Phase equilibrium study of mantle minerals at high pressure and high temperature.

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The author has developed the basic techniques for high pressure and high temperature experiments for pressures above 25 GPa and temperatures up to 2000ﾟC using an MA8 apparatus. A performance test has been adopted to evaluate different anvil materials and optimum conditions for high pressure generation have been explored. Moreover, improvements in the furnace assembly has been made to reach higher temperatures and to keep thermocouple failure minimum. Further, a digital recording system equipped with a micro-computer has been introduced, which has made phase equilibria studies to be conducted in greater accuracy than before.On the basis of these technical improvements in high pressure and temperature experiments, phase equilibrium studies have been made for major mantle minerals as follows: Diopside was found to decompose into assemblages of CaSiO_3 Perovskite plus MgSiO_3 components at pressures greater than 18 GPa. Precise determination of the phase relations in the system enstatite-pyrope has been made at pressures up to 27 GPa and temperatures at 1500ﾟC. Further, transformation mechanism of forsterite to spinal structures has been studied on the basis of both MA8 and transmission electron microscope (TEM) techniques.Phase transformations in pyrolite and MORB compositions have been estimated according to the present and recently determined phase equilibrium data by the authors. A mineral physics test has been made, in which seismic velocity and density changes associated with these phase transformations in a pyrolite composition are compared with those derived from geophysical observations. The results show that the calculated velocity and density profiles for the pyrolite mantle agree quite well with the observed distributions throughout the upper mantle and the transitions region.