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射线衍射研究了Mg_2SiO_4、MgSiO_3、Ca_<0.5>Mg_<0.5>SiO_3和软锰矿的相变。结果表明，与淬火实验相比，这些成分向MgSiO_3钙钛矿相关相的转变明显（~ 2 GPa）低。认为尖晶石后相变可能不是造成660 km地震不连续的原因，或者目前使用的压力标度在下地幔状态的P/T条件下可能是无效的。根据俯冲洋壳和热岩质地幔物质的密度和地震速度变化的计算，提出了一种可能的情景，使这些实验结果与观测到的地震剖面相一致。在高压和高温同时条件下，通过x射线原位观察，首次证实了Ca_<0.5>Mg_<0.5>SiO_3解离成MgSiO_3和CaSiO_3钙钛矿组合。在21 ~ 28 GPa的压力和1000 ~ 1800℃的温度下，钙钛矿中Mg和ca组分的相互溶解度非常有限。认为巴西下地幔金刚石包裹体可能形成于与板块俯冲作用有关的较低温度（<1200℃）。在本课程的最后一年进行了利用烧结金刚石砧在多砧装置中产生更高压力的技术开发。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，对MgAl_2O_4和Ca_<0.5>Mg_<0.5>SiO_3的相变和P-V-T关系进行了初步的实验研究。

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.