Akimotoite-bridgmanite phase transition explains depressed 660-km seismic discontinuity in subduction zones
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The seismic wave velocities abruptly increase at a depth near 660 km in the Earth’s interior, referred to as the 660-km seismic discontinuity, separating the Earth’s upper and lower mantle. This discontinuity is commonly interpreted by the dissociation of (Mg,Fe)2SiO4 ringwoodite to (Mg,Fe)SiO3 bridgmanite plus (MgFe)O ferropericlase. One of the prominent features of the 660-km discontinuity is a significant depression down to 750 km beneath cold subduction zones. The ringwoodite dissociation has a gentle P,T slope and cannot explain the considerable depression beneath cold subduction zones. Therefore, another phase transition with a steep negative P,T slope should cause the depression of the 660-km discontinuity. The akimotoite-bridgmanite transition in (Mg,Fe)SiO3 is one possible candidate.
We determined the boundaries of the ringwoodite dissociation and akimotoite−bridgmanite transition in the MgO-SiO2 systems over a temperature range of 1250-2085 K using advanced high-pressure-temperature experimental techniques combining large-volume presses with synchrotron in situ X-ray diffraction based on the strict definition of phase equilibrium. Our results demonstrate that the P,T slope of the ringwoodite dissociation is almost zero, whereas the akimotoite−bridgmanite boundary has a steep negative P,T slope, whose magnitude increases with decreasing temperature. Our results predict that beneath cold subduction zones, ringwoodite first dissociates into akimotoite plus periclase, and then akimotoite transforms to bridgmanite with increasing depth, explaining the deep depression of the 660-km discontinuity. Our prediction is also supported by seismological observations showing double reflections of the 660-km discontinuity beneath cold subduction zones in addition to the deep depression.
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.
