MODELING OF THE SOLUBILITY OF A ONE-COMPONENT H2O OR CO2 FLUID IN SILICATE LIQUIDS

The modelling of the solubility of water and carbon dioxide in silicat e liquids (flash problem) is performed by assuming mechanical, thermal , and chemical equilibrium between the liquid magma and the gas phase. The liquid phase is treated as a mixture of ten silicate components H2O or CO2, and the gas phase as a pure H2O or CO2. A general model f or the solubility of a volatile component in a liquid is adopted. This requires the definition of a mixing equation for the excess Gibbs fre e energy of the liquid phase and an appropriate reference state for th e dissolved volatile. To constrain the model parameters and identify t he most appropriate form of the solubility equations for each dissolve d volatile, a large number of experimental solubility determinations ( 640 for H2O and 263 for CO2) have been used. These determinations cove r a large region of the P-T-composition space of interest. The resulta nt water and carbon dioxide solubility models differ in that the water model is regular and isometric, and the carbon dioxide model is regul ar and non-isometric. This difference is consistent with the different speciation modalities of the two volatiles in the silicate liquids, p roducing a composition-independent partial molar volume of dissolved w ater and a composition-dependent partial molar volume of dissolved car bon dioxide. The H2O solubility model may be applied to natural magmas of virtually any composition in the P-T range 0.1 MPa-1 GPa and > 100 0 K, whereas the CO2 solubility model may be applied to several GPa pr essures. The general consistency of the water solubility data and thei r relatively large number as compared to the calibrated model paramete rs(11) contrast with the large inconsistencies of the carbon dioxide s olubility determinations and their low number with respect to the CO2 model parameters (22). As a result, most of the solubility data in the database are reproduced within 10% of approximation in the case of wa ter, and 30% in the case of carbon dioxide. When compared with the exp erimental data, the H2O and CO2 solubility models correctly predict ma ny features of the saturation surface in the P-T-composition space, in cluding the change from retrograde to prograde H2O solubility in albit ic liquids with increasing pressure, the so-called alkali effect, the increasing CO2 solubility with increasing degree of silica undersatura tion, the Henrian behavior of CO2 in most silicate liquids up to about 30-50 MPa, and the proportionality between the fugacity in the gas ph ase, or the saturation activity in the liquid phase, and the square of the mole fraction of the dissolved volatile found in some unrelated s ilicate liquid compositions.