Depressed 660-km discontinuity caused by akimotoite–bridgmanite transition

Abstract

The 660-kilometre seismic discontinuity is the boundary between the Earth’s lower mantle and transition zone and is commonly interpreted as being due to the dissociation of ringwoodite to bridgmanite plus ferropericlase (post-spinel transition)1–3. A distinct feature of the 660-kilometre discontinuity is its depression to 750 kilometres beneath subduction zones4–10. However, in situ X-ray diffraction studies using multi-anvil techniques have demonstrated negative but gentle Clapeyron slopes (that is, the ratio between pressure and temperature changes) of the post-spinel transition that do not allow a significant depression11–13. On the other hand, conventional high-pressure experiments face difficulties in accurate phase identification due to inevitable pressure changes during heating and the persistent presence of metastable phases1,3. Here we determine the post-spinel and akimotoite–bridgmanite transition boundaries by multi-anvil experiments using in situ X-ray diffraction, with the boundaries strictly based on the definition of phase equilibrium. The post-spinel boundary has almost no temperature dependence, whereas the akimotoite–bridgmanite transition has a very steep negative boundary slope at temperatures lower than ambient mantle geotherms. The large depressions of the 660-kilometre discontinuity in cold subduction zones are thus interpreted as the akimotoite–bridgmanite transition. The steep negative boundary of the akimotoite–bridgmanite transition will cause slab stagnation (a stalling of the slab’s descent) due to significant upward buoyancy14,15.

A New Approach Determining a Phase Transition Boundary Strictly Following a Definition of Phase Equilibrium: An Example of the Post-Spinel Transition in Mg2SiO4 System

“…Its combination with in situ X-ray diffraction can determine the Clapeyron slopes most accurately and precisely among various experimental and theoretical approaches [14]. One reason is the highly stable and homogenous temperature field produced by the resistively heated furnace, allowing a temperature resolution of 3 K at temperatures around 2000 K [15][16][17]. A second reason is the precise pressure determination by measuring relative volumes of pressurestandard materials by using X-ray diffraction.…”

“…A second reason is the precise pressure determination by measuring relative volumes of pressurestandard materials by using X-ray diffraction. The most advanced technology allows a pressure resolution of 0.05 GPa in the pressure range to 30 GPa [15][16][17]. A third is the nearly real-time observation of changes in phase assemblies and their proportions.…”

“…A third is the nearly real-time observation of changes in phase assemblies and their proportions. These advantages enable determining Clapeyron slopes robustly to discuss mantle structure and dynamics in detail based on these slopes [15][16][17][18][19][20][21][22][23][24][25][26][27].…”

“…Although this technique has provided us with the most robust results, there are discrepancies among previous studies due to various problems that occur during heating. To solve these problems, Chanyshev et al [17] developed a novel method using a multi-anvil press to determine a phase boundary strictly based on the definition of phase equilibrium and obtained the boundaries of the dissociation of ringwoodite to bridgmanite + periclase (post-spinel transition) in Mg 2 SiO 4 and akimotoite-bridgmanite transition in MgSiO 3 . In this paper, we explain the essential points of this technique in detail by taking the postspinel transition in Mg 2 SiO 4 as an example.…”

“…In this paper, we explain the essential points of this technique in detail by taking the postspinel transition in Mg 2 SiO 4 as an example. We note that the general experimental method for in situ X-ray diffraction in a multi-anvil press for this experiment was described in [17].…”