The Mechanics of Multiphase Magmas: A Perspective from the Scale of Crystals and Bubbles to Magma Reservoirs

The interplay between melt, crystals, andvolatile bubbles controls the physical properties of magmas in Earth's crust, the rate of phase separation, and, by extension, chemical differentiation. The mechanical processes that couple crystals, bubbles, and melt are nonlinear, and their expression in magmatic systems can vary greatly with the relative phase proportions in the magma. In this review we propose a multiscale perspective on multiphase magmas under crustal storage conditions, with a specific focus on phase separation mechanisms. We start with an inventory of forces acting on a single crystal or volatile bubble in a silicate melt. We follow with a discussion of different upscaling strategies to simplify the description of the dynamics at greater scales, relevant to the evolution of magma reservoirs.We discuss recent progress in the development of models to study the internal dynamics of magma reservoirs, highlight current challenges, and propose possible paths for further progress. ▪ The mechanical interaction between the constituents (melt, crystals, and bubbles) at the scale of crystals controls the properties of magmas. ▪ The choice of upscaling strategy is controlled by the processes that are considered. ▪ Melt-crystal separation processes and their efficiency vary with the relative proportion of the phases involved. ▪ Melt extraction by repacking is fast compared to compaction but stalls as the mush reaches the maximum packing.