Depletion of davemaoite in Earth's ambient lower mantle.

The mineralogy of the Earth's lower mantle is fundamental to understanding the planet's formation, composition, evolution, and dynamics. However, uncertainties in the elastic properties and phase transitions of CaSiO₃ davemaoite-the third most abundant phase in a pyrolitic lower mantle-have hindered quantitative links between seismic observations and lower-mantle mineralogy. Here we combine Brillouin spectroscopy with resistively heated diamond anvil cell experiments to measure the sound velocities of CaSiO₃ davemaoite up to 80 GPa and 973 K. We show that the tetragonal-cubic phase transition occurs below 900 K throughout the entire lower mantle. Fitting the thermoelastic properties of cubic davemaoite reveals substantially reduced shear-wave velocities at deep lower-mantle conditions. Because of these intrinsically slow velocities, increasing bridgmanite abundance alone cannot reconcile seismic observations for a normal lower mantle containing even small amount of davemaoite. Our results therefore suggest that davemaoite is depleted in the ambient lower mantle but concentrated within LLSVPs, likely as a consequence of late-stage crystallization during magma-ocean differentiation.