The nucleation and growth of mixed H2O–CO2 bubbles in magmas

Abstract

Volcanic eruptions are driven by the nucleation and growth of gas bubbles that form when volatile species dissolved in magma become supersaturated. Previous models for bubble growth have focussed on H₂O; however, CO₂ also plays a fundamental role in the nucleation and growth of gas bubbles. Here, we develop a numerical model to explore the nucleation and growth of bubbles containing both H₂O and CO₂ in magma of arbitrary composition. Nucleation is modelled as a Poisson process using classical nucleation theory with composition-appropriate solubility models for the mixed H₂O–CO₂ fluid. We find that CO₂ dramatically increases the depth of bubble nucleation compared with H₂O-only systems; for a case-study rhyolite (Krafla, Iceland) CO₂ increases nucleation depth from 130 m (H₂O-only) to 760 m if CO₂ is included (a factor of 6 increase in nucleation pressure); for a case-study basalt (Fagradalsfjall, Iceland), nucleation occurs at 13 km depth if CO₂ is included, but does not occur at all if H₂O is the only volatile species. Post-nucleation growth of the bubbles is investigated by extending a ‘shell model’ to include CO₂ as well as H₂O. The species are coupled via a mixed equation-of-state for the gas phase, introducing a co-dependence on their solubility that allows H₂O to exsolve at greater depth when CO₂ is present. As a result, exsolution of a small volume of CO₂ can trigger the exsolution of a much larger volume of H₂O, driving rapid, disequilibrium bubble growth. Our findings show that accounting for mixed H₂O–CO₂ volatile compositions is essential for accurate modelling of magma ascent and eruption dynamics.