Understanding the complexity of the 660-km seismic discontinuity

地幔占地球体积的70%，因此在地球的演化中起着关键作用。世纪以来，地幔中660 km深度的地震间断被认为是上、下地幔物质交换（包括水和二氧化碳）的重要过滤器。实验室测量表明，主要地幔矿物的相变解释了这660公里的不连续性。然而，在该深度附近存在多个相变，并且这些相变之间的相互作用还没有很好地理解。近年来的高分辨率地震学研究揭示了该不连续面附近的复杂结构和不连续面性质的强烈横向变化，这表明地幔中存在矿物相变之间的相互作用，为高分辨率实验室测量提供了动力。调查人员将充分利用以下方面的最新进展：（1）金刚石砧室（高压装置）技术，其允许精细控制压力，（2）激光悬浮合成技术，这使得研究人员能够设计类似于地幔岩石预期的起始材料的成分，以及（3）高分辨率同步加速器X射线衍射技术，可以明确地检测相变。研究结果将有助于地球物理学家理解660 km不连续面在地幔对流和化学分异中的作用。这项工作是及时的，因为从大型数字地震台网（如USARAY）获得的巨大地震数据集开始提供高分辨率图像，以解决660公里不连续附近的精细尺度结构。

The mantle represents 70% of the Earth's volume, and therefore plays a key role in the evolution of the Earth. Since early 20th century, a seismic discontinuity at 660-km depth in the mantle has been believed to be an important filter for material exchange (including water and carbon-dioxide) between the upper and lower mantle. Laboratory measurements have shown that the phase transitions in the dominant mantle minerals explain this 660-km discontinuity. However, there exist multiple phase transitions near this depth and the interactions among these phase transitions are not well understood. Recent high-resolution seismology studies reveal complex structures near the discontinuity and strong lateral variations in the properties of the discontinuity, suggesting interaction between mineral phase transitions occur in the mantle and providing motivation for high-resolution laboratory measurements.In order to measure the interactions among different phase transitions near the 660-km discontinuity, the investigators will take full advantage of recent developments in (1) the diamond-anvil cell (a high pressure device) technique, which allows fine control of pressure, (2) the laser levitation synthesis technique, which allows the investigators to engineer the compositions of starting materials similar to those expected for mantle rocks, and (3) high-resolution synchrotron X-ray diffraction techniques, which allow detection of the phase transitions unambiguously. The results from this study will help geophysicists to understand the role of the 660-km discontinuity in mantle convection and chemical differentiation. Ths proposed work is timely because huge seismic datasets available from large digital seismic networks, such as USARRAY, are beginning to provide high-resolution images to resolve fine-scale structures near the 660-km discontinuity.