Abstract

Authors: A. J. Turner, R. F. Katz, and M. D. Behn
Abstract: Grain size is a fundamental control on the rheology and permeability of the mantle. These properties, in turn, shape the transport and extraction of melt from the mantle source. It is therefore important to model the continuum grain-size field as a part of two-phase flow calculations that aim to capture the full spatial variability of melt transport in the upper mantle.
We first consider a two-dimensional, single-phase model to predict the steady-state grain size beneath a mid-ocean ridge.  The model employs a composite rheology of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a brittle stress limiter. Grain size is calculated using the paleowattmeter model of Austin & Evans (2007). We investigate the sensitivity of the grain size model to parameter variations. Our model predicts that permeability varies by two orders of magnitude due to the spatial variability of grain size within the expected melt region of a mid-ocean ridge.
We then consider a two-phase model to test the influence of spatially varying grain size on melt transport. We find that the rheological coupling of grain size has a greater influence on melt transport than the coupling through permeability. The model predicts that a spatially variable grain-size field can promote focusing of melt towards the ridge axis. This focusing is distinct from the commonly discussed sub-lithospheric decompaction channel. Furthermore, our model predicts that the shape of the partially molten region is sensitive to rheological parameters associated with grain size. The comparison of this shape with observations may help to constrain the rheology of the upper mantle beneath mid-ocean ridges.