Understanding of mantle seismic discontinuities is key to understanding the structure and dynamics of the Earth's mantle. The 660-km seismic discontinuity is the boundary between the upper and lower mantles; because the discontinuity depth is close to the pressure of the post-spinel transition (ringwoodite (Mg,Fe)2SiO4 decomposes to bridgmanite (Mg,Fe)SiO3 and ferropericlase (Mg,Fe)O), it is usually considered that the 660-km seismic discontinuity should be caused by the post-spinel transition. However, some anomalies at this boundary such as significant depressions of 660-km discontinuity at subduction zones cannot be explained by the post-spinel transition. These depressions can be explained by the akimotoite-bridgmanite phase transition since akimotoite is supposed to be the most abundant mineral in subduction zone at 20-24 GPa.
In this study, we determined the akimotoite-bridgmanite (MgSiO3) phase transition boundary at 1250-1650 K in a multi-anvil press with in-situ X-ray diffraction at P61b at DESY. A starting material was bridgmanite MgSiO3. A stable phase was judged by observing change in the relative intensity of coexisting akimotoite and bridgmanite. We successively determined transition pressures by bracketing the phase boundary with increasing temperature from 1250 to 1650 K with a 50-K interval to suppress pressure drop.
The phase boundary was determined at 23.3-25.5 GPa in the investigated temperature range according to the Vinet-based MgO equation of state. The Clapeyron slope was determined as -13.6 at T < 1400 K and -1.2 at T > 1400 K. Such a steep slope of the akimotoite-bridgmanite phase transition boundary at low temperatures allows us to explain the existence of the 660-km seismic discontinuity depressions up to 740 km for the coldest slabs, such as the Tonga slab.