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深部，最明显的是在下地幔的顶部和底部。地震观测显示，波速和各向异性都有显著差异，这表明不同类型的岩石混合不良。可能对这些观测结果负责的最重要的材料之一是硅酸钙，或称CaSiO3。它不仅是环境下地幔的主要组成部分，也是地壳表层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和表面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次要元素组成的函数，这是本项目的重点。这项工作还将使张皮和她的研究小组能够在德克萨斯农工大学开发一门研讨会课程，重点关注用于从地球表面到内部的地球和其他行星的现代同步加速器实验技术。德克萨斯农工大学没有开设过类似的课程，这样的课程将为学生提供研究机会，将潜在用户带到同步加速器设施，并培训美国的下一代研究人员和科学家。来自代表性不足群体的高中生将被招募到与PI合作的研究项目中，促进地球科学界的多样性。该项目将评估CaSiO_3钙钛矿(Davemao ite，Ca-PV)作为解释地球下地幔观测到的地震各向异性和不均一性的潜在候选者。立方和四方Ca-PV的热弹性性质将通过布里渊光谱学方法和金刚石顶压室技术相结合来测量。由于Ca-PV的不可猝灭特性以及CaSiO_3与波长为1024 nm的Nd：YAG激光之间的耦合较差，将采用CO_2激光加热的方法原位合成样品。用同步辐射X射线衍射法测定了Ca-PV的状态方程。利用立方和四方Ca-PV更好的约束热弹性性质，张皮和她的团队将评估Ca-PV在观测到的下地幔地震不均一性中的作用，并在结合Ca-Pv的结构测量后估计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智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.