New experimental data and semi-empirical parameterization of H₂O–CO₂ solubility in mafic melts

We present here new experimental data on H₂O–CO₂ solubility in mafic melts with variable chemical compositions (alkali basalt, lamproite and kamafugite) that extend the existing database. We show that potassium and calcium-rich melts can dissolve ∼1wt% CO₂ at 3500bar (350MPa) and 1200°C, whereas conventional models predict solubilities of 0.2–0.5wt%, under similar P–T conditions. These new data, together with those in the literature, stress the fundamental control of melt chemical composition on CO₂ solubility. We present a semi-empirical H₂O–CO₂ solubility model for mafic melts, which employs simplified concepts of gas–melt thermodynamics coupled with a parameterization of both chemical composition and structure of the silicate melt. The model is calibrated on a selected database consisting of 289 experiments with 44 different mafic compositions. Statistical analyses of the experimental data indicate that, in mafic melts, the chemical composition and therefore the structure of the melt plays a fundamental role in CO₂ solubility. CO₂ solubility strongly depends on the amount of non-bridging oxygen per oxygen (NBO/O) in the melt, but the nature of the cation bonded to NBO is also critical. Alkalis (Na+K) bonded to NBO result in a strong enhancement of CO₂ solubility, whereas Ca has a more moderate effect. Mg and Fe bonded to NBO have the weakest effect on CO₂ solubility. Finally, we modelled the effect of water and concluded that H₂O dissolution in the melt enhances CO₂ solubility most likely by triggering NBO formation. In contrast with CO₂ but in agreement with earlier findings, H₂O solubility in mafic melts is negligibly affected by melt composition and structure: it only shows a weak correlation with NBO/O.