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。自20世纪初以来，地幔660公里深度的地震不连续面一直被认为是上、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，主要地幔矿物中的相变解释了这种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还没有被很好地理解。最近的高分辨率地震学研究揭示了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，表明矿物相变之间的相互作用发生在地幔中，并为高分辨率实验室测量提供了动力。为了测量660公里不连续面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-砧腔(一种高压装置)技术，它可以精确控制压力；(2)激光悬浮合成技术，它使研究人员能够设计出类似于地幔岩石的初始材料的组成，以及(3)高分辨率同步加速器X射线衍射技术，它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理解660公里长的不连续面在地幔对流和化学分异中的作用。这项拟议的工作是及时的，因为从大型数字地震台网(如美国国家地震台网)获得的巨大地震数据集正开始提供高分辨率图像，以解决660公里断裂面附近的精细结构。

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.