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

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 H 2 O; however, CO 2 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 H 2 O and CO 2 in magma of arbitrary composition. Nucleation is modelled as a Poisson process using classical nucleation theory with composition-appropriate solubility models for the mixed H 2 O–CO 2 fluid. We find that CO 2 dramatically increases the depth of bubble nucleation compared with H 2 O-only systems; for a case-study rhyolite (Krafla, Iceland) CO 2 increases nucleation depth from 130 m (H 2 O-only) to 760 m if CO 2 is included (a factor of 6 increase in nucleation pressure); for a case-study basalt (Fagradalsfjall, Iceland), nucleation occurs at 13 km depth if CO 2 is included, but does not occur at all if H 2 O is the only volatile species. Post-nucleation growth of the bubbles is investigated by extending a ‘shell model’ to include CO 2 as well as H 2 O. The species are coupled via a mixed equation-of-state for the gas phase, introducing a co-dependence on their solubility that allows H 2 O to exsolve at greater depth when CO 2 is present. As a result, exsolution of a small volume of CO 2 can trigger the exsolution of a much larger volume of H 2 O, driving rapid, disequilibrium bubble growth. Our findings show that accounting for mixed H 2 O–CO 2 volatile compositions is essential for accurate modelling of magma ascent and eruption dynamics. • Numerical model for mixed H 2 O-CO 2 bubble nucleation and growth developed. • CO 2 shown to significantly increase nucleation pressure. • Disequilibrium degassing leaves geochemical signals in melt and gas phases.