Plastic Deformation of Wadsleyite and Olivine Under Deep Mantle Conditions Using a Rotational Drickamer Apparatus (RDA)

The top of the Earth's mantle transition zone is characterized by the transformation of olivine (Mg,Fe)2SiO4 to its high-pressure polymorph wadsleyite at ~14 GPa (410 km depth). If this transformation is accompanied by a change in rheological properties, it will have a dramatic influence on the role of the transition zone in material circulation in the deep earth. To determine the possible effects of the olivine to wadsleyite transformation on rheological properties, we have conducted large-strain plastic deformation experiments on wadsleyite (16 GPa, 1800 K) and olivine (11 GPa, 1800 K) using a rotational Drickamer apparatus (RDA) on beamline X17B-2 at the NSLS, Brookhaven National Laboratory. The olivine sample was San Carlos olivine hot-pressed at 5 GPa, 1500 K, and the wadsleyite sample was synthesized from the same starting material at 15 GPa, 1400 K, both with grain sizes of a few microns. In the RDA, a ring-shaped sample is compressed between two vertically-oriented, opposed tungsten carbide anvils with circular faces truncated to 4 mm diameter [1]. After compression and annealing at the run conditions, shear deformation in the RDA is achieved by rotating the upper anvil (the lower anvil is stationary). In this study, the olivine and wadsleyite samples were deformed by rotating the top anvil at a constant rate of 0.1 °/min. to a rotation angle of ~20°. We calculated sample stress using the d-spacings calculated from in-situ X-ray diffraction at six different azimuth angles, and we determined sample strain by monitoring the orientation of an X-ray opaque Re or Mo foil strain marker in X-ray radiograph images. The olivine and wadsleyite samples were deformed to strains of 0.6-0.7 at a strain rate of ~5 x 10-5 s-1. Under these conditions, olivine and wadsleyite reached steady-state stress by a strain of 0.2-0.25. The steady-state stress for wadsleyite at 16 GPa and 1800 K is 3.5 ± 0.3 GPa, while the steady state stress for olivine at 11 GPa and 1800 K is 1.8 ± 0.1 GPa. These experimental results show that wadsleyite is significantly stronger than olivine at comparable pressure and temperature. This observed difference in strength suggests that there is a significant increase in viscosity from the base of the upper mantle to the transition zone, if other conditions such as water content are similar. [1] Xu, Y., Nishihara, Y., and Karato, S. in Frontiers in High-Pressure Research: Applications to Geophysics (eds. Chen, J., Wang, Y., Duffy, T. S., Shen, G., and Dobrzhinetskaya, L. F.) 167-182 (Elsevier, Amsterdam, 2005).