Effects of pressure on diffusion creep in wet olivine aggregates

Abstract

We report the results of an experimental study to determine the influence of pressure on diffusion creep in wet olivine. In order to observe diffusion creep at high-pressure conditions at laboratory strain rates, we prepared ultra-fine grained (0.22–0.72 μm) olivine aggregates both from San Carlos olivine and from oxide mixtures using a solgel technique. All samples contain some water (~1000 ppm wt) presumably due to adsorption on fine-grained powder samples. We used Deformation-DIA (D-DIA) coupled with synchrotron X-ray facility (6-BM-B beamline at APS) to perform in-situ high pressure (3–10 GPa) deformation experiments in the temperature range 973–1123 K. The operation of diffusion creep in our samples was inferred from the weak (hkl) dependence of lattice strain, the much smaller strength of our samples compared to the strength expected for dislocation creep, and the estimate of stress exponent. Diffusion creep was also confirmed from a comparison to previous results on diffusion creep at lower pressures. We assume the flow low of the following form ε˙∝CWrd−mexp−PV∗RT (ε̇: strain rate, CW: water content, d: grain size) with r = 0.7–1.0 and m = 2 or 3, from which we obtain the activation volume (V*) of 2.5 +/− 0.6 cm3/mol. This activation volume is substantially smaller than that for dislocation creep implying that diffusion creep plays an important role in the deep upper mantle. Some implications for seismic anisotropy are also discussed.