The optimal depth of silicic subvolcanic magma chambers, a dynamic approach to the brittle-ductile transition

Neutral buoyancy and rheological transitions in the crust are commonly assumed to control where magmas accumulate and form subvolcanic magma chambers. However, the density of volatile-rich silicic magmas is typically lower than the surrounding crust, and the rheology of the crust alone does not define the depth of the brittle-ductile transition around a magma chamber. Yet, typical pressures inferred from geophysical inversions or petrological methods seem to cluster around 2 ± 0.5 kbar for subvolcanic storage in all tectonic settings and irrespective of the density structure of the host crust. We use thermo-mechanical modeling to study the conditions under which growing magma bodies (to form sustained chambers feeding eruptions) are susceptible to erupt. We show that there exists a critical range of storage pressure that is controlled by volatile exsolution and crustal rheology. At pressure 1.5 kbar, and for geologically realistic water contents, chamber volumes, and average recharge rates, the presence of an exsolved magmatic volatile phase hinders chamber growth because the volume of magma erupted over each eruption cycle is greater than the amount that is fed to the system by recharges. At pressures 2.5 kbar, the rheology of the crust in long-lived magmatic provinces is sufficiently compliant to inhibit most overpressure build-up during recharge and prevent eruptions. Sustainable eruptible silicic magma reservoirs are only able to develop within a relatively narrow range of pressures around 2 ± 0.5 kbar, where the amount of exsolved volatiles fosters growth while the high viscosity of the crust promotes the necessary overpressurization for eruption.