Shear waves split into fast and slow waves when they travel through elastically anisotropic media, and the anisotropy of the seismic velocity is recorded by seismic stations. In the Earth's deep interior, this is usually interpreted as the effect of crystallographic preferred orientation (CPO) of the constituent minerals.

In the uppermost lower mantle, seismic anisotropy is ubiquitous near subducting slabs, where shear waves with horizontal polarization propagate faster than those with vertical polarization (VSH > VSV).

Phase D, an elastically anisotropic hydrous mineral, is stable around cold subducting slabs at depths of mid mantle, potentially being the source of seismic anisotropy. To investigate this, a team of researchers in Japan performed well-controlled deformation experiments on phase D aggregates under conditions of the lower mantle transition zone.

The uniaxial and simple shear deformation experiments were conducted on Mg-endmember phase D and Al-bearing phase D aggregates at 20GPa and 500–1,000°C. Strong (0001) fabrics were observed in both uniaxially compressed and simple shear samples.

The results suggest that phase D tends to predominantly glide in (0001) crystallographic planes, developing significant CPO under high pressure and high temperature conditions. The seismic anisotropy observed in the mid-mantle in several cold subduction zones can be explained by the deformation of phase D.