As partially molten rocks deform, they develop melt preferred orientations, shape preferred orientations, and crystallographic preferred orientations (MPOs, SPOs and CPOs). We investigated the co-evolution of these preferred orientations in experimentally deformed partially molten rocks, then calculated the influence of MPO and CPO on seismic anisotropy. Olivine-basalt aggregates containing 2 to 4 wt% melt were deformed in general shear at a temperature of 1250°C under a confining pressure of 300 MPa at shear stresses of τ = 0 to 175 MPa to shear strains of γ = 0 to 2.3. Grain-scale melt pockets developed a MPO parallel to the maximum principal stress, s1, at γ < 0.4. At higher strains, the grain-scale MPO remained parallel to s1, but incipient, sample-scale melt bands formed at ~20° to s1. An initial SPO and CPO were induced during sample preparation, with [100] and [001] axes girdled perpendicular to the long axis of the sample. At the highest explored strain, a strong SPO was established, and the [100] axes of the CPO clustered nearly parallel to the shear plane. Our results demonstrate that grain-scale and sample-scale alignments of melt pockets are distinct. Furthermore, the melt and the solid microstructures evolve on different timescales: in planetary bodies, changes in the stress field will first drive a relatively rapid reorientation of the melt network, followed by a relatively slow realignment of the crystallographic axes. Rapid changes to seismic anisotropy in a deforming partially molten aggregate are thus caused by MPO rather than CPO.

Observations of rocks from exhumed shear zones clearly reveal that secondary phases strongly influence the mechanical and microstructural evolution of materials undergoing large-strain shear deformation. Through Zener pinning, secondary phases promote grain size sensitive creep, allowing deformation to localize in fine-grained regions. For the longevity of such shear zones over geological times, fine grain sizes must be maintained between episodes of deformation such that localization will continue in these regions in subsequent deformation events. Experimental studies of static grain growth on single phase materials demonstrate relatively fast rates of grain growth that would serve to undo grain size refinement under natural conditions. Although static grain growth experiments on samples composed of two or more phases indicate slower growth rates of each phase, such studies have typically been carried out on undeformed material. To investigate the effectiveness of secondary phases at inhibiting grain growth after large-strain deformation, analog samples were synthesized from olivine (Ol) and ferropericlase (Fp) powders with Ol:Fp ratios of 1:5 to 5:1. Samples were deformed in torsion in a triaxial gas-medium apparatus to shear strains of γ = 3 - 7 at P = 300 MPa and T = 1523 K to induce mixing between the two phases; specifically, the distribution of phase boundaries followed a random binomial distribution. Subsequently, sections of each sample were statically annealed at P = 0.1 MPa and T = 1523 K for 10 h or 100 h. Grain size measurements obtained via electron backscatter diffraction indicate that after 10 h of post-deformation annealing Ol grains are smaller by a factor of 1.5 and Fp grains are smaller by a factor of 3 than predicted from single-phase grain growth laws. Comparing the ratio of grain sizes of the two phases to the secondary phase fraction yields a power law fit with an exponent of ~ 0.4 in Ol-rich samples and ~ 1 in Fp-rich samples. These results, along with microstructural observations, indicate that secondary phase particles are primarily distributed along grain boundaries in the Ol-rich samples but are randomly dispersed in Fp-rich samples. Our results demonstrate that secondary phases are highly effective at pinning grain size during static annealing following significant deformation.

We calibrate a subgrain-size piezometer using electron backscatter diffraction (EBSD) data collected from experimentally deformed samples of olivine and quartz. Systematic analyses of angular and spatial resolution test the suitability of each dataset for inclusion in calibration of the subgrain-size piezometer. To identify subgrain boundaries, we consider a range of critical misorientation angles and conclude that a 1° threshold provides the optimal piezometric calibration. The mean line-intercept length, equivalent to the subgrain-size, is found to be inversely proportional to the von Mises equivalent stress for datasets both with and without the Holyoke and Kronenberg (2010) correction. These new piezometers provide stress estimates from EBSD analyses of polymineralic rocks without the need to discriminate between relict and recrystallised grains and therefore greatly increase the range of rocks that may be used to constrain geodynamic models.