Stress measurement under high pressure using Kawai-type multi-anvil apparatus combined with synchrotron radiation.

A system for stress measurement under high pressure has been developed at beamline BL04B1, SPring-8, Japan. A Kawai-type multi-anvil apparatus, SPEED-1500, was used to pressurize polycrystalline KCl to 9.9 GPa in a mechanically anisotropic cell assembly with the KCl sample sandwiched between dense Al(2)O(3) pistons. The variation of deviatoric stress was determined from the lattice distortion measured using two-dimensional X-ray diffraction with monochromatic synchrotron X-rays. The low-pressure B1 phase transformed to the high-pressure polymorph B2 during compression. The deviatoric stress increased with increasing pressure in both the B1 and B2 phases except for the two-phase-coexisting region at a pressure of 2-3 GPa. This new system provides one of the technical foundations for conducting precise rheological measurements at conditions of the Earth's lower mantle.

Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab.

The mineral akimotoite, ilmenite-structured MgSiO(3), exists at the bottom of the Earth's mantle transition zone and within the uppermost lower mantle, especially under low-temperature conditions. Akimotoite is thought to be a major constituent of the harzburgite layer of subducting slabs, and the most anisotropic mineral in the mantle transition zone. It has been predicted that if akimotoite crystals are preferentially oriented by plastic deformation, a cold subducted slab would be extremely anisotropic. However, there have been no studies of crystallographic preferred orientations and very few reports of plastic deformation experiments for MgSiO(3) ilmenite. Here we present plastic deformation experiments on polycrystalline akimotoite, which were conducted at confining pressures of 20-22 GPa and temperatures of 1,000-1,300 degrees C. We found a change in crystallographic preferred orientation pattern of akimotoite with temperature, where the c-axis maximum parallel to the compression direction develops at high temperature, whereas the c axes are preferentially oriented parallel to the shear direction or perpendicular to the compression direction at lower temperature. The previously reported difference in compressional-wave seismic anisotropy between the northern and southern segments of the Tonga slab at depths of the mantle transition zone can conceivably be attributed to the difference in the crystallographic preferred orientation pattern of akimotoite at varying temperature within the slab.