High pressure phase equilibria of constituent minerals of mantle and oceanic crust, and chemical heterogeneity of deep mantle

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1. High-pressure stability fields of calcium ferrite, hexagonal phase and hollandite were determined in the systems MgAl_2O_4-Mg_2SiO_4, MgAl_2O_4-NaAlSiO_4 and KAlSi_3O_8, respectively, based on high-pressure experiments, calorimetry, and lattice vibrational modeling using Raman spectra.2. Formation of oxygen vacancies in perovskites in the system MgSiO_3-MgAlO_<2.5> was established by studies using ^<27>Al-NMR spectroscopy and X-ray Rietveld analysis. We concluded that two types of ionic substitutions (Mg^<2+>+Si^<4+> = 2Al^<3+> and 2Si^<4+> + O^<2-> = 2Al^<3+> +V^0) occur in the aluminous perovskites.3. High-pressure phase relations in CaMgSi_2O_6, CaSiO_3 and in the system CaTiSiO_5-CaSi_2O_5 were determined in detail. In phase transition study on SrSiO_3, we found new hexagonal perovskite of SrSiO_3 and new BaGe_2O_5III-type SrSi_2O_<5->.4. Enthalpies of formation were measured and stability relations were determined for various high-pressure A^<2+>B^<4+>O_3 perovskites. We found systematic relations between the enthalpies of formation and cationic radii. The relations provide useful guide to estimate enthalpies of formation and synthesis pressures of unknown perovskites.5. Standard entropies of Mg_2SiO_4 modified spinel and spinel were measured, and used to calculate olivine-modified spinel-spinel transitions and postspinel transition. The results indicated that the postspinel transition occurs at pressure of 660 km depth at about 1900 K, which confirms that the postspinel transition is responsible for the 660 km-discontinuity.6. Post-perovskite transition boundary of CaIrO_3 has a strongly positive P-T slope, consistent with that of MgSiO_3.