Understanding the complexity of the 660-km seismic discontinuity

地幔占地球体积的70%，因此在地球演化中起着关键作用。自20世纪初以来，在地幔深处660公里处的地震不连续被认为是上地幔和下地幔之间物质交换（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。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尚未得到很好的理解。最近的高分辨率地震学研究揭示了结构不连续附近的复杂结构和结构不连续性质的强烈横向变化，表明地幔中发生了矿物相变之间的相互作用，并为高分辨率实验室测量提供了动力。为了测量660公里不连续区域附近不同相变之间的相互作用，研究人员将充分利用以下方面的最新发展：(1)钻石砧细胞（一种高压装置）技术，该技术可以精确控制压力；(2)激光悬浮合成技术，该技术允许研究人员设计与地幔岩石相似的起始材料组成；(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。这项研究的结果将有助于地球物理学家了解660公里的不连续在地幔对流和化学分异中的作用。这项工作是及时的，因为来自大型数字地震台网（如USARRAY）的大量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.