The Inner Workings of Crustal Distillation Columns; The Physical Mechanisms and Rates Controlling Phase Separation in Silicic Magma Reservoirs

Abstract

Igneous processes have a fundamental impact on how our planet is shaped: they contribute to the growth of continents, control volcanic activity, form ore deposits and supply most volatile elements to our atmosphere. In the course of this igneous differentiation, phase separation plays a key role, as in all distillation processes. How, and how fast, this phase separation occurs are therefore critical questions to address to better understand the inner workings of the Earth (and other planets). In this Perspectives article, we will review some of the most important aspects of the processes that govern igneous distillation, considering the effect of three distinct phases (crystals-melt-fluid, in decreasing order of viscosity and density) on mechanical separation processes in a gravity field. We will also discuss the potential impacts of external factors (e.g. tectonic forces, magma recharge, seismic waves) on phase separation. Regardless of the source of energy driving phase separation in crustal differentiation columns, crystal settling at low crystallinity and compaction at intermediate to high crystallinity play a major role in separating silicate minerals from melts and fluids. We suggest that compaction without any associated deformation of solids (herein referred to as 'crystal repacking') is an important process that can extract up to a few tens of per cent (volume) of melt from its crystalline matrix, particularly in shallow silicic reservoirs. Rates of melt extraction by compaction are probably relatively slow, requiring centuries to millennia to generate large crystal-poor pockets (<10s to 100s of km3 of silicic melt). Alternative processes, such as gas-driven filter pressing or melt segregation by shear or deformation, can enhance or inhibit phase separation, depending on specific conditions, but they are unlikely to be particularly efficient in silicic systems.