Large Silicic Eruptions, Episodic Recharge, and the Transcrustal Magmatic System

Abstract

AbstractLarge silicic eruptions result from rapid evacuation of large, upper crustal reservoirs of silicic magmas. These silicic magmas are thought to be replenished by melt extracted from underlying crystal‐rich source mushes, but the timescales and mechanisms of such melt extraction are unclear. Geochemical observations suggest that the replenishing melt is often more primitive than the eruptives and must thus cool and crystallize to generate the highly silicic magmas that eventually lead to large eruptions. Motivated by these observations, we use thermal models to explore the replenishment conditions capable of building an eruptible silicic reservoir to generate a large eruption. Results show that the minimum melt replenishment rate required for a silicic reservoir to start growing increases with the effective thermal diffusivity of the overlying crust and decreases with the depth of the reservoir. For an eruptible reservoir at 6 km depth to grow, the initial replenishment rate needs to be greater than 2 × 10−9 m/s. High replenishment rates are required to provide enough advected heat to counterbalance rapid heat loss and consequent freezing that would prevent the eruptible reservoir from growing. However, these high initial replenishment rates must then subside over time for the eruptible reservoir to cool, crystallize, and evolve to highly silicic melts. Thermal histories of some natural systems suggest assembly of large eruptible reservoirs in <20 kyr. The rapid replenishment followed by its decay suggests that replenishment was triggered by a pronounced but ephemeral increase in the porosity and permeability of the underlying crystal‐rich source mush, allowing for rapid melt expulsion. We speculate that this perturbation may be driven by the sudden incursion of deep‐seated, hotter magmas into the base of the crystal‐rich source mush.