image: (a, b) Images show deformation cell assemblage and microstructure of shear-deformed Al bearing-phase D aggregate under 20GPa, 800°C, conditions of lower mantle transition zone. Large deviation of strain marker and elongated grains indicate that significant strain was applied to the sample. (c) The inverse pole figure suggests that the (0001) lattice plane predominantly aligns in the shear plane after deformation.
Credit: Wentian Wu, Ehime University
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, we performed well-controlled deformation experiments on phase D aggregates under conditions of the lower mantle transition zone. Our 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.
Journal
Journal of Geophysical Research Solid Earth