Solubility of carbon dioxide in albitic melt

Abstract

Infrared spectroscopy has been used to measure the concentrations of molecular CO_2,
\nand carbonate in albitic (NaAlSi_3O_8) glasses quenched from melts equilibrated with CO_2,
\nvapor at high pressures (15-30 kbar) and temperatures (1450-1625 °C). At constant temperature,
\nthe concentrations of carbonate and molecular CO_2, as well as the CO^2_3/CO_2,
\nratio increase with increasing pressure under vapor-saturated conditions. The second derivative
\nof these species concentrations with respect to pressure under vapor-saturated
\nconditions at constant temperature is positive over the range of conditions studied. At
\nconstant pressure under vapor-saturated conditions, the solubility of molecular CO_2, decreases with increasing temperature, but the concentration of carbonate increases. The net
\neffect is that total CO_2, solubility is nearly independent of temperature. According to our
\nresults, CO_2 solubility and speciation change gradually over the range of conditions that
\nwe have studied and do not indicate major or abrupt structural changes in albitic melts
\nin this P-Z range.
\nOur results can be described thermodynamically in terms of two reactions. The first,
\nCO_2(vapor) = CO_2,molecular(melt), describes the heterogeneous equilibrium between melt
\nand vapor. The second, CO_2,molecular(melt) + O^2-(melt) = CO^2_3-(melt), models the homogeneous
\nequilibrium between melt species. Volume and enthalpy changes of these two
\nreactions have been constrained by our solubility and speciation data. We emphasize that
\nthe solubilities of volatile components that dissolve in melts in several different forms
\nmust be treated by such coupled heterogeneous and homogeneous equilibria and that
\nspectroscopic methods provide direct insights into them.