Seismic Anisotropy in the Lower Mantle Transition Zone Induced by Lattice Preferred Orientation of Akimotoite

Seismic anisotropy has been widely observed near the subducting slabs in the lower mantle transition zone (MTZ) and is often interpreted by the lattice preferred orientation (LPO) of constituent minerals. Akimotoite is one of the dominant minerals near the cold subducting slabs. Therefore, we conducted the well-controlled uniaxial and shear deformation experiments on the MgSiO3 akimotoite aggregates at 21-23 GPa and 900-1300 degrees C by using the D111-type Kawai-type multianvil apparatus. We observed strong LPOs and the most dominant slip system of akimotoite is suggested to be <10<(1)over bar>0>(0001). The elastic wave velocities of deformed samples were calculated to be strong azimuthal and polarization anisotropy with the velocities of horizontally polarized shear waves greater than that of vertically polarized shear waves for the horizontal mantle shearing. Our results provide important implications for the origin of observed seismic anisotropies and the mantle flow directions in the lower MTZ. Plain Language Summary Seismic anisotropy can provide insight into the dynamic process of the Earth's deep interior observed in the lower mantle transition zone (MTZ). The seismic anisotropy can be often caused by lattice preferred orientation (LPO) of elastically anisotropic minerals. But the dominant minerals in the lower MTZ, ringwoodite and majorite, are nearly isotropic. Thus, another candidate is necessary to explain the observed seismic anisotropy. Akimotoite, (ilmenite-structured (Mg, Fe) SiO3), is elastically anisotropic and it is one of the main constituting minerals near the subducting slabs in the lower MTZ, which is also the most likely location for the origin of observed seismic anisotropy. Therefore, the LPO of akimotoite is likely to contribute to the seismological observations of anisotropy. To investigate the LPO of akimotoite, we performed the well-controlled deformation experiments on the akimotoite aggregates under the conditions near the subducting slabs in the bottom of lower MTZ. We observed the LPO formed during deformation, and we calculated the elastic seismic wave anisotropy corresponding to the LPO of akimotoite. Our results indicate that the observed seismic anisotropy near the subducting slabs can be explained by the LPO of akimotoite.