TOWARD AN EXPERIMENTAL-STUDY OF DEEP MANTLE RHEOLOGY - A NEW MULTIANVIL SAMPLE ASSEMBLY FOR DEFORMATION STUDIES UNDER HIGH-PRESSURES AND TEMPERATURES

A new sample assembly for the multianvil high-pressure apparatus has b een developed that results in high-strain plastic deformation at high pressures and temperatures with minimal deformation during the initial pressurization stage. In this assembly, the sample is a thin disk whi ch is sandwiched between two pistons and oriented at 45 degrees to the ir long axis. The sample and pistons are surrounded by a Pt tube and a polycrystalline MgO cylinder. Upon pressurization, a uniaxial stress develops because of the anisotropy of mechanical properties. Deformati on during initial pressurization, which occurred in previous studies, is minimized by locating soft materials at the ends of the pistons and by the simple shear deformation geometry (as opposed to uniaxial comp ression) that allows sliding at the sample-piston interfaces at low pr essures. Large plastic strains, up to similar to 100% shear strain, ha ve been achieved in (Mg,Fe)(2)SiO4 phases at high pressures (up to 15 GPa) and high temperatures (up to 1900 K). A theoretical analysis has been made to evaluate the relative contributions to sample deformation from the relaxation of elastic strain in the sample column and from c ontinuing advancement of the multianvil guide blocks. The observed dep endence of strain on time, pressure and temperature suggests that defo rmation in the present experiments occurred mostly as a relaxation pro cess rather than at a constant strain rate caused by continuous piston movement. A comparison of the creep strength of olivine inferred from the strain relaxation data at similar to 15 GPa and similar to 1900 K with low-pressure data provides an estimate of the activation volume for creep of V=14 (+/-1) x 10(-6) m(3) mol(-1). The theoretical analy sis shows that constant strain rate deformation could result from the advancement of the guide blocks after complete stress relaxation, alth ough the total strain will be much less than that attained in the rela xation process. Possible applications of this technique to studies of high-pressure rheology and deformation microstructures in high-pressur e minerals are discussed, and strategies for future deformation experi ments under high pressures and temperatures are proposed.