The post-spinel transition pressure in Mg₂SiO₄ was determined with a Kawai-type multi-anvil press combined with in-situ X-ray diffraction. We obtained the highest transition pressure of 23.6-23.9 GPa at 1700 K so far by our advanced multianvil technology, such as forced pumping during heating and anvil tapering [Ishii et al. 2016, 2017, 2019]. These pressures agree with those for the 660-km discontinuity at an expected mantle temperature of 2000 K. The present study resolved a question of 20 years’ standing, a discrepancy between the post-spinel transition pressure and the 660-km discontinuity depth, and indicates the applicability of the widely accepted models of the mantle. The lower pressures reported by previous studies are experimental artifacts due to the pressure drop upon heating.

Figure. Phase relations in the system Mg₂SiO₄-Fe₂SiO₄ at a temperature of 1700 K. Brg: bridgmanite, fPc: ferropericlase, Rw: ringwoodite, St: stishovite. At the mantle composition, a width of the Rw + Brg + fPc loop is only 0.01 GPa in pressure, corresponding to 250 m in depth. This width becomes even smaller at the mantle temperature of 2000 K. The horizontal axis is the fraction of Mg₂SiO₄ component in percent.

Ishii, T., Huang, R., Fei, H., Koemets, I., Liu, Z., Maeda, F., Yuan, L., Wang, L., Druzhbin, D., Yamamoto, T., Bhat, S., Farla, R., Kawazoe, T., Tsujino, N., Kulik, E., Higo, Y., Tange, Y., Katsura, T., 2018. Complete agreement of the post-spinel transition with the 660-km seismic discontinuity. Sci Rep-Uk 8, 6358.