Co-evolution of melt and crystal phases in experimentally sheared partially molten rocks and generation of seismic anisotropy during rapid deformation

Partial melting often occurs alongside sites of rapid deformation in the Earth’s mantle and crust. Microstructural molten and crystalline components align in response to deformation, leading to anisotropies in mechanical, transport, and seismic properties detectable by remote sensing. Here, we investigate the co-evolution of melt, shape, and crystallographic preferred orientations (MPOs, SPOs & CPOs) at early stages of experimental shear deformation, constraining their contribution to observable signatures. We characterized the microstructures of partially molten (2-4 wt% melt) olivine-basalt aggregates deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa, at shear stresses of τ = 0-175 MPa and shear strains of γ = 0-2.3. We then used the Gassman poroelastic differential effective medium method to calculate resultant seismic anisotropy.

The grain-scale network of melt pockets developed a strong preferred orientation parallel to the maximum principal stress at γ < 0.4. At higher strains, the orientation of the grain-scale melt pockets remained parallel to the maximum principal stress, but incipient, sample-scale melt bands formed at ~25° antithetic to the direction of shear.  While the orientation of individual melt pockets evolved quickly, grain SPOs and CPOs required larger strains (γ > 2) to strengthen and change. A weak SPO and CPO were induced during sample preparation, with grain long axes oriented perpendicular to the direction of maximum principal stress and a- and c-axes girdled perpendicular to the long axis of the sample. At the highest explored shear strain, a strong SPO was established and the girdled a-axes of the CPO rotated to align nearly parallel to the shear plane, developing clusters parallel to the shearing direction.  

These results yield two key conclusions about the orientation of melt networks in deforming partially molten rocks. First, the grain-scale and sample-scale alignments of melt pockets are distinct. At the grain scale, melt pockets align approximately parallel to the maximum principal stress, but the en echelon arrangement of melt pockets yields a sample-scale MPO at ~25^o to the maximum principal stress (20^o to the shear plane). Second, the relative timescales of melt and solid microstructural evolution are different, and are directly reflected in changes to seismic anisotropy. The grain-scale MPO reacts to a change in the orientation of the maximum principal stress after only a small amount of strain; in contrast, CPOs and SPOs require much higher strains before responding to a change in stress conditions. Seismic anisotropy is greatest when olivine a-axes and the grain-scale orientation of melt pockets are in relatively close alignment, so anisotropy will quickly decrease with any change in the orientation of the stress field that results in a rotation of the MPO away from the orientation of olivine a-axes. Perturbations to a local stress field can thus be observed almost immediately due to a rapidly reorienting melt network, making MPOs a more valuable predictor of instantaneous change than CPO-driven anisotropies.