Optimal depth of subvolcanic magma chamber growth controlled by volatiles and crust rheology

Abstract

Storage pressures of magma chambers influence the style, frequency and magnitude of volcanic eruptions. Neutral buoyancy or rheological transitions are commonly assumed to control where magmas accumulate and form such chambers. However, the density of volatile-rich silicic magmas is typically lower than that of 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 storage pressures inferred from geophysical inversions or petrological methods seem to cluster around 2 ± 0.5 kbar in all tectonic settings and crustal compositions. Here, we use thermomechanical modelling to show that storage pressure is controlled by volatile exsolution and crustal rheology. At pressures $$\lesssim$$ 1.5 kbar, and for geologically realistic water contents, chamber volumes and recharge rates, the presence of an exsolved magmatic volatile phase hinders chamber growth because eruptive volumes are typically larger than recharges feeding the system during periods of dormancy. At pressures $$>rsim$$ 2.5 kbar, the viscosity of the crust in long-lived magmatic provinces is sufficiently low to inhibit most eruptions. Sustainable eruptible magma reservoirs are able to develop only 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. Volatile exsolution and crustal viscosity dictate that the optimum pressure for the growth of an eruptible magma reservoir is 2 kbar in all tectonic settings and crustal compositions, according to thermomechanical modelling.