Magma Storage, Differentiation and Transfer in Vertically Extensive Magmatic Systems

Abstract

Geophysical, geochemical, and petrological studies indicate that magma storage and chemical differentiation can occur at various depths in the continental crust. Examination of exposed ancient magmatic systems reveal a continuum from a refractory lower crust to a silica-rich upper crust, prompting a debate on the mechanisms governing magma storage and differentiation in vertically extensive magmatic systems. Conceptual models for magma storage and chemical differentiation within the crust range from (i) a low crystallinity magma body, where differentiation occurs primarily by fractional crystallisation and partial melting of the crust, to (ii) a trans-crustal high crystallinity ‘mush’ reservoir consisting of a porous crystal matrix with melt in the pore space. Chemical differentiation in these mush reservoirs is driven by the compaction of the crystal matrix, buoyant upward reactive flow of melt, and partial melting of the surrounding crust. We use a numerical model to investigate magma storage and chemical differentiation in the continental crust. The model assumes a magmatic system sustained by the intrusion of mantle-derived basalt into the lower crust.  We find that intrusion of basalt creates a high crystallinity reservoir in the lower crust. Reactive flow and compaction cause melt to accumulate at the top of the reservoir, creating a layer of low crystallinity, chemically differentiated magma.  Buoyancy overpressure causes this magma to  evacuate via dikes and we assume the magma intrudes the mid-crust to form a second high crystallinity mush reservoir. Reactive flow and compaction once again lead to the accumulation of low-crystallinity, evolved magma at the top of the reservoir that can evacuate and intrude the upper crust, driving volcanic eruptions or forming shallow plutons. The compositional evolution of magma as it ascends through the system is influenced by the flux of the parental basalt and the fertility of the crust. A higher basalt flux leads to warmer reservoirs that evacuate less evolved magma. Reservoirs in infertile crust that cannot melt are formed only of hot, parental magma, and also evacuate less evolved magma. Partial melting of fertile crust allows melt to percolate upwards and accumulate in cooler crust, leading to the evacuation of evolved (silicic) magma. In most of our simulation cases, the reservoirs remain as discrete bodies in the lower- and mid-crust, with magma transfer occurring via dikes; a trans-crustal reservoir does not form. The lower-crust reservoir evacuates mafic to intermediate magma, whilst the cooler mid-crust reservoir primarily evacuates evolved, silicic magma, consistent with vertical compositional trends in exposed, ancient magmatic systems. Both reservoirs comprise primarily low-melt fraction mush, consistent with geophysical imaging of contemporary systems.  Layers of accumulated low-crystallinity magma are transient and likely too thin to resolve in geophysical data. The predicted volume, composition, and frequency of episodic magma intrusions into the upper crust are consistent with observed data from large volcanic eruptions. Our results suggest that reactive flow in multiple mush reservoirs controls magma storage and differentiation.