The influence of spreading rate and permeability on melt focusing beneath mid-ocean ridges

At mid-ocean ridges, oceanic crust is emplaced in a narrow neovolcanic region on the seafloor, whereas basaltic melt that forms this oceanic crust is generated in a wide region beneath as suggested by a few geophysical surveys. The combined observations suggest that melt generated in a wide region at depths has to be transported horizontally to a small region at the surface. We present results from a suite of two-phase models applied to the mid-ocean ridges, varying half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the litho-sphere-asthenosphere boundary (LAB). Three distinct melt focusing mechanisms are recognized in these models: 1) melting pressure focusing, 2) decompaction layers and 3) ridge suction, of which the first two play dominant roles in focusing melt. All three of these mechanisms exist in the fundamental two phase flow formulation but the manifestation depends largely on the choice of rheological model. The models also show that regardless of spreading rates, the amount of melt and melt transport patterns are sensitive to changes in intrinsic permeability, K-0. In these models, the LAB is delineated by the melt-rich decompaction layers, which are essentially defined by the temperature dependent rheological and freezing boundaries. Geophysical observations place the LAB at a steeper incline as compared to the gentler profile suggested by most of our models. The models suggest that one way to reconcile this discrepancy is to have stronger melting pressure focusing mechanism as it is the only mechanism in these models that can focus melt before reaching the typical model thermal LAB. The apparent lack of observable decompaction layers in the geophysical observations hints at the possibility that melting pressure focusing could be significant. These models help improve our understanding of melt focusing beneath mid-ocean ridges and could provide new constraints for mantle rheology and permeability.