MafiCH: a general model for H₂O–CO₂ solubility in mafic magmas

The solubility of CO₂ in mafic magmas is strongly dependent on magma composition, which ultimately affects magma storage conditions and eruptive behavior. Recent experimental work showed that previously published volatile solubility models for mafic magmas are not well calibrated at mid-crustal pressures (400–600 MPa). Using a simple thermodynamic model, here we construct a general CO₂ solubility model for mafic magmas by establishing the compositional dependence of two key thermodynamic parameters. The model is calibrated using experimental data from 10 magma compositions that span a range of pressures as well as silica (44–53 wt.%) and total alkali (2–9 wt.%) contents. We also survey the experimental literature for relevant H₂O solubility data to determine how to model H₂O solubility for these magmas. We combine these separate CO₂ and H₂O solubility models into a single general model for mixed-fluid (H₂O–CO₂) solubility in mafic magmas called MafiCH. We test the MafiCH model using experiments from three compositions that fall both within and beyond the calibrated range, and find that the model accurately constrains the CO₂ solubility of depolymerized magmas. Sensitivity tests identify that Na, Ca, and Al have the largest effect on CO₂ solubility while Si and Mg do not play a strong role in CO₂ solubility in mafic, depolymerized melts. Overall, saturation pressures calculated using the new model presented here are typically lower than those predicted by previous models. The model provides a new framework to interpret volcanic data from mafic magma compositions for which no experimental data is available.