Plastic deformation of wadsleyite and olivine at high-pressure and high-temperature using a rotational Drickamer apparatus (RDA)

Large-strain plastic deformation experiments of wadsleyite and olivine were conducted using a rotational Drickamer apparatus (RDA) up to pressure and temperature conditions corresponding to the Earth's mantle transition zone. Sintered ring-shaped (Mg,Fe)2SiO4 wadsleyite and olivine samples were deformed at P ∼ 16 GPa and T = 1600 and 1800 K, and P ∼ 11 GPa and T = 1800 K, respectively, with equivalent strain rate of ε˙E∼6×10−5 s−1. In situ observations of deforming samples were carried out using the synchrotron radiation facility at Brookhaven National Laboratory, NSLS, X17B2. Stress was measured by X-ray diffraction at six different angles with respect to the compression axis. The stress estimated by X-ray diffraction was in good agreement with the stress estimated from dislocation density (for olivine). Strain was determined using X-ray radiographs of a strain marker (Re or Mo foil). Deformation of samples with a RDA involves both uniaxial compression and simple shear. A new formulation is developed to analyze both components to determine the rheological properties of a sample. Stress–strain curves show strain-hardening up to the equivalent strain of ɛE ∼ 0.2 followed by the quasi-steady state deformation. Wadsleyite is found to be stronger than olivine compared at similar conditions and the creep strength of olivine at P ∼ 11 GPa is much higher than those at lower pressures.