Lower mantle seismic anisotropy and heterogeneities - insight from the thermoelastic properties of CaSiO3 perovskite

越来越多的证据表明，地幔对流的运动并不能有效地混合地球的深部内部，特别是在下地幔的顶部和底部。地震观测表明，波速和各向异性都有显著变化，表明不同岩石类型的混合不良。可能导致这些观察结果的最重要的材料之一是硅酸钙钙钛矿或CaSiO 3。它不仅是环境下地幔的主要组成部分，也是地壳物质向下地幔输送的重要阶段。因此，估计整个地球下地幔中CaSiO 3的含量可以阐明地球内部与地表之间的物质交换过程。这需要测量通过这种矿物的波速作为方向和微量元素组成的函数，这是该项目的重点。这项工作还将使PI Zhang和她的研究小组能够在德克萨斯州A M大学开发一个研讨会课程，重点是用于地球和其他行星从表面到内部的现代同步加速器实验技术。德州农工大学还没有开设类似的课程，这样的课程将为学生提供研究机会，将潜在的用户带到同步加速器设施，并培养美国下一代研究人员和科学家。该项目将评估CaSiO 3钙钛矿（davemaoite，Ca-pv）作为解释观测到的地球下地幔地震各向异性和不均匀性的潜在候选材料。利用布里渊光谱技术结合金刚石对顶砧技术测量了立方晶系和四晶系Ca-pv的热弹性性质。由于Ca-pv的不可淬灭性以及CaSiO 3与1024 nm波长的Nd：YAG激光之间的耦合较差，因此将使用CO2激光加热方法原位合成样品。将使用同步加速器X射线衍射确定Ca-pv的状态方程。利用立方和四元Ca-pv较好的约束热弹性性质，张丕和她的小组将评估Ca-pv在观测到的下地幔地震非均匀性中的作用，以及在结合Ca-的纹理测量之后估计Ca-pv产生的地震各向异性。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响进行评估，被认为值得支持审查标准。

Growing evidence shows that the motions of mantle convection do not effectively mix the Earth’s deep interior, most notably at the top and the bottom of the lower mantle. Seismic observations show significant variations in both wave speeds and anisotropy, indicative of poor mixing of different rock types. One of the most important materials that may be responsible for these observations is calcium silicate perovskite, or CaSiO3. It is not only a major component of the ambient lower mantle, but also an important phase in the surface crustal materials that are transported into the lower mantle. Thus, estimating the amount of CaSiO3 throughout the Earth’s lower mantle can elucidate the material exchange process between the Earth’s interior and the surface. This requires measurement of the wave speeds through this mineral as a function of direction and minor element composition, which is the focus of this project. This work will also enable PI Zhang and her research group to develop a seminar course at Texas A&M University focusing on modern synchrotron experimental techniques used to the Earth and other planets from the surface to the interior. No similar class has been offered at Texas A&M University, and such class will open enable research opportunities for students, bringing potential users to synchrotron facilities, and training the next-generation researchers and scientists in US. High school students from underrepresented groups will be recruited to work with PI on research projects, promoting diversity of the geoscience community.This project will evaluate CaSiO3 perovskite (davemaoite, Ca-pv) as the potential candidate for explaining the observed seismic anisotropy and heterogeneities in the Earth’s lower mantle. The thermoelastic properties of cubic and tetragonal Ca-pv will be measured through Brillouin spectroscopy methods combined with diamond anvil cell technique. Due to the unquenchable nature of Ca-pv and the poor coupling between CaSiO3 and Nd:YAG laser with the 1024 nm wavelength, the sample will be synthesized in-situ using the CO2 laser heating method. Equation of state of Ca-pv will be determined using synchrotron X-ray diffraction. Utilizing the better constrained thermoelastic properties of cubic and tetragonal Ca-pv, PI Zhang and her group will evaluate the role of Ca-pv in the observed seismic heterogeneities in the lower mantle, as well as estimate the seismic anisotropy produced by Ca-pv after combining with textural measurements of Ca-pv.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.