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The Earth's mantle is characterized by an abrupt velocity increase at a depth of 660-km, referred to as the 660-km discontinuity. Mineral physics shows that ringwoodite decomposes to bridgmanite plus ferropericlase around a pressure corresponding to the 660-km depth, referred to as the post-spinel transition. It has been therefore considered that the post-spinel transition causes the 660-km discontinuity.

One of the 660-km discontinuity features is that it occurs over an interval less than 2 km only, corresponding to 0.1 GPa in pressure. This extreme sharpness was not able to be duplicated by any laboratory experiment. One reason is that the pressure resolutions in previous studies were limited to 0.2~0.5 GPa. Another is that sample pressures dropped more than 2 GPa even with keeping temperature and press load. We developed high-precision pressure measurement to 0.05 GPa by in situ X-ray diffraction and attempted to keep the pressure constant by finely controlling the press load. By combining such state-of-the-art high-pressure experiments and thermochemical calculation, we have demonstrated that the pressure interval is only 0.01 GPa, corresponding to the discontinuity thickness of 250 m (Fig. 1).

These results are more than enough to interpret the discontinuity sharpness and provide new support for whole mantle convection in a chemically homogeneous pyrolitic mantle. The present work also proposes a new method to detect flows between the upper and lower mantles based on discontinuity sharpness because the interval should increase by a rapid flow across the 660-km discontinuity.

Fig. 1. 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.

Takayuki Ishii, Rong Huang, Robert Myhill, Hongzhan Fei, Iuliia Koemets, Zhaodong Liu, Fumiya Maeda, Liang Yuan, Lin Wang, Dmitry Druzhbin, Takafumi Yamamoto, Shrikant Bhat, Robert Farla, Takaaki Kawazoe, Noriyoshi Tsujino, Eleonora Kulik, Yuji Higo, Yoshinori Tange, Tomoo Katsura, 2019. Sharp 660-km discontinuity controlled by extremely narrow binary post-spinel transition, Nature Geoscience, 12, https://doi.org/10.1038/s41561-019-0452-1