Ultrasonic Velocity and Density Measurements on Silicate Melts at Upper Mantle Pressures

熔融体存在于地球内部，在火山喷发和熔岩流动、地幔岩浆的形成和迁移、液态外核对流等各种地质过程中发挥着重要作用。例如，对熔体声速的精确了解有助于我们诊断地球深处是否存在熔体，而关于熔体和晶体密度对比的信息决定了地球演化过程中岩浆中晶体的沉降/漂浮。然而，表征地球上地幔深处熔体的行为和特性仍然是一项具有挑战性的任务。拟议的项目将(i)开发利用超声波干涉法对地球上地幔压力下的液体/熔体进行精确声速测量的新技术，以及（ii）对地球上地幔中最丰富的矿物橄榄石熔体进行测量。目前，使用静态下沉-浮子和冲击波方法进行密度测量是在地幔压力下限制熔体密度以测试密度交叉的主要工具，例如在碱性硅酸盐液体和共存的橄榄石之间。迄今为止，熔体的声速测量仅在环境压力下使用变样本深度技术进行；然而，由于缺乏精确测量熔体厚度或改变样品深度的方法，高压下的测量一直受到阻碍。pi已经通过使用x射线成像技术开发了克服这些困难的技术。他们建议对Mg2SiO4-Fe2SiO4的铁端元以及中间成分进行测量。这些测量和导出的状态热方程将补充并提供静态（下沉-浮子，x射线吸收），冲击波和计算机模拟的基准数据，从而可以精确表征上地幔，过渡区和下地幔中晶体/液体密度交叉的位置。

Melts exist in the Earth's interior and play important roles in various geological processes, such as volcanic eruption and lava flow, the formation and migration of magmas in the mantle, and the convection of liquid outer core. For instance, precise knowledge of sound speed of melts helps us diagnose the presence of melts in the Earth's deeper depths, and the information about the density contrasts between melts and crystals determines the sink/float of crystals in magmas in the evolution of the Earth. Characterization of the behavior and properties of melts at the Earth's upper mantle depths, however, remains a challenging task. The proposed project will (i)develop new techniques for precise sound speed measurements using ultrasonic interferometric method on liquids/melts at pressures of the Earth's upper mantle, and (ii)conduct measurements on melts of olivine, the most abundant mineral of the Earth's upper mantle. Currently, density measurements using static sink-float and shock wave methods have been the major tools in constraining melt densities at mantle pressures to test the density crossovers, such as between the basic silicate liquids and the co-existing olivine. To date, sound velocity measurements on melts have only been conducted at ambient pressure using varying-sample-depth technique; measurements at high pressures, however, have been hindered by the lack of means of precisely measuring melt thickness or varying the sample depths. The PIs have developed the techniques to overcome these difficulties by using an X-radiographic imaging technique. They propose to conduct measurements on the iron end-members as well as intermediate compositions of Mg2SiO4-Fe2SiO4. These measurements and the derived thermal equation of states will complement and provide benchmarking data for static (sink-float, X-ray absorption), shock wave, and computer simulations, allowing for a refined characterization of the locations of crystal/liquid density crossovers in the upper mantle, transition zone, and lower mantle.