Shear Deformation of Wadsleyite in the Rotational Drickamer Apparatus

We have performed shear deformation experiments on samples of polycrystalline wadsleyite at 13-18 GPa and 1300-1800 K in the rotational Drickamer apparatus. Our goal is to collect quantitative data on the rheological properties of wadsleyite at transition zone conditions. Our experiments build on and extend the results of Nishihara et al. (2008) and Kawazoe et al. (2009) on deformation of olivine and wadsleyite. Our previous studies showed strong evidence for dynamic recrystallization and the resultant operation of some grain-size sensitive mechanisms of deformation. In order to investigate the role of grain-size sensitive creep in wadsleyite, we prepared fine-grained wadsleyite samples. The starting material, San Carlos olivine, was the same as in the previous studies, but had a smaller initial grain size (<2 μm). Wadsleyite was synthesized from this fine-grained olivine in a Kawai-type multianvil; the initial average grain size of the wadsleyite was approximately 1 μm. The initial water content of the undeformed wadsleyite was determined to be dry (<50 ppm H/Si). Deformation experiments were conducted at the X17B2 beamline of the National Synchrotron Light Source, using white x-rays to collect energy-dispersive diffraction patterns at a fixed 2θ of 6.7° over ten azimuth angles. X-ray diffraction spectra were collected in 300-900 s exposures over the course of each experiment. In situ determinations of strain were made from radiographs of the sample using a Mo foil inserted into the sample assembly as a strain marker. Uniaxial and shear stresses were determined from the observed d-spacing of multiple wadsleyite lattice planes as a function of azimuth angle, utilizing the theory of Singh (1993). Samples of wadsleyite were deformed at three different constant anvil rotation rates, corresponding to strain rates of approximately 10-4 - 10-5 s-1, in an effort to determine the stress exponent. Experiments were also conducted at three different constant furnace power conditions, corresponding to temperatures from 1300 to 1800 K.