Phase Transformations and Thermodynamic Properties of Constituent Minerals of Slabs in the Lower Mantle

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High-pressure phase relations of minerals consisting of MORE and pyrolite in slabs subducting into the lower mantle were investigated up to pressure-temperature conditions of the uppermost lower mantle. Calorimetric measurements were also performed, and phase relations were determined using the obtained thermodynamic data. Main results are as follows.(1) High-pressure phase relations in the systems MgAl_2O_4-NaAlSiO_4 and MgAl_2O_4-CaAl_2O_4 were clarified up to 30 Gpa, and a new phase with hexagonal symmetry was found. Structural analysis of the hexagonal phase showed that this phase has a new structure related to calcium-ferrite structure. This phase may be an important host of alkali elements and aluminum in the lower mantle. Phase transitions in NaAlSiO_4 were also examined by high-pressure experiments and calorimetry, and the phase boundary for transition to NaAlSiO_4 calcium ferrite was determined.(2) Phase relations in the system Mg_4Si_4O_<12>-Mg_3Al_2Si_3O_<12> were examined at 1000 ℃ up to 27 Gpa. It was clarified that garnet solid solution transforms to ilmenite solid solution which further transforms to perovskite solid solution. Enthalpies of the garnet-ilmenite-perovskite transitions were measured, and the phase boundaries were calculated. Based on the results, it was established that the garnct-ilmcnitc-pcrovskitc transitions occur in pyrolite in subducting slabs at the uppermost lower mantle.(3) Enthalpies of perovskite solid solutions in the systems CaSiO_3-CaTiO_3 and CaSiO_3-CaGeO_3 were measured, and the enthalpy was estimated for CaSiO_3 perovskite which cannot be recovered at ambient conditions. Phase relations of CaSiO_3 perovskite, one of major constituent minerals of the lower mantle, were thermodynamically calculated.