Plastic deformation experiments to high strain on mantle transition zone minerals wadsleyite and ringwoodite in the rotational Drickamer apparatus

We report the results of plastic deformation experiments on polycrystalline wadsleyite and ringwoodite performed at 15–23 GPa and 1300–2100 K conducted using the rotational Drickamer apparatus (RDA). Wadsleyite was synthesized from fine-grained (∼2 μm) San Carlos olivine in a Kawai-type multianvil apparatus; the average grain size of the resulting wadsleyite was 1.2 μm. The initial water content of the undeformed wadsleyite was 24,000–26,000 H/10⁶ Si but the final water content is variable and less than the initial water content. Ringwoodite was synthesized from wadsleyite in situ in the RDA. Both strain and stress were measured in situ using a synchrotron x-ray facility. Determinations of strains and strain rates were made from x-ray radiographs of the sample, using a Mo foil strain marker in the sample assembly. The state of stress was determined from the observed d-spacing of multiple lattice planes as a function of azimuth angle. Samples were deformed at various strain rates at around 10⁻⁴-10⁻⁵ s⁻¹. Deformation mechanisms were inferred from the stress exponent and the microstructures. We determined the stress exponent n for wadsleyite to be 6±3, suggesting dislocation creep was the dominant deformation mechanism in wadsleyite. At grain sizes of ∼1 μm, our samples were still deforming primarily by dislocation creep. However, small dislocation-free grains are also observed suggesting that diffusion creep may operate in some parts of our samples.