Post-deformation grain growth in polymineralic rocks of olivine + ferropericlase

Abstract:

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.