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

Lars N Hansen,Thomas Breithaupt,Kathryn M Kumamoto,David L Goldsby,Christopher A Thom,Cameron D Meyers,William B Durham,David L Kohlstedt,David Wallis

doi:10.1029/2018jb016736

Abstract

AbstractPlastic 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.

Highlights

The strength of Earth's lithosphere directly influences a wide variety of large‐scale geodynamic processes
The strength of Earth's lithosphere is dependent on the microphysical mechanisms of plastic deformation in olivine
We developed and calibrated a new constitutive model for low‐temperature plasticity in olivine that accounts for grain size effects and strain magnitude, using physical arguments based on local and long‐range interactions of lattice dislocations

Summary

Introduction

The strength of Earth's lithosphere directly influences a wide variety of large‐scale geodynamic processes. This research established models for the variation in general deformation mode as a function of depth into Earth's interior, suggesting that the integrated strength of the lithosphere depends intimately on the strength of ductile rocks in deeper and hotter regions. At the high temperatures characteristic of Earth's deep interior, dislocation motion is likely limited by diffusive processes, those dislocations involved in dislocation climb (e.g., Karato et al, 1993; Kohlstedt & Hansen, 2015; Weertman, 1970). At the colder temperatures characteristic of the strongest portions of the lithosphere, diffusion is less active and deformation is limited by dislocation glide. Ductile deformation in this regime is often referred to as low‐temperature plasticity The maximum stress supported by the lithosphere depends strongly on deformation by low‐temperature plasticity in the constituent minerals

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Discussion

Conclusion