Heat sources and the evolution of multiple magma reservoirs in the continental crust

Thermal models of crustal magma reservoirs usually assume that heating occurs due to the intrusion of mantle-derived sills or dykes. In these models, intrusion of large magma volumes during an 'incubation period' is required to heat the crust to produce a persistent reservoir (where the melt fraction is above zero between successive intrusions). However, crustal heating is also caused by asthenospheric upwelling, due to events such as delamination or crustal thinning. Moreover, multiple magma reservoirs may be present in the crust, transferring heat between them. Thus, models assuming a single reservoir and an initially undisturbed geotherm may be incorrectly predicting timescales of formation and duration of crustal magma reservoirs.

Through numerical modelling, we examine magma reservoir formation and evolution in the continental crust. We assume upwelling brings hot mantle to the base of the crust, capture the conduction of this heat and model intrusion of mantle-derived basalt into the mid- to lower-crust. Results confirm the combination of the two heat sources creates a persistent reservoir. Within this reservoir, we model heat transfer and reactive percolative flow of melt driven by buoyancy. Results show reactive flow creates thin (10²m) layers of evolved, low crystallinity magma at the top of the reservoir, overlying a thick (10³m) layer of high crystallinity 'mush'. The low crystallinity magma is buoyant, and we model its migration to a shallower reservoir. Despite magma leaving the deeper reservoir, reactive flow causes a new magma layer to form, leading to periodic delivery of magma to the shallower reservoir at a rate controlled by processes in the deeper reservoir.

Our results suggest that a mantle-induced thermal anomaly can notably reduce the incubation period and extend the life of a mid- to lower-crustal magma reservoir,but only when the intrusion of basalt occurs later in the life of the anomaly. However, shallower reservoirs are likely transient, as magma is not transferred rapidly enough from the deeper reservoir to produce a persistent reservoir in the upper crust, even with the presence of a mantle thermal anomaly. Thus, volcanoes may show no direct evidence of magma in a shallower reservoir, but still have an active reservoir at depth that can supply magma for future eruptions.