Determination of post-spinel and post-majorite transitions by in situ X-ray observations with synchrotron radiation.

用<0.5><0.5>同步辐射X射线衍射原位研究了Mg_2SiO_4、MgSiO_3、Ca_ Mg_ SiO_3和软锰矿的相变。结果表明，在所有这些成分中，向MgSiO_3钙钛矿相的转变都明显低于淬火实验所测得的值（约1.2GPa）。这表明，后尖晶石相变可能不是负责660公里的地震不连续，或目前使用的压力标度可能是无效的下地幔体制的P/T条件。根据<0.5><0.5>高压高温条件下的X射线原位观测，首次证实了Ca_Mg_SiO_3分解为MgSiO_3和CaSiO_3钙钛矿组合的事实。在所研究的压力（21 ~ 28 GPa）和温度（1000 ~ 1800℃）下，Mg和Ca组分在这些钙钛矿中的互溶度非常有限。巴西报告的下地幔金刚石包裹体可能是在与板块俯冲有关的较低温度（&lt;1200℃）下形成的。在本计划的最后一年，继续开发使用烧结金刚石砧在多砧装置中产生更高压力的技术。因此，现在可以在40 GPa的压力和1500℃的温度下进行原位X射线观测。对MgAl_2O_4和Ca_ Mg_ SiO_3在此条件下的相变和P-V-T关系进行了初步的实验研究<0.5><0.5>。

Phase transitions in Mg_2SiO_4, MgSiO_3, Ca_<0.5>Mg_<0.5>SiO_3 and pyrolite were studied by in situ X-ray diffraction using synchrotron radiation. It was found that the transitions to MgSiO_3 perovskite-related phases in all of these compositions were significantly (〜2 GPa) lower than those determined by quench experiments. It is suggested that the postspinel phase transition may not be responsible for the 660 km seismic discontinuity, or currently used pressure scales may be invalid at the P/T conditions of the lower mantle regime. A possible scenario to reconcile these experimental results with the observed seismic profile is proposed based on the calculations of the density and seismic velocity changes in subducting oceanic crust and pyrolitic mantle materials.The dissociation of Ca_<0.5>Mg_<0.5>SiO_3 into an assemblage of MgSiO_3 and CaSiO_3 perovskites was firstly confirmed on the basis of in situ X-ray observation under simultaneous high pressure and high temperature conditions. The mutual solubilities of Mg- and Ca-components in these perovskite were found to be very limited at the pressures (21〜28 GPa) and temperatures (1000〜1800℃) studied. It is suggested that the diamond inclusions of lower mantle origin reported in Brazil may be formed at relatively low temperatures (<1200℃) related to the subduction of slabs.Development of technology to produce higher pressures in multianvil apparatus using sintered diamond anvils was pursued in the final year of the present program. As a result, in situ X-ray observations are now possible at pressures to 40 GPa and temperatures to 1500℃. Preliminary experimental studies have been made on the phase transitions and P-V-T relations in MgAl_2O_4 and Ca_<0.5>Mg_<0.5>SiO_3 under these conditions.