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

用同步辐射原位X射线衍射法研究了Mg_2SiO_4、MgSiO_3、Ca_&lt；0.5&gt；Mg_&lt；0.5&gt；SiO_3和热解石的相变。结果表明，所有成分向钙钛矿型镁钛矿相的转变均明显低于淬火实验所测得的转变温度(~2 GPa值)。认为尖晶石后的相变可能不是造成660公里地震不连续的原因，或者目前使用的压力表在下地幔的P/T条件下可能是无效的。基于俯冲洋壳和热解地幔物质的密度和地震速度变化的计算，提出了一种可能将这些实验结果与观测地震剖面相吻合的情景。在高压和高温同时进行的现场X射线观测的基础上，首次证实了Ca&lt；0.5&gt；mg&lt；0.5&gt；SiO_3解离为一组由MgSiO_3和CaSiO_3组成的组合。在所研究的压力(21~28 Gpa)和温度(1000~1800℃)下，钙镁组分在这些钙钛矿中的相互溶解度非常有限。据认为，巴西报道的下地幔成因的金刚石包裹体可能是在与板岩俯冲有关的相对较低的温度(&lt；1200℃)下形成的。在本计划的最后一年，开发了利用烧结金刚石锤在多压锤设备中产生更高压力的技术。因此，现在可以在40 Gpa的压力和1500℃的温度下进行现场X射线观测。在此条件下，初步研究了MgAl_2O_4和Ca_&lt；0.5&gt；Mg_&lt；0.5&gt；SiO_3的相变和P-V-T关系。

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.