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Solubility of CO2 in a Ca-rich leucitite: effects of pressure, temperature, and oxygen fugacity

Research paper by Yves Thibault, John R. Holloway

Indexed on: 01 Mar '94Published on: 01 Mar '94Published in: Contributions to mineralogy and petrology. Beitrage zur Mineralogie und Petrologie



Abstract

The solubility of carbon dioxide in a Ca-rich leucitite has been investigated as a function of pressure (0.1–2.0 GPa), temperature (1200–1600°C), and oxygen fugacity. The experiments were done in a rapid-quench internally-heated pressure vessel (0.1 GPa) and a piston cylinder (0.5–2.0 GPa). The leucitite glass, previously equilibrated at NNO, and silver oxalate were loaded in Fe-doped Pt capsules (oxidized conditions) and graphitelined Pt capsules (reduced conditions). Secondary Ion Mass Spectrometry and bulk carbon analyses were used to determine the amount of dissolved carbon. Speciation of carbon was characterized by Fourier transform microinfrared spectroscopy. At oxidized conditions, only CO32- is observed as a dissolved species. The solubility is high with CO2 contents in the melt attaining 6.2 wt% at 2.0 GPa and 1350°C. The solubility increases with pressure and shows a significant negative temperature dependence. An excellent correlation is obtained when the data are fit to a model, based on the simplified solubility reaction CO2(vapor)+O2-(melt)⇒CO32-(melt), which describes the solubility of CO2 as a function of pressure, temperature and fCO2. At reduced conditions, the amount of carbon dissolved is significantly lower, and CO32- is still the only species present in the melt. If the solubility model established at oxidized conditions is applied, the carbon dissolved appears to be essentially a function of fCO2 alone although divergence increases in a consistent manner with pressure and temperature. This could suggest a low but significant solubility of CO with a positive temperature dependence or a departure of the calculated fluid compositions determined by the equation of state from the actual ones. The strong preferential solubility of carbon in its oxidized C4+ form, even at reduced conditions, implies that ascending melts with high CO2 solubility can experience significant oxidation through degassing. This could reconcile the oxidized nature of some Ca-rich alkaline magmas with more reduced mantle source regions.