Low-temperature plasticity of olivine during high stress deformation of peridotite at lithospheric conditions — An experimental study

Deformation experiments on natural peridotite from the Almklovdalen complex, Norway, were carried out in a Griggs-type apparatus at temperatures of 20, 300, and 600 °C, confining pressures of 1.0 to 2.5 GPa, and strain rates of 3·10−6 to 8 · 10−5 s−1. The experiments yield maximum differential stresses in the range of 1.0 to 2.9 GPa falling mostly between Byerleeʹs law and Goetzeʹs criterion and thus indicating semi-brittle behaviour. Whereas strength of samples deformed at 20 °C increases significantly with increasing confining pressure, a systematic pressure-dependence of strength is not obvious at 300 and 600 °C. The intracrystalline deformation features of the main constituent olivine were analysed by light and electron microscopic techniques (SEM/EBSD, TEM). Deformation microstructures systematically vary with temperature, but are insensitive to confining pressure. Samples deformed at 20 °C reflect predominantly brittle failure by intragranular microcracks and shear zones. Microstructures from samples deformed at higher temperatures show evidence of low-temperature plasticity of olivine in the form of pronounced undulatory extinction associated with high dislocation densities. Pile-up of dislocations leads to the formation of either fracture arrays at 300 °C or deformation lamellae parallel (100) and cellular structures at 600 °C, indicating intragranular work hardening. A gradual increase in glide-controlled crystal–plastic deformation of olivine at increasing temperature is interpreted to be responsible for the variation in mechanical behaviour and microstructural characteristics. The mechanical data and microstructural observations consistently suggest a temperature for the transition from the strength-controlling dominance of brittle to crystal–plastic deformation mechanisms close to 600 °C. The tested peridotite samples show a lower strength than quartzite samples at comparable experimental conditions, possibly related to crystallographic differences of olivine and quartz. The agreement between microfabrics of experimentally and naturally deformed peridotites demonstrates the importance of low-temperature plasticity of olivine during high-stress deformation at lithospheric conditions related in particular to seismic activity in the mantle.