Silicic magma reservoirs in the Earth's crust

Magma reservoirs play a key role in controlling numerous processes in planetary evolution, including igneous differentiation and degassing, crustal construction, and volcanism. For decades, scientists have tried to understand what happens in these reservoirs, using an array of techniques such as field mapping/petrology/geochemistry/geochronology on plutonic and volcanic lithologies, geophysical imaging of active magmatic provinces, and numerical/experimental modeling. This review paper tries to follow this multi-disciplinary framework while discussing past and present ideas. We specifically focus on recent claims that magma columns within the Earth's crust are mostly kept at high crystallinity ("mush zones"), and that the dynamics within those mush columns, albeit modulated by external factors (e.g., regional stress field, rheology of the crust, pre-existing tectonic structure), play an important role in controlling how magmas evolve, degas, and ultimately erupt. More specifically, we consider how the chemical and dynamical evolution of magma in dominantly mushy reservoirs provides a framework to understand: (1) the origin of petrological gradients within deposits from large volcanic eruptions ("ignimbrites"); (2) the link between volcanic and plutonic lithologies; (3) chemical fractionation of magmas within the upper layers of our planet, including compositional gaps noticed a century ago in volcanic series (4) volatile migration and storage within mush columns; and (5) the occurrence of petrological cycles associated with caldera-forming events in long-lived magmatic provinces. The recent advances in understanding the inner workings of silicic magmatism are paving the way to exciting future discoveries, which, we argue, will come from interdisciplinary studies involving more quantitative approaches to study the crust-reservoir thermo-mechanical coupling as well as the kinetics that govern these open systems.