Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: 1. Mechanical Behavior

To understand the processes involved in rheological weakening due to phase mixing in olivine + orthopyroxene aggregates, we conducted high-strain torsion experiments on two-phase samples at a temperature of 1200°C and a confining pressure of 300 MPa. Samples composed of iron-rich olivine plus 26% orthopyroxene were deformed to outer radius shear strains of up to γ ≈ 26. Values for the stress exponent, n, and grain size exponent, p, were determined based on least squares fits of the strain rate, stress, and grain size data to a power law creep equation both at smaller strains (γ ≤ 3) and at larger strains (γ ≥ 24). Microstructural observations demonstrate that with increasing shear strain, grain size decreased and mixtures of small, equant grains of olivine and pyroxene developed. The values of n and p combined with associated changes in microstructure demonstrate that our samples deformed (i) by dislocation-accommodated grain-boundary sliding with subgrains present at lower strains and (ii) by dislocation-accommodated grain-boundary sliding with subgrains absent at higher strains. The evolution of both the mechanical and the microstructural properties observed in this study provide insights into the dynamic processes associated with rheological weakening and strain localization in the plastically deforming portion of the lithosphere.