Elasticity of Mantle Minerals at High Pressures by Brillouin Scattering

了解地幔的结构、组成和演化是地球物理研究的中心目标。关于地球内部结构的最直接信息来自对地震波的分析。对地震数据的解释需要详细了解声速以及矿物的弹性和非弹性特性如何随压力、温度、晶体结构和成分而变化。弹性刚度张量表征了晶体的各向异性弹性响应。弹性模量也是一系列固态现象的基础，包括机械稳定性、原子间相互作用、材料强度、可压缩性和相变机制。在这个项目中，普林斯顿大学的Thomas Duffy正在使用布里渊散射来确定地壳和地幔中选定矿物的高压弹性特性。高压下布里渊散射测量特别有用，因为它们不仅能够约束体模量和剪切模量，而且还能约束弹性各向异性随压缩的变化。橄榄石多晶岩和辉石是上地幔中最丰富的矿物。有许多基本的未解决的问题与这些相的弹性有关。近年来，越来越多的人认识到橄榄石多晶具有容纳相当数量的氢作为结构缺陷的能力。即使是少量氢的存在也可能强烈影响地幔的一些重要性质。然而，目前还没有测量方法来限制氢在高压下对橄榄石或其高压多晶体瓦德斯莱石弹性刚度的影响。同样，水的掺入对其他上地幔矿物（如正辉石）弹性的影响尚不清楚，但具有潜在的重要地球物理意义。在这个提议中，Duffy正在测量氢的结合如何影响橄榄石多形物的弹性，直至相当于地球400公里深处的压力。他和他的学生们也在类似条件下测量与地幔相关的辉石矿物的弹性特性。在高压下测量了石英和硅辉石的弹性特性。对于硅辉石，达菲和他的学生们正在将单晶弹性测量扩展到高达50 GPa，对应于地球下地幔的压力。他们还使用多晶布里渊散射来研究矿物在20-100 GPa下的聚集体弹性。利用这些数据，达菲和他的学生们根据组成矿物的成分和晶体结构来解释地球深处的地震数据。这项研究的结果使我们对行星深处的结构、组成和演化有了更好的了解。

Understanding the structure, composition, and evolution of Earth's mantle is a central goal of geophysical research. The most direct information about the interior structure of the Earth comes for analysis of seismic waves. Interpretation of seismic data requires a detailed understanding of how the sound velocities and hence elastic and anelastic properties of minerals vary with pressure, temperature, crystal structure, and composition. The elastic stiffness tensor characterizes the anisotropic elastic response of crystals. Elastic moduli are also fundamental to a range of solid-state phenomena including mechanical stability, inter-atomic interactions, material strength, compressibility, and phase transition mechanisms.In this project, Thomas Duffy of Princeton University is using Brillouin scattering to determine the high-pressure elastic properties of selected minerals of the Earth's crust and mantle. Measurements by Brillouin scattering at high pressures are especially useful because of their ability to constrain not only the bulk and shear modulus but also the variation of elastic anisotropy with compression. Olivine polymorphs and pyroxenes are among the most abundant minerals in the upper mantle. There are a number of fundamental unsolved problems connected to the elasticity of these phases. In recent years, there has been growing recognition that olivine polymorphs have the capacity to accommodate appreciable amounts of hydrogen as structural defects. The presence of hydrogen even in small amounts might strongly influence a number of important properties of the mantle. However, there are no measurements yet to enable us to constrain the effects of hydrogen on the elastic stiffnesses of olivine or its high-pressure polymorph, wadsleyite, at high pressure. Similarly, the effects of H2O incorporation on the elasticity of other upper mantle minerals such as orthopyroxene are poorly known but have potentially important geophysical implications. In this proposal, Duffy is measuring how hydrogen incorporation affects the elasticity of olivine polymorphs up to pressures corresponding to 400-km depth in the Earth. He and his students are also measuring the elastic properties of mantle-relevant pyroxene minerals to similar conditions. The elastic properties of quartz and stishovite are being measured at high pressures. For stishovite, Duffy and his students are extending single-crystal elasticity measurements to as high as 50 GPa, corresponding to pressures of Earth's lower mantle. They are also using polycrystalline Brillouin scattering to study the aggregate elasticity of minerals at 20-100 GPa. Using such data, Duffy and his students are interpreting seismic data for the deep Earth in terms of the composition and crystal structure of the constituent minerals. The results of this research are yielding a better understanding of the structure, composition, and evolution of deep planetary interiors.