H₂O–CO₂ solubility in alkali-rich mafic magmas: new experiments at mid-crustal pressures

Volatile solubility in magmas depends on several factors, including composition and pressure. Mafic magmas with high concentrations of alkali elements are capable of dissolving larger quantities of H₂O and CO₂ than subalkaline basalt, which possibly contributes to large explosive eruptions. Existing volatile solubility models for alkali-rich mafic magmas are well calibrated below ~ 200 MPa, but at greater pressures the experimental data are sparse. To fill in this gap, we conducted a set of mixed H₂O–CO₂ volatile solubility experiments between 400 and 600 MPa at 1200 °C in six mafic compositions with variable alkali contents (Stromboli, Etna, Vesuvius, Erebus, Sunset Crater, and the San Francisco Volcanic Field). Results from our experiments indicate that existing volatile solubility models for alkali-rich mafic magmas, if extrapolated beyond their calibrated ranges, do not accurately describe CO₂ solubility at mid-crustal pressures. We adapt an existing thermodynamic model to reflect our higher-pressure experimental data by determining model parameters ΔVr0,m (partial molar volume change of CO₂ reaction) and K₀ (equilibrium constant) for each studied composition. In these compositions, ΔVr0,m is found to vary between ~ 15 and ~ 25 cm³ mol⁻¹, while ln K₀ ranges from − 14.9 to − 14.0. The calculated solubility curves show good agreement with CO₂ solubility data from the experiments and provide a more accurate description of CO₂ solubility than purely empirical fits. These new experiments indicate while CO₂ solubility is increased in alkali-rich mafic magmas, it is not simply controlled by total alkali content but rather the full multicomponent magma composition.