The influence of microstructure on deformation of olivine in the grain‐boundary sliding regime

Abstract

Although microstructural evolution is critical to strain‐dependent processes in Earth's mantle, flow laws for dunite have only been calibrated with low‐strain experiments. Therefore, we conducted a series of high‐strain torsion experiments on thin‐walled cylinders of iron‐rich olivine aggregates. Experiments were performed in a gas‐medium apparatus at 1200°C and constant strain rate. In our experiments, each at a different strain rate, a peak stress was observed followed by significant strain weakening. We first deformed samples to high enough strain that a steady state microstructure was achieved and then conducted strain rate stepping tests to characterize the creep behavior of each sample with constant microstructure. A global fit to the data yields a stress exponent of 4.1 and a grain‐size exponent of 0.73, values which agree well with those from previous small‐strain experiments conducted on olivine in the dislocation‐accommodated grain‐boundary sliding (GBS) regime. Strong crystallographic preferred orientations provide support for GBS accommodated by movement of (010)[100] dislocations. The observed strain weakening is not entirely explained by grain‐size reduction; thus, we propose that the remaining 30% reduction in stress is related to CPO development. To incorporate microstructural evolution in a constitutive description of GBS in olivine, we (1) derive a flow law for high‐strain deformation with steady state microstructure, which results in an apparent stress exponent of 5.0, and (2) present a system of evolution equations that recreate the observed strain weakening. Our results corroborate flow‐law parameters and microstructural observations from low‐strain experiments and provide a means for incorporating strain weakening into geodynamic simulations.