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Melting beneath mid‐ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust. Magma is focused into this zone by lateral transport. This focusing has typically been explained by dynamic pressure gradients associated with corner flow, or by a sublithospheric channel sloping upward toward the ridge axis. Here we discuss a novel mechanism for magmatic focusing: lateral transport driven by gradients in compaction pressure within the asthenosphere. These gradients arise from the covariation of melting rate and compaction viscosity. The compaction viscosity, in previous models, was given as a function of melt fraction and temperature. In contrast, we show that the viscosity variations relevant to melt focusing arise from grain‐size variability and non‐Newtonian creep. The asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the East Pacific Rise. More generally, we find that although grain‐size and non‐Newtonian viscosity are properties of the solid phase, their effect on melt transport beneath mid‐ocean ridges is more profound than their effect on the mantle corner flow.

Magmatic Focusing to Mid‐Ocean Ridges: The Role of Grain‐Size Variability and Non‐Newtonian Viscosity