TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 51 (No. 3), pp. 251-262, 2017

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

Shumpei Yoshimura,1* Michihiko Nakamura2 and Hisayoshi Yurimoto3,4

1Department of Earth and Planetary Sciences, Hokkaido University, Sapporo 060-0810, Japan
2Department of Earth Science, Tohoku University, Sendai 980-8578, Japan
3Department of Natural History Sciences, IIL, Hokkaido University, Sapporo 001-0021, Japan
4ISAS/JAXA, Sagamihara 252-5210, Japan

(Received June 27, 2016; Accepted December 27, 2016)

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 (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.
Key words: rhyolite, CO2, solubility, species

*Corresponding author E-mail: shumpyos@sci.hokudai.ac.jp

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