Combined X-ray and Ultrasonic Measurements Under P-T Conditions of the Mantle Transition Region

We have developed techniques to perform simultaneous in situ X-ray and ultrasonic measurements of polycrystalline high-pressure phases at pressures to 19 GPa and temperatures to 1400C, using a combination of synchrotron radiation and a large-volume multianvil apparatus at SPring-8. High-resolution CCD image of the sample yielded precise determination of the sample length under high pressure and temperature, while the ultrasonic travel time was simultaneously measured by a pulse echo overlapping using the transfer function method developed by Li et al. (2004). The pressure and temperature were independently evaluated by means of the unit-cell volume measurements of the pressure marker and of the thermocouple e.m.f., respectively. The first attempt on (Mg0.9Fe0.1)2SiO4 ringwoodite in this method was highly successful, and we have also been conducting precise P-V-T-Vp-Vs measurements on a polycrystalline majorite sample with the O`pyrolite minus olivineO_L composition. By combining the elastic velocity data on these two major high-pressure phases, we can tightly constrain the chemical composition and constitution of the mantle transition region. We also expect substantial extension of the pressure range in these measurements toward the lower mantle regime can be made either using smaller cell assembly or using harder anvil materials, such as sintered polycrystalline bodies of cBN or diamond.