1. Institute of Mineralogy, Petrology, and Economic Geology
2. Department of Geophysics, Tohoku University, Sendai 980-8578, Japan
3. Department of Earth Sciences, Chiba University, Chiba 263-8522, Japan
4. Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan

Correspondence to: Rei Shiraishi¹ Correspondence and requests for materials should be addressed to R.S. (Email: siraisir@ganko.tohoku.ac.jp).

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.

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