Large Strain Mechanical Behavior of Olivine + Periclase Rocks

Although polymineralic rocks are commonly present in shear zones on Earth, due to their complexity, few experimental studies have examined their mechanical behavior at high strain. To investigate the mechanical behavior of two-phase rocks at large strains, we performed a series of experiments on samples of olivine (Ol) + periclase (Per). This analog system provides insight into the mechanical behavior of two-phase rocks composed of a strong (Ol) and a weak (Per) phase. Samples containing fPer = 0.1 to 0.8, along with both single-phase Ol and Per samples, were deformed in a high-resolution gas-medium torsion apparatus at T = 1523 K, P = 300 MPa, and at a constant shear strain rate of d/dt = 1.5 10-4 s-1 up to strains of 4. Our experimental results demonstrate different behavior for samples with fPer < 0.5 and fPer > 0.5. Samples with fPer < 0.5 reach a peak stress of = 140 - 160 MPa by < 0.5, followed by gradual strain weakening of = 20 MPa up to = 6. Similar mechanical behavior is observed in single-phase Ol samples deformed via dislocation-accommodated grain boundary sliding, however a larger drop in stress occurs of 40 MPa. The difference between single-phase Ol and samples with fPer < 0.5 is consistent with the relatively small grain size reduction and minimal geometric softening in the two-phase samples. In contrast, samples with fPer > 0.5 yield at = 70 - 80 MPa and = 0.1, then continue to strain harden, resulting in an increase in stress of = 50 - 60 MPa by = 6. Although dislocation creep of single-phase Per does not exhibit strain hardening beyond = 1, the similar behavior up to = 1 suggests samples with fPer > 0.5 also deformed via dislocation creep. The continued hardening at high strain is likely due to interactions between mobile and immobile dislocations built up during deformation. Over the full range of fPer, the final sample strength is 10 - 40 MPa greater than that of either constituent phase. Our results demonstrate that the mechanical behavior of two-phase materials is determined by the majority phase and that, even if the majority phase is the weaker phase, the two-phase mixture can be stronger than predicted by a rule of mixtures.