A focus on melt focusing

Mid-oceanic ridges are the location of over 90% of global magmatism. Geophysical and geochemical observations at mid-oceanic ridges gives us valuable information such as the structure and composition of the crust and mantle beneath [Gregg et al, 2012 ]. To make better interpretations of the available observations, quantitative models are necessary. While the region of melt production beneath mid-oceanic ridges is quite large, oceanic crust is produced in an infinitesimally narrow region. This observation suggests that there must be some process for focusing melt at depth to the ridge axis. A host of mechanisms have been suggested along with arguments on the geochemical signatures of focusing. Here, we present a new two-phase model for magma generation and transport beneath mid-oceanic ridges using TerraFERMA, The Transparent Finite Element Rapid Model Assembler [Wilson et al, 2017]. This model incorporates a range of rheologies including diffusion, dislocation and depth-dependent Von-mises as well as a simplified phase diagram for magma production and crystalization [Katz et al, 2003], both of which are coupled to the solid and melt flow. Preliminary results suggest that magma can focus well below the thermal boundary layer due to a range of effects arising from variable viscosity and thermal consequences. Unlike other models that produce high-permeability melt channels near the base of the thermal boundary layer due to freezing, in these models, melts appear to be focused due to compaction pressure gradients induced by melting and a large bulk viscosity at small porosities. In addition, these models demonstrate a thermal channeling instability due to coupling melting to thermal advection by the melt. We will quantify the effects of the various focusing processes as well as their observational consequences for crustal production and possibly trace element variability.