Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab

Abstract

The mineral akimotoite, ilmenite-structured MgSiO₃, exists at the bottom of the Earth’s mantle transition zone and within the uppermost lower mantle, especially under low-temperature conditions¹. Akimotoite is thought to be a major constituent of the harzburgite layer of subducting slabs, and the most anisotropic mineral in the mantle transition zone^2,3,4. It has been predicted that if akimotoite crystals are preferentially oriented by plastic deformation, a cold subducted slab would be extremely anisotropic⁵. However, there have been no studies of crystallographic preferred orientations and very few reports of plastic deformation experiments for MgSiO₃ ilmenite. Here we present plastic deformation experiments on polycrystalline akimotoite, which were conducted at confining pressures of 20–22 GPa and temperatures of 1,000–1,300 °C. We found a change in crystallographic preferred orientation pattern of akimotoite with temperature, where the c-axis maximum parallel to the compression direction develops at high temperature, whereas the c axes are preferentially oriented parallel to the shear direction or perpendicular to the compression direction at lower temperature. The previously reported difference in compressional-wave seismic anisotropy between the northern and southern segments of the Tonga slab at depths of the mantle transition zone⁶ can conceivably be attributed to the difference in the crystallographic preferred orientation pattern of akimotoite at varying temperature within the slab.