Abstract The phase transformations of MgSiO3 bridgmanite control the structure, dynamics and chemistry of the Earth’s mantle. Formation of bridgmanite occurs at a depth of about 660 km causing the strong and abrupt seismic discontinuity. Previous experimental studies have revealed that this discontinuity is caused by ringwoodite dissociation in the average mantle. However, the cause of the 660‐km seismic discontinuity beneath hotspots remains unclear. Here we determine the phase relations in the MgSiO3 system near the 660‐km seismic discontinuity conditions. At 2,200–2,350 K with decreasing pressure, MgSiO3 bridgmanite first transforms to akimotoite and then to garnet. The akimotoite‐bridgmanite boundary has almost no temperature dependence, whereas the garnet–akimotoite transition has a very steep positive boundary slope. Based on these slopes, we calculated the garnet–bridgmanite boundary slope. Depending on the temperature regime, the akimotoite‐bridgmanite or the garnet–bridgmanite transition may occur in ascending plume beneath hotspots near the 660 km depth.

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