Longevity of strain localization associated with dynamic recrystallization of olivine in mantle rocks

Abstract:

The processes that cause strain localization in the lithosphere are fundamental to our understanding of plate tectonics. The relationship between localization and grain size reduction by dynamic recrystallization (DRX) is ubiquitous in natural shear zones, but whether localization by DRX persists to high strains is debated because of the potential role of grain growth. If DRX is associated with surface energy-driven grain boundary migration (γGBM), for example, grains will grow after recrystallization and localization will be cyclical or temporary. At high stress, low temperature conditions, however, migration is theoretically predicted to be driven by strain energy (ρGBM). At these conditions, grain growth may be suppressed even in monophase aggregates, and localization will persist over geologic timescales. Here we examine the conditions at which DRX leads to permanent, as opposed to transient, localization by evaluating the role of grain growth in olivine aggregates. We deformed as-is Balsam Gap dunite in axial compression using a Griggs rig and molten salt cell. We conducted 12 experiments at 1100 and 1200°C, a strain rate of 10^-5 s^-1, and a confining pressure of 1300 MPa. After samples reached strains of ~30%, we relaxed the stress, either with the motor off or at a strain rate of 10^-6 s^-1, for specific time increments before quenching. We perform detailed microstructural analyses to identify the transition from ρGBM to γGBM at these experimental conditions. We predict the microstructure (at time t₁) will reflect ρGBM, with negligible net grain growth. After strain energy has been substantially reduced (time t_1+n), evidence of γGBM will be observed in the microstructure as 120° triple junctions and increased grain sizes. By isolating the conditions at which γGBM becomes more important than ρGBM, we can infer the grainsize-stress conditions under which DRX may promote permanent strain localization in the lithosphere.