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

Abstract:

At mid-ocean ridges, oceanic crust is emplaced in a narrow neo-volcanic 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 lithosphere 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 show that regardless of spreading rates, the amount of melt and melt transport patterns are sensitive to changes in intrinsic permeability, K0. Geophysical observations place the LAB at a steeper incline as compared to the gentler profile suggested by modeling efforts. The decompaction melt-rich layer roughly follows and itself can define the lithosphere-asthenosphere boundary (LAB), which without the melt layer, would be along the temperature dependent rheological and freezing boundaries. Melting pressure focusing is the only focusing mechanism that can focus melt before reaching the typical model thermal LAB. The lack of the decompaction layers in the geophysical observations hint at the possibility that melting pressure focusing could be significant, which could provide constraints for mantle rheology, permeability and the lithosphere-asthenosphere boundary.