Structure and thermal elastic properties of calcium silicate perovskite

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Calcium silicate perovskite (CaPv) occurs as the third most abundant phase in the Earth’s lower mantle, next to (Mg,Fe)SiO3 bridgmanite and (Mg,Fe)O. It is considered a candidate mineral to explain some lower-mantle seismic observations and anomalies, as well as the main host for large cations in the lower mantle. Seismic properties of CaPv at mantle conditions, however, remain poorly constrained, as there are often contradictory theoretical and experimental predictions and determinations in the literature. Experiments and calculations have revealed multiple crystal structures of CaPv, but its phase transition boundary and elastic properties in the pressure-temperature-composition (P-T-X) space of relevance to the mantle are still under debate. This project will contribute to the following fundamental questions: (1) the fate of subducted oceanic crust and CaPv’s roles in lower mantle mineralogy and seismology, and (2) the origin of mantle’s seismic anomalies and heterogeneity. The project will directly contribute to the training of one graduate student at in-house laboratory and large synchrotron facilities and will also actively involve two early-career scientists. The research team will continue to mentor undergraduate students or high school students, particularly traditionally underrepresented minorities, i.e., Native Hawaiians and other Pacific Islanders, to study geosciences and planetary sciences by active involvement in ongoing research. The results from the proposed project will be disseminated via publications, national and international meetings, public lectures and news media.The proposed project aims to accurately measure the crystal structure and identify the phase transition boundary of (Al,Ti)-bearing CaSiO3 pervoskite in the P-T-X space of Earth’s mantle, employing laser-heated and externally-heated diamond anvil cell (EHDAC) and synchrotron X-ray diffraction techniques. The team will approach this goal by developing in-situ methods of growing non-quenchable silicate single crystals, and combining single-crystal/multigrain X-ray diffraction measurements with a newly developed EHDAC setups to reach temperatures above 1200 K. The proposed research aims to (1) determine the crystal structure and phase transformations of pure and (Ti,Al)-bearing CaPv, and (2) measure the thermal equation of state of CaPv at pressure-temperature conditions relevant to Earth's transition zone and lower mantle. The proposed measurements on the crystallographic phase transitions and thermal elastic properties of CaSiO3 will break open a bottleneck on the quest into the mantle mineralogy, and thus offer new understanding of CaPv’s contribution in elucidating the lower-mantle seismic observations and anomalies.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.