# Magmatic focusing to mid-ocean ridges: the role of grain size variability and non-Newtonian viscosity

## TL;DR

This paper proposes a new mechanism for magmatic focusing at mid-ocean ridges driven by gradients in compaction pressure caused by grain size variability and non-Newtonian viscosity, improving understanding of melt transport.

## Contribution

It introduces a novel model highlighting the role of grain size and non-Newtonian viscosity in magmatic focusing, contrasting with traditional explanations based on corner flow.

## Key findings

- Grain size variability influences melt focusing significantly.
- Non-Newtonian viscosity affects melt transport more than mantle flow.
- Model predictions align with electrical resistivity data from the East Pacific Rise.

## Abstract

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.