Yield Stress of Periclase at High Pressure and Low Temperature

Magnesiowustite (Mg,Fe)O is a significant constituent of the Earth lower mantle. In a follow-up investigation of the rheological properties of MgO periclase as an analog for magnesiowustite, one powder specimen of MgO (micrometric grain size) has been investigated at pressures up to 10 GPa and temperatures ranging from 25\deg C to 300\deg C. The experiment was performed in a multi-anvil (SAM85, DIA-type) high-pressure apparatus, with boron epoxy as pressure medium and a BN capsule enclosing the specimen. Temperature was measured with a W3%Re-W25%Re thermocouple situated next to the sample, while the cell pressure (NaCl Decker's scale) and the stress in the sample (diffraction peak broadening, a review in Weidner, 1998) were monitored by using in-situ synchrotron X-ray diffraction at the superconductor wiggler beam line of the NSLS facility (Brookhaven, NY). The run product was investigated by Transmission Electron Microscopy (MET) using a Jeol 200 CX microscope. From the X-ray diffraction data, we infer that periclase exhibits at room temperature a transition from elastic to plastic deformation at differential stresses higher than 3 GPa. A yield stress value of just over 4 GPa was measured under these conditions. Slight sample relaxation over time was also observed during heating at 300\deg C. Although the specimen average grain size was larger than typically 150 nm during the run, the run-product exhibits a very small average grain size (about 40 nm measured by X-ray diffraction), which suggests that nano-cracking occurred in the highly stressed material during decompression. Investigation of the run product by TEM confirms the X-ray diffraction data. Most crystals exhibit, indeed, nanometric sub-domains and numerous defects. Among these defects, the expected <110>\{1/line{1} 0\} slip systems have been identified and seems to dominate the deformation at high pressure and low temperature, although cleavage along the \{001\} planes may also have occurred at low pressure during decompression. Weidner (1998) Rheological studies at high pressure, in "Ultrahigh-pressure mineralogy: Physics and chemistry of the Earth's deep interior", R. J. Hemley Ed., Reviews in Mineralogy, vol.37, Mineralogical Society of America, Washington D.C., pp.493-524.