Subducting oceanic lithosphere delivers significant amounts of mid-ocean ridge basalt (MORB) to the lower mantle. Parts of this crust may detach and accumulate at the upper–lower mantle boundary, forming chemically distinct layers that are linked to seismic anomalies: reduced shear wave velocities and increased electrical conductivity. Davemaoite makes up about 23 volume percent of subducted oceanic crust and is one of the slowest seismic phases in the lower mantle. Therefore, its phase transition from tetragonal to cubic structure, accompanied by spontaneous strain and softening of elastic moduli, can significantly affect seismic velocities. However, experimental data on the transition temperature were contradictory: early measurements with external heating gave about 600 K, whereas theoretical calculations predicted 1200–2000 K. The work stands apart from its neighbors in the archive, which mostly study magnetic materials and superconductivity, but thematically it is close to studies of graphene and collective behavior, as it also concerns structural distortions and phase transitions in condensed matter.
The authors used a diamond anvil cell with laser heating (LHDAC) combined with synchrotron X-ray diffraction at the SPring-8 source (beamline BL10XU). Samples of pure CaSiO3 and glass of composition CaSi0.75Ti0.25O3 were placed between gold foils (pressure marker) and films of boron-doped diamond (laser absorber), surrounded by KCl as a pressure-transmitting medium. Heating was performed with a fiber laser from both sides, and temperature was measured by spectroradiometric pyrometry with an accuracy of about 10%. Pressure was determined from the lattice parameter of gold. Diffraction patterns were analyzed by fitting peaks to cubic (Pm-3m) and tetragonal (P4/mmm) models. The transition was detected by a sharp change in the full width at half maximum of the (200) peak and by a jump in the c/a ratio. Additionally, synchrotron infrared Fourier-transform spectroscopy (FTIR) was performed in an externally heated cell up to 1000 K to confirm the transition via changes in vibrational spectra.
For pure CaSiO3, the transition from tetragonal to cubic phase was observed at temperatures from 1470 K at 50 GPa to 1990 K at 84 GPa. This is 500–1000 K higher than in previous experiments with external heating, but consistent with theoretical predictions. For the composition CaSi0.75Ti0.25O3, the transition boundary turned out to be even higher: from 1300 K at 15 GPa to 2410 K at 68 GPa, that is, about 600–700 K higher than for pure davemaoite at the same pressures. Thus, titanium stabilizes the tetragonal phase. Extrapolation to realistic titanium content in subducted MORB (5–10 mol%) gives transitions at depths from 700 to 1200 km along a cold slab geotherm. Thermal equations of state were also determined: for cubic CaSiO3 V0 = 45.54 ų, K0 = 243 GPa, γ0 = 1.47; for tetragonal CaSi0.75Ti0.25O3 V0 = 48.9 ų, K0 = 214 GPa. The density of davemaoite with 25% titanium is 1.2–1.5% lower than that of pure davemaoite, but still 1.5–3.4% higher than the density of the surrounding mantle.
The results resolve a long-standing discrepancy between experiments and theory: the transition indeed occurs at high temperatures, as predicted by calculations. This changes our understanding of where davemaoite is in the cubic phase in the mantle. In cold subducted slabs, it remains tetragonal to depths greater than 2000 km, while in hot regions near the core–mantle boundary it may transform to the cubic phase. Such sensitivity to temperature and composition can explain seismic scatterers in the mid-lower mantle, where shear wave velocity reductions of up to 12% are observed, and contributions to large low-shear-velocity provinces (LLSVPs) at the base of the mantle. The work differs from its archive neighbors in that it studies a phase transition in a mineral under extreme pressures and temperatures, rather than magnetic or superconducting properties, and for the first time directly measures the transition boundary in a laser-heated cell, rather than extrapolating it from low-temperature data.
The topic may develop toward more detailed studies of the influence of other impurities, such as aluminum and hydrogen, on the transition temperature. It is also important to measure the elastic properties of davemaoite near the transition to quantitatively link it to seismic velocities. Advances in laser heating and synchrotron diffraction techniques will allow reaching even higher pressures and temperatures, approaching conditions at the core–mantle boundary itself. Furthermore, modeling mantle convection with the new phase transition will help test hypotheses about the origin of LLSVPs.
The results will impact geophysics and mineralogy of the lower mantle, especially the interpretation of seismic data and models of chemical heterogeneity. They are also important for understanding the fate of subducted oceanic crust and its role in the thermal and chemical evolution of the Earth.
The next step will be to measure sound velocities in davemaoite at high pressures and temperatures near the phase transition. It is also necessary to investigate the effect of realistic titanium concentrations (5–10%) and other impurities on the transition boundary.
The work is directly related to the problem of explaining seismic anomalies in the lower mantle, particularly scatterers in the mid-mantle and large low-shear-velocity provinces near the core–mantle boundary. It also addresses the question of chemical heterogeneity of the mantle and the fate of subducting slabs, which remains one of the key unsolved problems in geophysics.
Davemaoite was officially recognized as a mineral only in 2021, although its existence in the lower mantle had been hypothesized for decades. The name honors geophysicist Ho-Kwang (David) Mao, who made enormous contributions to the study of minerals at high pressures.