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

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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.