This research paper investigates the nature of the 660-km seismic discontinuity in Earth's mantle, which marks the boundary between the upper and lower mantle. The key findings and implications are:

1. We conducted high-pressure, high-temperature experiments to study the post-spinel transition, where ringwoodite breaks down into bridgmanite and ferropericlase.
2. We found that this transition occurs over an extremely narrow pressure interval of only about 0.01 GPa (equivalent to about 250 meters in depth) at mantle temperatures and compositions.
3. This extremely sharp transition can explain the observed sharpness of the 660-km seismic discontinuity, which seismological studies have shown to be less than 2 km thick.
4. The sharpness of the transition suggests that there is no need for a compositional difference between the upper and lower mantle to explain the 660-km discontinuity.
5. This supports the idea of whole-mantle convection in a chemically homogeneous mantle, rather than layered convection with distinct upper and lower mantles.
6. We also discuss how the thickness of the discontinuity might be affected by mantle dynamics, particularly the latent heat released during the phase transition in areas of vertical mantle flow.
7. We suggest that mapping variations in the sharpness of the 660-km discontinuity globally could provide insights into mantle dynamics and areas of vertical mantle flow.

This study provides important constraints on the nature of Earth's mantle and its dynamics, supporting models of whole-mantle convection and a chemically homogeneous mantle.

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.

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, 869–872. 10.1038/s41561-019-0452-1