High Pressure Phase Transformations Down to the Lower Mantle Conditions and Implications for the Nature of the Seismic Discontinuities.

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High pressure and high temperature experiments have been made on mantle materials using both quench method and in situ X-ray diffraction measurement. The phase transformations in a pyrolite composition were studied by the former method at pressures to 28 GPa and temperatures along a representative mantle geotherm.It was demonstrated that iron partitioning of the coexisting phases in this composition affects significantly the width of the pressure interval between the modified spinel and spinel phases of olivine, which is consistent with the nature of the 410km discontinuity revealed by the recent seismological studies. On the other hand, we presented some data on the partitioning of iron between perovskite and magnesiowustite under the lower mantle conditions, and showed that the iron partition coefficient approaches unity with increasing pressure as a result of the increase of aluminum in the perovskite phase. The sharpness of the 660km seismic is discussed on the basis of these experimental data.The phase boundary between spinel and the postspinel phase has been determined by in situ X-ray measurements using a combination of the synchrotron source (SPring-8) and a multianvil device. The boundary was determined at temperatures up to about 2000ﾟC,demonstrating that the postspinel phase boundary has a negative Clapeyron slope as estimated by quench experiments and thermodynamic analyzes. The boundary was located at about 21 GPa, at 1600ﾟC,which is about 2GPa lower than the earlier estimations based on other high pressure studies.