RHEOLOGY MEASUREMENTS AT HIGH PRESSURE AND TEMPERATURE J. Ando, D. J. Weidner, Y. Wang, G. Chen, CHiPR and Dept. of Earth and Space Sciences, U. S. B., Stony Brook, NY 11794; 516-632-8241; email: dweidner@sunysb. edu

Measurement of rheological properties of Earth materials in a multi-anvil devices is accomplished with the aid of a synchrotron generated x-ray probe. Deviatoric stresses are generated by compressing a polycrystalline powder as a result of the interaction between individual grains. The stress is quantified by measurement of the broadening of the diffraction peaks. Generally, the magnitude of the deviatoric stress will exceed the nominal pressure unless the sample fails or flows. The change of the peak width with time defines the strain rate of the sample. Taken together, these data provide constraints on the rheological properties for these conditions.

In situ x-ray diffraction measurements of MgAl₂O₄ spinel at 10 GPa confining pressure and temperatures up to 1100C provide an example of such data. Strain rates of the order of 10^-7 s^-1 were measured at 600C with deviatoric stresses of the order of a few GPa. The total plastic strain in the sample amounted to a few per cent. The stress-strain rate-temperature conditions quantitatively agree with those for spinel given by Frost and Ashby (Deformation-Mechanism Maps The Plasticity and Creep of Metals and Ceramics) and correspond to the plasticity-power law creep boundary for this material.

The current limits of this approach are defined by the resolution of x-ray energy. Future improvements will come using monochromatic sources with high resolution detection systems. This will enable lower stress determinations and hence higher temperatures.