The compositional dependence of the saturation surface of H2O+CO2 fluids in silicate melts

The volatile saturation surface in H2O-CO2-silicate melt systems is modeled by applying thermodynamic equilibrium between gaseous and liquid volatile components. The whole database of existing saturation data in the C-O-H-silicate liquid systems has allowed us to re-calibrate a previously developed fully multicomponent H2O-CO2 saturation model [Papale, P., 1999. Modeling of the solubility of a two-component H2O + CO2 fluid in silicate liquid. Am. Mineral., 84, 477-492]. The new database nearly doubles the previous one, greatly improving the performances of the whole model, which now adopts a significantly lower number of model parameters with respect to the previous calibration. The multicomponent H2O + CO2 saturation model is fully non-ideal, the only assumption being that the excess Gibbs free energy of the silicate mixture can be represented by an expansion of first-order symmetric interaction terms. No a-priori assumption is made on the P-T dependence of the volatile-oxide interaction terms, meaning that no assumption is made on the partial molar volume and enthalpy of the dissolved volatiles. The whole treatment is evaluated by restrictive statistical algorithms, which confirm the model validity on an extended database. The model allows to investigate extensively the dependence of the complex volatile saturation surface on composition. In order to explore the non-linear behaviors implicit in the physics of the dissolution process, the model is employed in a series of calculations aimed at illustrating some of the compositional features of the volatile saturation surface in both one-component and two-component volatile conditions. The results show compositional-dependent minima and maxima, some of which are known from the experiments. Non-ideal behavior is enhanced in two-component fluid phase conditions and pressures above a few hundreds MPa, where calculated isobaric H2O-CO2 saturation curves reveal the possible existence of a maximum in CO2 saturation at non-zero H2O contents. Due to the compositional dependence of the volatile saturation surface, it is outlined the important role played by redox conditions, especially in iron-rich melt systems like basalts.