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研究了地幔矿物的相平衡和硅酸盐熔体与矿物之间的分配关系，以揭示地幔矿物的组成和化学演化。现将本项目取得的重要成果介绍如下：1.测定了Mg_2SiO_4-Fe_2SiO_4系统中尖晶石的离散界，并对其钙钛矿进行了量热法测量。利用上述数据计算的尖晶石后转变界面表明，尖晶石与钙钛矿和镁闪锌矿的解离界面斜率为-3*1 Mpa/K，富镁尖晶石在相对较低的温度下首先解离为镁硅灰石和辉钛矿。石榴石在27 GPa时分解成钙钛矿和刚玉，在37 GPa时结合成单一的钙钛矿。在该体系中，石榴石固溶体向钙钛矿型…转变。更多的是通过两相场。这些结果表明，铝在下地幔中的寄主相为钙钛矿。石榴石-钙钛矿过渡界面斜率为2*1 Mpa/K，增强了地幔对流。Fe_3Al_2Si_3O_12&gt；石榴石在21 Gap时分解为组分氧化物。在CaAl_2O_4-MgAl_2O_4系统中发现了一种新的富镁六方相，并确定了其结构。得出结论：在以往的天然石榴石和MORB高压实验研究中观察到的未知富铝相与新发现的六方相具有相同的结构。钙钛矿固溶体在CaTiO_3-CaSiO_3体系中在15 Gpa以上是稳定的。钙钛矿中CaSiO_3的摩尔分数小于66%，成功地恢复到常温条件下。基于对回收钙钛矿的量热测量，估算了CaSiO_3钙钛矿的热值，并用来确定CaMgSi_2O_6辉石的离解边界。在硅酸盐熔体和矿物之间的元素分配实验中，所有元素在实验中都达到了1h以上的化学平衡。熔体与橄榄石或镁闪石之间的分配系数(D)在离子半径65 pm左右出现最大值。熔体和石榴石之间的D值在40-90 PM范围内几乎相似，高于这一范围时，D值突然降低。熔体与CaSiO_3钙钛矿之间的D值在50和100点左右出现两个极大值，两者之间存在一个极小值。较少

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