Solubility of carbon dioxide in albitic melt

Abstract

Infrared spectroscopy has been used to measure the concentrations of molecular CO_2, and carbonate in albitic (NaAlSi_3O_8) glasses quenched from melts equilibrated with CO_2, vapor at high pressures (15-30 kbar) and temperatures (1450-1625 °C). At constant temperature, the concentrations of carbonate and molecular CO_2, as well as the CO^2_3/CO_2, ratio increase with increasing pressure under vapor-saturated conditions. The second derivative of these species concentrations with respect to pressure under vapor-saturated conditions at constant temperature is positive over the range of conditions studied. At constant pressure under vapor-saturated conditions, the solubility of molecular CO_2, decreases with increasing temperature, but the concentration of carbonate increases. The net effect is that total CO_2, solubility is nearly independent of temperature. According to our results, CO_2 solubility and speciation change gradually over the range of conditions that we have studied and do not indicate major or abrupt structural changes in albitic melts in this P-Z range. Our results can be described thermodynamically in terms of two reactions. The first, CO_2(vapor) = CO_2,molecular(melt), describes the heterogeneous equilibrium between melt and vapor. The second, CO_2,molecular(melt) + O^2-(melt) = CO^2_3-(melt), models the homogeneous equilibrium between melt species. Volume and enthalpy changes of these two reactions have been constrained by our solubility and speciation data. We emphasize that the solubilities of volatile components that dissolve in melts in several different forms must be treated by such coupled heterogeneous and homogeneous equilibria and that spectroscopic methods provide direct insights into them.

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