Rheological weakening due to phase mixing in olivine + orthopyroxene rocks

Abstract:

To understand the processes involved in rheological weakening due to phase mixing, we conducted torsion experiments on samples composed of iron-rich olivine + orthopyroxene at a temperature of 1200°C and a pressure of 300 MPa using a gas-medium apparatus. The samples were deformed to total shear strains up to γ ≈ 26.

The stress exponent of n ≈ 3 and grain size exponent of p ≈ 1 at lower strain (1.9 ≤ γ ≤ 4.2) and n ≈ 2 and p ≈ 2.5 at larger strain (γ ≥ 26) were determined from a linear least-squares fit to the strain rate, stress, and grain size data using a power-law creep equation assuming a single power-law equation with a grain size dependence. These values of n and p indicate that our samples deformed primarily by dislocation-accommodated grain boundary sliding at lower strain, with an increased contribution of diffusion-accommodated grain boundary sliding at higher strain. Dynamic recrystallization occurred with significant grain size reduction of both phases. In samples deformed to low strain (1.9 ≤ γ ≤ 4.2), olivine and pyroxene grains are elongated sub-parallel to the shear direction. In contrast, in samples deformed to higher strains (γ ≥ 26), olivine and pyroxene are rounded and small. Orthopyroxene grains pin olivine grain boundaries and inhibit grain growth, and thereby keep deformation in a grain-size sensitive region. The microstructure and mechanical properties observed in this study are an important step toward understanding dynamical processes of strain localization and rheological weakening under upper mantle conditions