Attenuation and modulus softening at high pressure and high temperature

We report an experimental method to study the anelastic properties of materials at high pressure and high temperature. The multi-anvil high pressure deformation device, used to apply a cyclic loading force onto the sample, can reach 15 GPa and 2000K. A synchrotron X-ray radiation source provides time resolved images of the sample and reference material. The images yield stress and strain as a function of time, stresses are derived from the reference material, strains from the sample. Strain as small as 10-5 can be resolved. We have obtained experimental results which exhibit resolvable attenuation factor (Q-1) and Young's modulus (E) at deep Earth conditions. Samples of San Carlos olivine, MgO, and a two phase mixture of olivine and ringwoodite for Fo30 have been studied. For San Carlos, these results are in quantitative agreement with previously reported lower pressure data and suggest that temperature and grain size have dominating effect on these properties with a very small pressure dependence of attenuation. The two phase region of olivine - ringwoodite suggest that bulk modulus softening results from driving the phase transformation by the oscillating load. This latter observation is consistent with a diffusion based model of Jackson. This model further predicts a softening of seismic P waves in regions of coexisting phases in the Earth.