This article presents research on the 660-kilometer seismic discontinuity in Earth's mantle, which marks the boundary between the lower mantle and transition zone. The key findings are:

1. We determined the phase transition boundaries for two reactions:
   1. Ringwoodite to bridgmanite + ferropericlase (RBP transition)
   2. Akimotoite to bridgmanite (AB transition)
2. We found that the RBP boundary has almost no temperature dependence, while the AB boundary has a very steep negative slope at lower temperatures.
3. The steep AB boundary can explain the large depressions of the 660-km discontinuity observed in cold subduction zones, down to 740 km depth.
4. In cold slabs (T < 1260 K), the AB transition likely causes the depressed 660-km discontinuity, rather than the RBP transition as previously thought.
5. The steep negative slope of the AB transition may cause slab stagnation due to significant upward buoyancy.
6. In warmer regions, the RBP transition still explains the discontinuity at around 660 km depth.

We used advanced multi-anvil techniques with in situ X-ray diffraction to accurately determine these phase boundaries. Our findings provide new insights into mantle dynamics, particularly in subduction zones, and help explain seismological observations of depressed and multiple discontinuities in these regions.

Figure. Comparison of the ringwoodite dissociation (dot dashed black curve) and akimotoite−bridgmanite (colid violet curve) phase transition boundaries. Ak − MgSiO₃ akimotoite, Brg − MgSiO₃ bridgmanite, Pc − MgO periclase, Rw − Mg₂SiO₄ ringwoodite. Green and red circles indicate Rw→Ak+Pc dissociation at 670 km depth and Ak→Brg transition at 730-740 km depth, respectively.

Chanyshev, A., Ishii, T., Bondar, D., Bhat, S., Kim, E.J., Farla, R., Nishida, K., Liu, Z., Wang, L., Nakajima, A., Yan, B., Tang, H., Chen, Z., Higo, Y., Tange, Y., Katsura, T., 2022. Depressed 660-km discontinuity caused by akimotoite–bridgmanite transition. Nature 601, 69-73. 10.1038/s41586-021-04157-z