Mineral measurements could explain mantle seismology

As tectonic plates shift and collide, slabs of cold, dense oceanic crust get pushed down into Earth’s mantle. Due to the heat and pressure at those depths, the crust’s minerals can undergo transformations into an assemblage of different structures. One suspected transformation would rearrange calcium and silicon, common elements in oceanic crust, into calcium silicate with a cubic perovskite (CaPv) structure. Petrological studies suggest that CaPv should constitute 30% of crustal material that makes it to the boundary between upper and lower mantle. The presence of cubic CaPv could explain the observed slowing of seismic waves approaching that boundary from the surface. But cubic CaPv is difficult to hold stable under conditions accessible to experiments, so researchers have had trouble measuring sound velocity through the mineral, and extrapolations based on tetragonal CaPv do not match seismic observations.

Now researchers at Ehime University and the Tokyo Institute of Technology in Japan have made the first sound velocity measurements of cubic CaPv. Steeve Gréaux and colleagues compressed a CaSiO₃ glass rod in a multi-anvil press at mantle-level pressures and temperatures (23 GPa and 1700 K) to form CaPv. The researchers measured the speed at which ultrasonic waves travelled through the sample, and they irradiated it with x rays to confirm its cubic structure. They found that cubic CaPv is 25% less rigid than predicted and sound waves therefore travel through it more slowly than previously thought. The sound velocities match seismic observations at the upper–lower mantle boundary depth of about 660 km.

The results provide evidence that CaPv-rich crust may accumulate in the uppermost lower mantle. The new measurements could help geologists trace how former crust has been transformed in Earth’s mantle and then returned to the surface as fresh crust. (S. Gréaux et al., Nature 565, 218, 2019 .)