A High-strain Flow Law for Olivine: Grain-size Sensitivity and Crystallographic Fabric

The dislocation-accommodated grain-boundary sliding regime, which has recently been recognized in deformation experiments on olivine aggregates, has important implications for the rheological characteristics of the upper mantle and for the interpretation of microstructures in naturally deformed rocks. However, previous experimental studies suffer from both the difficulty in synthesizing samples with a desired mean grain size and the limitation that crystallographic fabrics have not fully developed at low strains. We alleviate these difficulties in a series of innovative deformation experiments. Aggregates of iron-bearing olivine, hot pressed as hollow cylinders, were deformed in torsion at 1200°C in a gas-medium apparatus at constant strain rate until a steady-state shear stress was reached, which occurred by a shear strain of ~5. Since recrystallized grain size is a function of shear stress, we were able to systematically vary the mean grain size among samples by changing the controlling shear-strain rate. Above a shear strain of ~5, strain rate stepping tests were performed to determine the stress exponent. Between each strain-rate step, the strain rate was returned to the original strain rate to reset the microstructure. Experiments were stopped when the total shear strain reached ~11. Our results yield a stress exponent of ~4 and a grain size exponent of ~1, both of which agree with previous small-strain compression experiments on olivine in the dislocation-accommodated grain-boundary sliding regime. If the dependence of strain rate on grain size in the power-law flow law is removed by inserting the grain-size piezometer, the apparent stress exponent in the resulting grain size independent flow law increases to 5.2. Microstructural analyses indicate that very strong crystallographic-preferred orientation (CPO) fabrics are developed with M-indices of ~0.6. The CPO fabrics have [100] maxima sub-parallel to the shear direction and [010] maxima sub-perpendicular to the shear plane, consistent with activation of the (010)[100] slip system. These results confirm the validity of grain-size exponents previously determined in compression experiments and demonstrate that grain-boundary sliding can form very strong CPO fabrics at high strain. Thus, the observation of a pronounced CPO in naturally deformed rocks may indicate the activation of grain-boundary sliding rather than simply dislocation creep, and both the grain-size and the deformation conditions need to be compared to newly derived flow laws to determine the dominant deformation mechanism.