A CO2 solubility model for silicate melts from fluid saturation to graphite or diamond saturation

A model based on a thermodynamic framework for CO2 concentrations and speciation in natural silicate melts at graphite/diamond-saturated to fluid-saturated conditions is presented. The model is simultaneously calibrated with graphite-saturated and fluid-saturated conditions allowing for consistent model predictions across the CCO buffer. The model was calibrated using water-poor (≤1 wt% H2O) silicate melts from graphite- to CO2-fluid-saturation over a range of pressure (P = 0.05-3 GPa), temperature (T = 950-1600 °C), composition (foidite-rhyolite; NBO = 0.02-0.92; wt% SiO2 ~ 39-77, TiO2 ~ 0.1-5.8, Al2O3 ~ 7.5-18, FeO ~ 0.2-24 MgO ~ 0.1-24, CaO ~ 0.3-14, Na2O~1-5, K2O ~ 0-6), and fO2 (~QFM +1.5 to ~QFM −6). The model can predict CO2 concentrations for a wide range of silicate melt compositions from ultramafic to rhyolitic compositions, i.e., melts that dissolve carbon only as carbonate anions CO32- and those that dissolve carbon both as CO32- and as molecular CO2mol as a function of pressure, temperature, and oxygen fugacity. The model also does a reasonable job in capturing CO2 solubility in hydrous silicate melts with ≤2-3 wt% H2O. New CO2 solubility experiments at pressures >3 GPa suggest that the newly developed CO2 solubility model can be satisfactorily extrapolated to ~4-5 GPa. Above 5 GPa the model poorly reproduces experimental data, likely owing to structural change in silicate melt at pressures above 5 GPa. An Excel spreadsheet and a Matlab function are provided as online supplementary materials for implementing the new CO2 solubility model presented here.