Comparison of Olivine Grain Growth during Dynamic Recrystallization, Post-deformational Annealing, and Static Annealing

Strain localization is associated with dynamic recrystallization in shear zones. However, whether localization persists to form long-lived plate boundaries is debated because of the possible counteracting effects of grain growth. We deformed Balsam Gap dunite, a natural olivine aggregate, under axial compression to examine the relative contributions of strain energy and surface energy in facilitating grain boundary migration (GBM) and grain growth. Experiments were conducted at 1100-1200°C, 10-4-10-5 s-1, and 1300 MPa confining pressure. Samples that were allowed to recover after deformation show abundant evidence of surface energy-driven GBM, but average grain size does not increase significantly compared to that in samples quenched prior to annealing. In contrast, samples that continued to deform at a reduced strain rate (for the same time as the annealed samples) show both strain energy- and surface energy-driven GBM, and an increased grain size. These observations suggest that growth is enhanced by continued deformation at low stress. More sluggish grain growth in deformed samples that annealed under static conditions may result from continued recrystallization during stress relaxation, grain boundary pinning by accessory chromite, or melt along grain boundaries and at triple junctions. To facilitate comparison of our results to published grain growth laws for olivine, we also conducted a hydrostatic grain growth experiment on 10-20 μm powders of Balsam Gap dunite and San Carlos olivine separated by a Pt disc and annealed for 24 hours at 1100°C. A similar experiment was run at 1000°C for 10 minutes to constrain the initial grain size before annealing. Both powdered materials exhibit grain growth after 24 hours. However, although they had the same starting grain size, the annealed San Carlos olivine is slightly coarser than the annealed Balsam Gap dunite, which suggests that grain growth is inhibited in the dunite.