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 ofCO(2 )often present in mafic systems. In this study, we integrate the thermodynamic effects of dissolved and exsolved H2O and CO(2 )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),CO(2 )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.