Silicic magma reservoirs in the Earth’s crust

Abstract

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 multidisciplinary 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.

Tephra glass chemistry provides storage and discharge details of five magma reservoirs which fed the 75 ka Youngest Toba Tuff eruption, northern Sumatra

“…The presence of abundant PV pumice and matrix glass, alongside PIV matrix glass in the oldest (lowest) proximal YTT ignimbrites indicates that these compositions were plentiful in early erupted material. PIV and PV magmas equilibrated at the highest pressures (~150–160 MPa), and their early eruption may relate to the destabilisation of the entire Toba system as one or both of these reservoirs gradually became primed and started to erupt, or that the eruption process was initiated by the arrival of hot magma from depth (Bachmann et al , ; Wolff et al , ; Bachmann and Huber, ; Tramontano et al , ). The frequent mingling of PIV and PV in pumices (see Table ), and their compositional association (Figs.…”

A mineral and cumulate perspective to magma differentiation at Nisyros volcano, Aegean arc

“…Such hybrid andesites are believed to tap shallow, crystalrich mush systems that have stalled in the upper crust due to decompression-induced degassing and rapid crystallisation (e.g., Blundy and Cashman 2005;Annen et al 2006;Bachmann and Huber 2016). Due to their high viscosity, these mush systems spend the majority of the time close to the solidus in an uneruptible state, but can be rapidly remobilised and erupted in response to recharge by a volatilerich mafic magma (e.g., Cooper and Kent 2014;Parmigiani et al 2014;Bergantz et al 2015).…”

“…Hence, Nisyros is in good agreement with the notion of Annen et al (2006) that significant decoupling between the geochemical and petrographic characteristics of arc magmas is probably the norm rather than the exception in volcanic rocks in arcs built on (thinned) continental crust. Whereas differentiation in the lower arc crust can exert a dominant control on the geochemical composition of the magmas (e.g., Hildreth and Moorbath 1988;Davidson et al 2007), textural and petrographic aspects are predominantly obtained during shallow storage (e.g., Eichelberger 1978;Annen et al 2006;Bachmann and Huber 2016). This provides an elegant solution to the common misfit between observed phenocryst phases and cumulate assemblages in arc lavas and the fractionating mineral assemblages required to reproduce whole rock major element variations in arc suites (e.g., Annen et al 2006;Melekhova et al 2015).…”

The mechanics of shallow magma reservoir outgassing

“… Schematic representation of modeling approach for outgassing shallow reservoir (modified from Bachmann and Huber []). We use a multiscale numerical approach to study the physics of outgassing in shallow magma reservoirs.…”