Accurate determination of phase equilibrium diagrams by calorimetric measurements of high pressure silicate minerals and applications to structure of the earth's mantle.

本研究项目的重要科学成果概括如下：1.用高温量热法测定了Mg_2SiO_4-Fe_2SiO_4系统中橄榄石改性尖晶石-尖晶石转变焓。由本热化学数据计算的相图与Ringwood和Major（1970）及Kawada（1977）的相图不太一致，但与Katalina和Ito（1989）用精密高压实验测定的相图非常一致。从计算的相图可以看出，橄榄石-尖晶石相变发生在地幔中390-410 km处，而尖晶石-尖晶石相变发生在地幔中490-540 km处。390和650公里处的温度估计分别为1400和1600摄氏度。用同样的量热法测定了Mg_4Si_4O_3-Mg_3Al_2Si_3O_3体系中辉石-石榴石转变的焓<12><12>。在修正压力标度后，计算的相图与Akaogi和Akimoto（1977）的高压实验结果符合得很好。相图表明，在300-500 km深度范围内，地幔中的辉石-石榴子石转变是逐渐发生的，且没有明显的地震速度不连续性.通过使用具有烧结金刚石尖端的新型Bridgman砧（直径26 mm）产生超过18 GPa的高压。利用这些新顶砧研究了β-Zn_2SiO_4向ZnSiO_3钛铁矿加ZnO的转变.根据ZnSiO_3中辉石-钛铁矿转变和Ni_2SiO_4尖晶石分解的实验数据，计算了这些相平衡边界。

The important scientific results of this research project are summarized as follows.1. Enthalpies of the olivine-modified spinel-spinel transitions in the system Mg_2SiO_4-Fe_2SiO_4 were measured by high temperature calorimetric techniques. The phase diagrams calculated form the present thermochemical data are not well consistent with those by Ringwood and Major (1970) and Kawada (1977), but agree very well with that determined with precise high pressure experiments by Katsura and Ito (1989). From the calculated phase diagrams, it is concluded that the olivine-modified spinel transition occurs at about 390-410 km in the mantle, while the modified spinel-spinel transition at about 490-540 km. Temperatures at 390 and 650 km are estimated to be 1400 and 1600 ﾟC, respectively.2. Enthalpy of the pyroxene-garnet transition in the system Mg_4Si_4O_<12>-Mg_3Al_2 Si_3O_<12> was measured by the same calorimetric techniques. The calculated phase diagram was in good agreement, with that by high pressure experiments by Akaogi and Akimoto (1977), when the pressure scale was revised. It was concluded from the phase diagram that the pyroxene-garnet transition in the mantle occurs gradually with depth in the depth range of about 300-500 km and shows no sharp seismic velocity discontinuity.3. High pressures over 18 GPa were generated by using new Bridgman-anvils (26 mm in diameter) with sintered diamond tips. The transition of beta-Zn_2SiO_4 to ZnSiO_3 ilmenite plus ZnO was examined using these new anvils.4. From the measured enthalpies of the pyroxene-ilmenite transition in ZnSiO_3 and of the disproportionation of Ni_2SiO_4 spinel, these phase equilibrium boundaries were calculated thermodynamically.