The Effect of Secondary-Phase Fraction on the Deformation of Olivine plus Ferropericlase Aggregates: 2. Mechanical Behavior

To study the mechanical behavior of polymineralic rocks, we performed deformation experiments on two-phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (f(Per)) between 0.1 and 0.8. Each sample was deformed in torsion at T = 1523 K, P = 300 MPa at a constant strain rate to a final shear strain of ? = 6 to 7. The stress-strain data and calculated values of the stress exponent, n, indicate that Ol in our samples deformed by dislocation-accommodated sliding along grain interfaces while Per deformed via dislocation creep. At shear strains of ? < 1, the strengths of samples with f(Per) > 0.5 match model predictions for both phases deforming at the same stress, the lower-strength bound for two-phase materials, while the strengths of samples with f(Per) < 0.5 are greater than predicted by models for both phases deforming at the same strain rate, the upper-strength bound. These observations suggest a transition from a weak-phase supported to a strong-phase supported regime with decreasing f(Per). Above ? = 4, however, the strength of all two-phase samples is greater than those predicted by either the uniform-stress or the uniform-strain rate bound. We hypothesize that the high strengths in the Ol + Per system are due to the presence of phase boundaries in two-phase samples, for which deformation is rate limited by dislocation motion along interfacial boundaries. This observation contrasts with the mechanical behavior of samples consisting of Ol + pyroxene, which are weaker, possibly due to impurities at phase boundaries.

Automated summary

In order to understand the mechanical behavior of polymineralic rocks, the deformation experiments were carried out on two-phase aggregates of olivine + ferropericlase. The study found that olivine deformed by dislocation-accommodated sliding along grain interfaces, while ferropericlase deformed via dislocation creep. The strength of the samples with higher ferropericlase fractions matched the model predictions, indicating a weak-phase supported regime, while the samples with lower ferropericlase fractions showed greater strength than predicted, suggesting a transition to a strong-phase supported regime. The presence of phase boundaries in the two-phase samples limited dislocation motion and resulted in higher strengths. Impurities at phase boundaries in samples consisting of olivine + pyroxene may lead to weaker mechanical behavior.