Solubility, speciation and dissolution mechanisms for CO2 in melts on the NaAlO2-SiO2 join

CO2 solubility and speciation in melts along the NaAlO2-SiO2 join from Ne ( NaAlSiO4) to a model rhyolite (Ry) composition (NaAlSi6O14), have been inve stigated as a function of pressure (10 to 25 kb) and temperature (1450 to 1 700 degrees C). Quenched glasses have been analysed using LECO bulk carbon analysis in conjunction with Fourier transform infrared (FTIR), Raman, and C-13 Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopi c measurements. In agreement with previous studies, the CO2 solubility was found to increas e as a function of total pressure for the Ne, Jd (NaAlSi2O6), Ab (NaAlSi3O8 ), and "Eu" (NaAlSi4O10) compositions. The temperature dependence of the CO , solubility was also investigated. For the Ne (15 kbar) composition, a sli ght increase in solubility was noted with increasing temperature, whereas a broad minimum in solubility was noted for the Jd composition (15 and 20 kb ar) in the 1550 to 1650 degrees C range. No obvious dependence of the solub ility on run temperature was noted for the high silica compositions Eu to R y at 15 kbar. Infrared (IR) and NMR spectra of the quenched glasses show that the type an d relative amounts of carbon-bearing species change systematically as a fun ction of composition. The relative and absolute abundance of carbonate (CO3 2-) groups increases rapidly with decreasing Si/(Na + Al) ratio, whereas th e relative and absolute abundance of molecular CO2 decreases. The result is that, for a given pressure and temperature, the CO2 solubility remains app roximately constant or decreases slightly with decreasing Si/(Na + Al) rati o between Ry and Jd compositions, but increases rapidly between Jd and Ne c ompositions, at 15 kbar and 1600 degrees C. In the most silica-rich composi tions nearly all the dissolved CO2 is in the form of molecular CO2. The LR and Raman spectra of dissolved molecular CO2 indicate some interaction with the silicate melt structure, which changes over the compositional range st udied. Four different types of dissolved carbonate groups with differing de grees of distortion have been identified by NMR spectroscopy, the relative proportions changing systematically with glass composition. Two of these ca rbonate groups are dominant in silica-poor, carbonate-rich compositions and correlate with distinguishable features in the IR spectra. The structural changes in both the carbonate and the molecular CO2 species as a function o f bulk composition along the join, result in changes in the IR extinction c oefficients for these species. The degree to which the observed quenched gl ass species reflect the situation in the melt at run conditions are discuss ed. Dissolved carbon monoxide (CO) has been identified from the NMR and FTIR sp ectra of glasses for experiments carried out under (unintentionally) reduci ng conditions. The experimental results indicate that considerable caution is required in preparing nominally "CO2-saturated" glasses. The absence of GO-related spectral features can be used to ensure that the experimental P- CO2 is in fact equal to P-tot. Copyright (C) 1999 Elsevier Science Ltd.