  Melting beneath mid-ocean ridges occurs over a region that is much broader
than the zone of magmatic emplacement to form 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
sub-lithospheric 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
co-variation 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 ex- planation of the electrical resistivity structure beneath one
location on the East Pacific Rise. More generally, although grain size and
non-Newtonian viscosity are properties of the solid phase, we find that in the
context of mid-ocean ridges, their effect on melt transport is more profound
than their effect on the mantle corner-flow.
