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 zone2,3,4. 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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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sport, and Technology of the Japanese Government.
Author Contributions R.S. performed experiments and took the lead in writing the manuscript. E.O. and A.S. designed the study. K.K. performed EBSD analyses. D.Z. worked on the seismological aspects of this study. All co-authors took part in the discussion and interpretation of the results and improving the manuscript.
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Shiraishi, R., Ohtani, E., Kanagawa, K. et al. Crystallographic preferred orientation of akimotoite and seismic anisotropy of Tonga slab. Nature 455, 657–660 (2008). https://doi.org/10.1038/nature07301
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DOI: https://doi.org/10.1038/nature07301
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