GAS PERCOLATION IN UPPER-CRUSTAL SILICIC CRYSTAL MUSHES AS A MECHANISM FOR UPWARD HEAT ADVECTION AND REJUVENATION OF NEAR-SOLIDUS MAGMA BODIES

Abstract

Several crystal-rich, intermediate to silicic magmas erupted at arc volcanoes record a reheating event shortly prior to eruption: they provide evidence for remobilization of crystal mushes by mafic magmas. As hybridization between the mush and the mafic magma is often limited, bulk mixing could not be the dominant process in transferring heat. Conductive heating from a basaltic underplate plays a role, but a few characteristics of these rejuvenated mushes suggest that reheating occurs faster than predicted by conduction. In the upper crust, a process that can transport heat faster than conduction, and still remain chemically nearly imperceptible, is the upward migration of a hot volatile phase ("gas sparging") that originates in underplated mafic magmas. Using numerical simulations, we quantified the thermal effects of two-phase flow (a silicic melt phase and a H2O-CO2 fluid phase) in the pore space of shallow silicic mushes that have reached their rheological lock-up point (i.e., rigid porous medium, crystallinity ≥50 vol.%). Results show that the reheating rates are significantly faster than conduction for volatile fluxes >0.1 m3/m2 yr. Considering that volatiles can be rapidly exsolved from the underplated mafic magma, these high fluxes can be promptly reached, leading to fast reheating; sill-like batches of mushes with volumes similar to the 1995-present eruption of the Soufrière Hills (Montserrat, W.I.) can be reheated by a few tens of degrees and remobilized within days to weeks. At these high fluxes, a considerable volume of volatiles is needed (similar to the volume of mush being reheated). Large silicic systems (>100-1000 km3) require unrealistic amounts of volatiles to be reheated in a continuous, high-flux sparging event. Rejuvenation of batholithic mushes therefore requires multiple sparging episodes separated by periods dominated by near-conductive heat transfer at low-flux sparging (<0.1 m3/m2 yr) and may take up to 100-200 ky. © 2005 Elsevier B.V. All rights reserved.