Effects of a Volatile Phase and Dike Transport on the Creation and Dynamics of Crustal Magma Reservoirs: A Three-Phase Numerical Model Study

We use a one-dimensional numerical model to investigate how the presence of a volatile phase (e.g., H₂O) impacts the formation and dynamics of crustal magma reservoirs. The system is driven by the repeated intrusion of mantle-derived basalt sills containing several weight percent volatiles. The model solves for conductive and advective heat transfer within a three-phase framework. The solid matrix and silicate melt are coupled as a two-phase porous/particulate flow system, capturing compaction at low melt fractions and hindered settling at high melt fractions. In contrast, the transport of the volatile component is simplified and modeled via one-directional (upward) diffusion, which facilitates flux melting of the crust. The chemical model incorporates three components (high/low SiO₂ and volatile), with solidus and liquidus temperatures dependent on bulk SiO₂ and melt volatile content. Volatile exchange between solid and melt phases is described by a partition coefficient, and a free volatile phase exsolves when melt saturation is exceeded.  Our results show that magma can accumulate at the top of a reservoir and we also capture rapid upwards transport of this magma via dykes if a critical buoyancy threshold is exceeded.

We find that the nonlinear coupling between volatile content, phase equilibria, two-phase melt-solid dynamics, latent heat, and dike transport, generates complex system behavior. A general conclusion is that reservoir growth is strongly controlled by crustal fertility and strength; if the crust can melt in response to added heat and volatiles, and dike initiation is inhibited by tectonic compression or high crust strength, the top of the reservoir migrates upward via partial melting to create a vertically extensive, mush dominated system. However, if the crust is infertile and magma frequently evacuates via dykes, then magmatism is primarily observed at distinct depth intervals separated by solid rock.

In systems where rapid vertical dike transport is inhibited and sill intrusion rates are high (>~2 mm/year, parameter-dependent), upwards migration halts at mid-crustal depths. The reservoir develops as a thick mush column hosting numerous transient, thin, high melt-fraction layers that are evolved and volatile-rich, interspersed with refractory material. These layers propagate, merge, and split, but remain confined to the mid-to-lower crust.

In systems where dyke transport is efficient, the system evolves differently.  Reservoir supplied by high parental magma intrusion rates converge toward a behavior similar to those with slow intrusion rates (≤1 mm/year). High melt-fraction material is continuously extracted upward leading to the formation of a shallow silicic system, culminating in a single, dominant silicic melt layer near the top of the reservoir at approximately 5–7 km depth which cools for form a silicic pluton.

Overall, our model predicts that crustal magmatic systems are highly dynamic, with melt fraction varying significantly in time and space. The presence of volatiles and the efficiency of vertical transport are first-order controls on how and where magma is stored and transported through the crust.