Grain-size dynamics beneath mid-ocean ridges: Implications for permeability and melt extraction

TL;DR

This study models the steady-state mean grain size beneath mid-ocean ridges to understand its impact on mantle permeability and melt focusing, providing insights into melt transport and seismic observations.

Contribution

It introduces a 2D model linking grain size, permeability, and melt focusing beneath mid-ocean ridges, incorporating various mantle rheologies and sensitivities.

Findings

Permeability varies with grain size, forming high and low regions.

Steep permeability boundaries focus melt towards the ridge axis.

Model results align with seismic and magnetotelluric observations.

Abstract

Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady-state mean grain size beneath a mid-ocean ridge. The mantle rheology is modelled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the piezometric relationship of Austin and Evans [2007]. We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean mean grain-size field in the context of permeability. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions forms a boundary that is steeply sloped toward the ridge axis. We hypothesise that such a permeability structure generated from the variability of the mean grain size may be able to focus melt towards the ridge axis, analogous to a Sparks and Parmentier [1991]-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise [Baba et al., 2006, Key et al., 2013].