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)(2)SiO4 wadsleyite and olivine samples were deformed at P similar to 16 GPa and T=1600 and 1800 K, and P similar to 11 GPa and T=1800 K, respectively, with equivalent strain rate of epsilon(over dot)(E) similar to 6 x 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 theological properties of a sample. Stress-strain curves show strain-hardening up to the equivalent strain of epsilon(E) similar to 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 similar to 11 GPa is much higher than those at lower pressures. (C) 2008 Elsevier B.V. All rights reserved.