On the causes of melt focusing at mid-ocean ridges: Ridge suction versus permeability barrier

Emplacement of magmatic crust at mid-ocean ridges (MORs) is confined in a narrow neovolcanic zone on the seafloor, whereas geophysical observations suggest that mantle melting occurs over a broad region. How melt is transported horizontally towards the ridge axis, i.e. melt focusing, remains incompletely understood. Here we present numerical models, theoretical decomposition, and scaling analysis, to isolate melt focusing mechanisms, and focus in particular on ridge suction and on the permeability barrier. We show that shear deformation induced dynamic pressure leads to large decompaction pressure, which increases porosity, instead of generating ridge suction as previously expected. We further demonstrate that a permeability barrier resulting from cold lithosphere systematically leads to a horizontal compaction pressure gradient that focuses melt toward the ridge axis, which may explain widespread melt focusing at global MORs as well as the three-dimensional melt distribution at ultra-slow spreading centers.