An experimental test of the viscous anisotropy hypothesis for partially molten rocks

TL;DR

This study experimentally tests the hypothesis that viscous anisotropy in partially molten rocks causes melt segregation during shear deformation, supporting models that explain planetary differentiation processes.

Contribution

It provides experimental validation for the viscous anisotropy hypothesis and its role in melt migration in partially molten rocks under shear.

Findings

Radial melt fraction gradient observed in experiments.

Melt segregation increases with strain, matching theoretical predictions.

Supports the role of microstructural anisotropy in large-scale geodynamics.

Abstract

Chemical differentiation of rocky planets occurs by melt segregation away from the region of melting. The mechanics of this process, however, are complex and incompletely understood. In partially molten rocks undergoing shear deformation, melt pockets between grains align coherently in the stress field; it has been hypothesized that this anisotropy in microstructure creates an anisotropy in the viscosity of the aggregate. With the inclusion of anisotropic viscosity, continuum, two-phase-flow models reproduce the emergence and angle of melt-enriched bands that form in laboratory experiments. In the same theoretical context, these models also predict sample-scale melt migration due to a gradient in shear stress. Under torsional deformation, melt is expected to segregate radially inward. Here we present new torsional deformation experiments on partially molten rocks that test this prediction. Microstructural analyses of the distribution of melt and solid reveal a radial gradient in melt fraction, with more melt toward the centre of the cylinder. The extent of this radial melt segregation grows with progressive strain, consistent with theory. The agreement between theoretical prediction and experimental observation provides a validation of this theory, which is critical to understanding the large-scale geodynamic and geochemical evolution of Earth.