Phase Transitions and Melting of Minerals at Ultrahigh Pressures and Chemical Evolution of the Mantle

通过高压实验和量热法，阐明了地幔矿物的相平衡和硅酸盐熔体与矿物之间的分配，从而揭示了地幔的组成和化学演化。本项目取得的重要成果如下：1。测定了尖晶石在mg2sio_4 - fe_2sio_4体系中的解离边界，并对MgSiO_3-FeSiO_3体系中的钙钛矿进行了量热测定。利用上述数据计算的尖晶石后过渡边界表明，尖晶石与钙钛矿和镁辉石的解离边界斜率为-3*1MPa/K，富镁尖晶石在较低温度下首先解离为镁辉石和镁辉石2。Mg_3Al_2Si_3O_<12 bbb_，石榴石在27 GPa时解离成钙钛矿和刚玉，在37 GPa时结合成单相钙钛矿。在Mg_4Si_4O_<12>-Mg_3Al_2Si_3O_<12>体系中，石榴石固溶体经两相场转变为钙钛矿。这些结果表明，下地幔中Al的主相为钙钛矿。石榴石-钙钛矿过渡边界斜率为2* 1MPa/K，增强了地幔对流。Fe_3Al_2Si_3O_<12>石榴石在21gpa下解离成组分氧化物。在CaAl_2O_4-MgAl_2O_4体系中发现了一种新的富镁六方相，并确定了其结构。结果表明，以往在天然石榴石和MORB高压实验中观察到的未知富al相与新发现的六方相具有相同的结构。钙钛矿在CaTiO_3-CaSiO_3体系中的完全固溶体在15 GPa以上时是稳定的。成功地回收了CaSiO_3含量低于66 mol%的钙钛矿。根据回收的钙钛矿的量热测量，估计了casio3钙钛矿的焓，并用于确定CaMgSi_2O_6辉石的解离边界。在硅酸盐熔体与矿物的元素分配实验中，在1小时以上的实验中，所有元素都达到了化学平衡。熔体与橄榄石或镁辉石之间的分配系数(D)在离子半径约65pm时达到最大值。熔体与石榴石之间的D值在40 ~ 90pm范围内基本一致，在此范围以上D值突然减小。熔体与CaSiO_3钙钛矿之间的D值在50 pm和100 pm左右出现两个最大值，两者之间存在一个最小值。少

Phase equilibria of mantle minerals and partitioning between silicate melt and the minerals were clarified by means of high pressure experiments and calorimetry in order to shed light to constitution and chemical evolution of the mantle. The importnat results obtained in this project are described below.1. Dissciation boundaries of spinel in the system Mg_2SiO_4-Fe_2SiO_4 were determined, and calorimetric measurements of perovskites in the system MgSiO_3-FeSiO_3 were performed. The post-spinel transition boundaries calculated using the above data indicate that the spinel dissociation boundary to perovskite and magnesiowustite has a slope of-3*1MPa/K and that Mg-rich spinel first dissociates into magensioswustite and stishovite at relatively low temperatures.2. Mg_3Al_2Si_3O_<12>, garnet dissociates at 27 GPa into perovskite and corundum, which combine into a single phase perovskite at 37 GPa. In the system Mg_4Si_4O_<12>-Mg_3Al_2Si_3O_<12> garnet solid solution transforms to perovskite … More via the two-phase field. These results indicate that host phase of Al in the lower mantle is perovskite. The garnet-perovskite transition boundary has a slope of 2* 1MPa/K, which enhances mantle convection. Fe_3Al_2Si_3O_<12> garnet dissociates into component oxides at 21 GPa.3. A new Mg-rich hexagonal phase was found in the system CaAl_2O_4-MgAl_2O_4, and the structure was determined. It is concluded that unknown Al-rich phases observed in previous studies on high pressure experiments of natural garnet and MORB have the same structure as the newly found hexagonal phase.4. Complete solid solutions of perovskite are stable in the system CaTiO_3-CaSiO_3 at above 15 GPa. The perovskites with less than 66 mol% CaSiO_3 were successfully recovered to ambient conditions. Based on calorimetric measurements of the recovered perovskites, enthalpy of CaSiO3perovskite was estimated and used to determine the dissociation boundary of CaMgSi_2O_6 pyroxene.5. In the element partitioning experiments between silicate melt and minerals, chemical equilibrium in respect to all the elements was achieved in the experiments longer than 1 hour. The partition coefficients (D) between melt and olivine or magnesiowustite showed a maximum at around 65 pm of ionic radius. The D values between melt and garnet showed almost similar values in the range of 40-90 pm, above which they suddenly descreased. The D values between melt and CaSiO_3 perovskite showed two maxima at about 50 and 100 pm, and a minimum was present between them. Less