Depletion of davemaoite in Earth’s ambient lower mantle
2026-08-15
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.