Low‐Temperature Plasticity in Olivine: Grain Size, Strain Hardening, and the Strength of the Lithosphere

Abstract

Plastic deformation of olivine at relatively low temperatures (i.e., low‐temperature plasticity) likely controls the strength of the lithospheric mantle in a variety of geodynamic contexts. Unfortunately, laboratory estimates of the strength of olivine deforming by low‐temperature plasticity vary considerably from study to study, limiting confidence in extrapolation to geological conditions. Here we present the results of deformation experiments on olivine single crystals and aggregates conducted in a deformation‐DIA at confining pressures of 5 to 9 GPa and temperatures of 298 to 1473 K. These results demonstrate that, under conditions in which low‐temperature plasticity is the dominant deformation mechanism, fine‐grained samples are stronger at yield than coarse‐grained samples, and the yield stress decreases with increasing temperature. All samples exhibited significant strain hardening until an approximately constant flow stress was reached. The magnitude of the increase in stress from the yield stress to the flow stress was independent of grain size and temperature. Cyclical loading experiments revealed a Bauschinger effect, wherein the initial yield strength is higher than the yield strength during subsequent cycles. Both strain hardening and the Bauschinger effect are interpreted to result from the development of back stresses associated with long‐range dislocation interactions. We calibrated a constitutive model based on these observations, and extrapolation of the model to geological conditions predicts that the strength of the lithosphere at yield is low compared to previous experimental predictions but increases significantly with increasing strain. Our results resolve apparent discrepancies in recent observational estimates of the strength of the oceanic lithosphere.

Constraints on the Rheology of the Lithosphere From Flexure of the Pacific Plate at the Hawaiian Islands

“…However, it seems that the LTP rheology, which appears to govern the strongest portion of the lithosphere (Figures 1a and 1b), may also play an important role in generating plate tectonics. Recent studies in mineral physics suggest that the LTP deformation mechanism may strengthen in regions that undergo large amounts of strain and grain-size reduction (Hansen et al, 2019;Kumamoto et al, 2017), such as plate boundaries, which we suggest may complicate an understanding of how to initiate and maintain plate tectonics. Similarly, Hunter and Watts (2016) report that laboratory flow laws for LTP fit the observations of flexure at subduction zones but are too strong at the Hawaiian Islands.…”

Dislocation interactions during low-temperature plasticity of olivine strengthen the lithospheric mantle

“…Recent laboratory experiments have led to a new hypothesis regarding the cause of differences in lithospheric strength. Hansen et al (2019) investigated low-temperature plasticity of single crystals and aggregates of olivine through experiments in a deformation-DIA (D-DIA) apparatus. Experiments were conducted mostly at room temperature to inhibit processes involving long-range diffusion, such as diffusion creep and dislocation climb.…”

“…Both single crystals and aggregates exhibited strain hardening of 1-2 GPa between the yield stress and flow stress. Importantly, Hansen et al (2019) conducted experiments in which the samples were subjected to cycles of shortening and extension. These experiments revealed that the yield stress during the initial loading was consistently greater in magnitude than those on subsequent cycles.…”

“…These interactions manifest as back stresses among dislocations that reduce the apparent yield stress upon load reversal. Based on these mechanical data, Hansen et al (2019) formulated a new flow law for low-temperature plasticity in olivine that characterises strain hardening by accumulation of back stress. Lithospheric-strength profiles for 80…”

“…In the present paper, we analyse the microstructures of samples of olivine deformed by low-temperature plasticity to test the hypothesis that strain hardening results primarily from long-range elastic interactions between dislocations. We utilise samples deformed at room temperature in a D-DIA apparatus by Hansen et al (2019) and in nanoindentation experiments by Kumamoto et al (2017) to assess the processes operating in both sets of experiments. We employ HR-EBSD to simultaneously map densities of GNDs and heterogeneity in residual stress, along with scanning transmission electron microscope (STEM) images of dislocation arrangements.…”