英語

species

2017-03-22

Geochemical Society of Japan

Geochemical Journal

51(3)

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 (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.

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