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

研究了俯冲至下地幔上部压力-温度条件下，俯冲到下地幔的板片中由More和热解石组成的矿物的高压相关系。还进行了量热测量，并利用所获得的热力学数据确定了相关系。主要研究结果如下：(1)明确了高达30 Gpa的MgAl_2O_4-NaAlSiO_4和MgAl_2O_4-CaAl_2O_4体系的高压相关系，发现了具有六方对称性的新相。对六方相的结构分析表明，该相具有与钙铁氧体结构相关的新结构。这一阶段可能是下地幔中碱元素和铝的重要寄主。本文还用高压实验和量热法研究了NaAlSiO_4的相变，确定了向NaAlSiO_4铁酸钙转变的相界。(2)在1000℃~27 Gpa范围内，研究了Mg_4Si_4O_&lt；12-gt；-Mg_3Al_2Si_3O_2-lt；12&gt；明确了石榴石固溶体转变为钛铁矿固溶体，钛铁矿固溶体进一步转变为钙钛矿固溶体。测量了石榴石-钛铁矿-钙钛矿相变的焓，并计算了相界。(3)测定了CaSiO_3-CaTiO_3和CaSiO_3-CaGeO_3系钙钛矿固溶体的相变热，并估算了CaSiO_3钙钛矿在常温下不能还原的相变热。计算了下地幔主要组成矿物CaSiO_3钙钛矿的相关系。

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.