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总结如下：1.用高温量热法测定了Mg_2SiO_4-Fe_2SiO_4系橄榄石改性尖晶石-尖晶石相变的热焓。根据目前的热化学数据计算出的相图与Ringwood和Mayer(1970)和Kawada(1977)的相图不是很一致，但与Katsura和Ito(1989)的精密高压实验所确定的相图非常吻合。根据计算的相图，橄榄石变质尖晶石相变发生在地幔390-410公里处，而变质尖晶石-尖晶石相变发生在490-540公里处。390公里和650公里处的温度估计分别为1400和1600ﾟC。用同样的量热法测定了Mg_4Si_4O_(12)-Mg_3Al_2Si_3O_(12)&gt；体系中辉石-石榴石的转变热。计算的相图与Akaogi和Akimoto(1977)高压实验的相图符合得很好。从相图可以看出，地幔中辉石-石榴石的转变是逐渐发生的，深度在300-500公里左右，没有明显的地震速度间断。高压超过18 Gpa是通过使用新的布里奇曼锤子(直径26毫米)和烧结的钻石尖端产生的。利用这些新的研究手段，考察了β-Zn_2SiO_4向ZnSiO_3钛铁矿+氧化锌的转变。根据测定的硅酸锌中辉石-钛铁矿的转变热和Ni2SiO4尖晶石的歧化热，从热力学上计算了这些相平衡界面。

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.