Two Competing Effects of Volatiles on Heat Transfer in Crystal-rich Magmas: Thermal Insulation vs Defrosting

Abstract

The build-up and eruption of crystal-rich ignimbrites, commonly referred to as ‘Monotonous Intermediates’, has been attributed to the incremental addition of new magma batches and the episodic partial melting and reactivation of rheologically locked crystal-rich magma bodies (mushes). In this study, we explore the role of volatiles exsolved from a hot intrusion underplating a crystal mush on the thermal evolution of the coupled mush–intrusion system. We solve the enthalpy conservation equation for the mush and the intrusion and investigate the exsolution of volatiles from the intrusion and their transport through the mush in one dimension. Our calculations span a range of pressures (from 1 to 4 kbar), mush composition (andesite to rhyolite) and initial water contents (from 1·5 to 6 wt %). The mobility of volatiles in the mush is controlled by the volume fraction of the pore space they occupy, and, as a consequence, by the amount of melting and the injection rate of volatiles from the intrusion to the mush. We find that volatiles affect the melting of the mush (or defrosting) in two opposite ways depending on pressure and the initial water content of the intrusion. When the intrusion volatile content is high and pressure relatively low (>4 wt % H2O at 2 kbar), the mass transfer of volatiles from the intrusion to the mush carries enthalpy beyond the melting front and can thus enhance defrosting and possibly remobilization of the mush. For lower initial volatile contents (< 4 wt % H2O) in the intrusion and/or higher pressure (3–4 kbar), volatiles stall at the interface between the two magma bodies and prevent defrosting as they thermally insulate the mush from the intrusion. We propose that the dual role played by volatiles during the thermal evolution of crystal-rich magmas reheated by an underplating intrusion can explain the presence of crystal-rich ignimbrites in arcs and their absence in drier hotspots or extensional regions.