CO2 bubble generation and migration during magma-carbonate interaction

We conducted quantitative textural analysis of vesicles in high temperature and pressure carbonate assimilation
experiments (1200 °C, 0.5 GPa) to investigate CO2 generation and subsequent bubble migration from carbonate
into magma. We employed Mt. Merapi (Indonesia) and Mt. Vesuvius (Italy) compositions as magmatic starting
materials and present three experimental series using (1) a dry basaltic-andesite, (2) a hydrous basaltic-andesite
(2 wt% H2O), and (3) a hydrous shoshonite (2 wt% H2O). The duration of the experiments was varied from 0 to
300 s, and carbonate assimilation produced a CO2-rich fluid and CaO-enriched melts in all cases. The rate of carbonate assimilation, however, changed as a function of melt viscosity, which affected the 2D vesicle number,
vesicle volume, and vesicle size distribution within each
experiment. Relatively low-viscosity melts (i.e. Vesuvius experiments) facilitated efficient removal of bubbles
from the reaction site. This allowed carbonate assimilation to continue unhindered and large volumes of CO2 to beliberated, a scenario thought to fuel sustained CO2-driven eruptions at the surface. Conversely, at higher viscosity
(i.e. Merapi experiments), bubble migration became progressively
inhibited and bubble concentration at the reaction site caused localised volatile over-pressure that can eventually trigger short-lived explosive outbursts. Melt
viscosity therefore exerts a fundamental control on carbonate assimilation rates and, by consequence, the style of
CO2-fuelled eruptions.