Accurate determination of phase equilibrium diagrams by calorimetric measurements of high pressure silicate minerals and applications to structure of the earth's mantle.

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The important scientific results of this research project are summarized as follows.1. Enthalpies of the olivine-modified spinel-spinel transitions in the system Mg_2SiO_4-Fe_2SiO_4 were measured by high temperature calorimetric techniques. The phase diagrams calculated form the present thermochemical data are not well consistent with those by Ringwood and Major (1970) and Kawada (1977), but agree very well with that determined with precise high pressure experiments by Katsura and Ito (1989). From the calculated phase diagrams, it is concluded that the olivine-modified spinel transition occurs at about 390-410 km in the mantle, while the modified spinel-spinel transition at about 490-540 km. Temperatures at 390 and 650 km are estimated to be 1400 and 1600 ﾟC, respectively.2. Enthalpy of the pyroxene-garnet transition in the system Mg_4Si_4O_<12>-Mg_3Al_2 Si_3O_<12> was measured by the same calorimetric techniques. The calculated phase diagram was in good agreement, with that by high pressure experiments by Akaogi and Akimoto (1977), when the pressure scale was revised. It was concluded from the phase diagram that the pyroxene-garnet transition in the mantle occurs gradually with depth in the depth range of about 300-500 km and shows no sharp seismic velocity discontinuity.3. High pressures over 18 GPa were generated by using new Bridgman-anvils (26 mm in diameter) with sintered diamond tips. The transition of beta-Zn_2SiO_4 to ZnSiO_3 ilmenite plus ZnO was examined using these new anvils.4. From the measured enthalpies of the pyroxene-ilmenite transition in ZnSiO_3 and of the disproportionation of Ni_2SiO_4 spinel, these phase equilibrium boundaries were calculated thermodynamically.