The Effect of Water on the Solubility and Speciation of CO2 in Aluminosilicate Glasses Along the Join SiO2-NaAlO2

Abstract

C O 2 and H20 are the most important volatiles in the majority of magmatic systems and although the C02 is usually subordinate in concentration, it is often the first component to reach saturation due to the much higher solubility of water in the melt. C02 vesiculation can therefore be the driving force in many eruptive situations and it is important to investigate quantitatively the effect that varying amounts of water have on CO2 saturation. A number of previous studies have suggested that the solubility of CO2 is greater in hydrous melts than in the anhydrous equivalents. This effect has been observed for a variety of polymerized and depolymerized compostions at high pressures (e.g. Brey and Green, 1975; Mysen, 1976). For albite and other simple sodium aluminosilicates, Mysen (1976) suggested that there is a maximum in the solubility as a function of the molar CO2/CO2 + H20 ratio in the capsule. In contrast, Blank et al. (1993), studying the solubility of CO2 in rhyolite at much lower pressures and temperatures found that the solubility of CO2 was not enhanced by the presence of water. The aim of this study is to examine the effect of water on the solubility and speciation of CO2 in albite and jadeite melts. The speciation information is important if changes in the physical properties of magmas under such conditions are to be predicted. Albite is the main composition studied as there is considerable data available on the solubility and speciation of the pure components (e.g. Stolper et al., 1987; Kohn et al., 1989; Silver and Stolper, 1989) as well as theoretical predictions of solubility mechanisms. In addition, our data will provide a direct test of the conclusions of Mysen (1976).