Melt-preferred orientation, anisotropic permeability, and melt-band formation in a deforming, partially molten aggregate

TL;DR

This paper develops a model of anisotropic permeability in partially molten rocks, exploring its effects on melt-band formation and orientation during shear deformation, with implications for Earth's mantle dynamics.

Contribution

It introduces a simple, general model of anisotropic permeability and analyzes its impact on melt-band angles using linear and nonlinear methods.

Findings

Anisotropic permeability influences melt-band angles.

Low-angle bands can emerge under certain anisotropic conditions.

Experimental conditions for these regimes are narrowly constrained.

Abstract

Shear deformation of partially molten rock in laboratory experiments causes the emergence of melt-enriched sheets (bands in cross-section) that are aligned at about 15-20 degrees to the shear plane. Deformation and deviatoric stress also cause the coherent alignment of pores at the grain scale. This leads to a melt-preferred orientation that may, in turn, give rise to an anisotropic permeability. Here we develop a simple, general model of anisotropic permeability in partially molten rocks. We use linearised analysis and nonlinear numerical solutions to investigate its behaviour under simple-shear deformation. In particular, we consider implications of the model for the emergence and angle of melt-rich bands. Anisotropic permeability affects the angle of bands and, in a certain parameter regime, it can give rise to low angles consistent with experiments. However, the conditions required for this regime have a narrow range and seem unlikely to be entirely met by experiments. Anisotropic permeability may nonetheless affect melt transport and the behaviour of partially molten rocks in Earth's mantle.