Investigating the Impact of an Exsolved H2O‐CO2 Phase on Magma Chamber Growth and Longevity: A Thermomechanical Model

Magmatic volatiles drive pressure, temperature, and compositional changes in upper crustal magma chambers and alter the physical properties of stored magmas. Previous studies suggest that magmatic H2O content influences the growth and longevity of silicic chambers through regulating the size and frequency of eruptions and impacting the crystallinity‐temperature curve. However, there has been comparatively little exploration of how CO2 impacts the evolution of magma chambers despite the strong influence of CO2 on H2O solubility and the high concentrations of CO2 often present in mafic systems. In this study, we integrate the thermodynamic effects of dissolved and exsolved H2O and CO2 with the mechanics of open‐system magma chambers that interact thermally and mechanically with the crust. We applied this model to investigate how intrinsic variations in magmatic H2O‐CO2 content influence the growth and longevity of silicic and mafic magma chambers. Our findings indicate that even with a tenfold increase in CO2 content (up to 10,000 ppm), CO2 plays a minimal role in long‐term chamber growth and longevity. While CO2 content affects the magma compressibility, the resulting changes in eruption mass are balanced out by a commensurate change in eruption frequency so that the time‐averaged eruptive flux and long‐term chamber behavior remain similar. In contrast, H2O content strongly influences chamber growth and longevity. In silicic systems, high H2O contents hinder magma chamber growth by increasing the total eruptive flux and steepening the slope of the crystallinity‐temperature curve. In mafic systems, high H2O contents promote magma chamber growth by flattening the slope of the crystallinity‐temperature curve. Plain Language Summary Water and carbon dioxide play an important role in the size and frequency of volcanic eruptions. They are also important in determining how much magma chambers can grow and for how long they can remain eruptible. Here, we present computer simulations that allow us to study the feedback between thermodynamic (heat and chemistry related) and mechanical (pressure build‐up and loss related) processes in magma chambers with different amounts of carbon dioxide and water and different magma compositions to understand how magma chambers grow. The outputs of these simulations track how temperature, pressure, volume, and the proportion of crystals, melt, and gas in the magma chamber change over time. Through these outputs, we found that carbo