Magmatic reservoirs located in the upper crust have been shown to result from the repeated intrusions of new magmas, and spend much of the time as a crystal-rich mush. The geometry of the intrusion of new magmas may greatly affect the thermal and compositional evolution of the reservoir. Despite advances in our understanding of the physical processes that may occur in a magmatic reservoir, the resulting architecture of the composite system remains poorly constrained. Here we performed numerical simulations coupling a computational fluid dynamics and a discrete element method in order to illuminate the geometry and emplacement dynamics of a new intrusion into mush and the relevant physical parameters controlling it. Our results show that the geometry of the intrusion is to first order controlled by the density contrast that exists between the melt phases of the intrusion and resident mush rather than the bulk density contrast as is usually assumed (Fig. 1). When the intruded melt is denser than the host melt, the intrusion pounds at the base of the mush and emplaced as a horizontal layer (Anim. 2). The occurrence of Rayleigh-Taylor instability leading to the rapid ascent of the intruded material through the mush was observed when the intruded melt was lighter than the host one and was also unrelated to the bulk density contrast (Anim. 3). In the absence of density contrasts between the two melt phases, the intrusion may fluidize the host crystal network and slowly ascend through the mush (Anim. 1). The effect of the viscosity contrast between the intruded and host materials was found to have a lesser importance on the architecture of intrusions in a mush. Analyzing the eruptive sequence of well documented eruptions involving an intrusion as the trigger shows a good agreement with our modeling results, highlighting the importance of specifically considering granular dynamics when evaluating magmas and mush physical processes.
Figure 1: Distribution of the runs as a function of the Atwood number and the dimensionless viscosity contrast. On the abscissa, the black and red coordinates indicate the Atwood numbers computed with the melt phase densities and with the bulk densities, respectively. Each square corresponds to a run and its color depends on the observed regime (blue: rising; black: fluidization; red: lateral spreading). All runs displayed on this figure were performed with the same dimensionless injection velocity.
Animation 1: Fluidization regime. The color of the particles depends on the magnitude of the contacts forces helping to highlight the force chains. The red color corresponds to the injected melt.
Animation 2: Lateral spreading regime.
Animation 3: Rising regime.
Related publication: Carrara, A., Burgisser, A., Bergantz, G.W., 2020. The architecture of intrusions in magmatic mush, Earth and Planetary Science Letters, 549, 116539. https://doi.org/10.1016/j.epsl.2020.116539