Sharp 660-km discontinuity controlled by extremely narrow binary post-spinel transition

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Abstract

The Earth’s mantle is characterized by a sharp seismic discontinuity at a depth of 660 km that can provide insights into deep mantle processes. The discontinuity occurs over only 2 km—or a pressure difference of 0.1 GPa—and is thought to result from the post-spinel transition, that is, the decomposition of the mineral ringwoodite to bridgmanite plus ferropericlase. Existing high-pressure, high-temperature experiments have lacked the pressure control required to test whether such sharpness is the result of isochemical phase relations or chemically distinct upper and lower mantle domains. Here, we obtain the isothermal pressure interval of the Mg–Fe binary post-spinel transition by applying advanced multi-anvil techniques with in situ X-ray diffraction with the help of Mg–Fe partition experiments. It is demonstrated that the interval at mantle compositions and temperatures is only 0.01 GPa, corresponding to 250 m. This interval is indistinguishable from zero at seismic frequencies. These results can explain the discontinuity sharpness and provide new support for whole-mantle convection in a chemically homogeneous mantle. The present work suggests that distribution of adiabatic vertical flows between the upper and lower mantles can be mapped on the basis of discontinuity sharpness.

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Fig. 1: Phase relations in the system Mg2SiO4–Fe2SiO4.
Fig. 2: Backscattered electron images of the samples recovered from 23.86 GPa and 1,700 K (M2268).
Fig. 3: Expansion of discontinuity thickness of the post-spinel transition boundary by the Verhoogen effect26.

Data availability

Details of the cell assembly used, representative X-ray diffraction patterns, a backscattered electron image of Mg2SiO4 recovered sample, parameters for the thermodynamic calculations and supplementary discussion of thermodynamic calculations regarding the effects of secondary components can be found in the Supplementary Information. Any additional data can be requested by e-mailing the corresponding author.

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Acknowledgements

We appreciate H. Fischer, S. Übelhack, R. Njul, H. Schulze, U. Trenz and S. Linhardt at Bayerisches Geoinstitut for their technical assistance. We acknowledge N. Tomioka, A. Shatskiy, G. Manthilake, S.-M. Zhai, K. Saito, K. Kawabe, E. Ito, A. Kubo, S. Okita, T. Okishio, M. Sugita, M. Matsui, A. Kuwata, M.-S. Song and S. Yokoshi for their participation in the early stage of this study (2003–2004). This work was supported by the research project approved by DFG (KA 3434/7-1, KA3434/8-1, KA3434/9-1) and BMBF (05K16WC2) to T. Katsura and DFG (IS 350/1-1) to T.I. This project has been also supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (proposal no. 787 527). T.I. has been supported by a research fellowship for scientific research from the Japan Society for the Promotion of Science (JSPS) for Young Scientists, an overseas research fellowship from the Scientific Research of the JSPS for Young Scientists and an Alexander von Humboldt Postdoctoral Fellowship. The synchrotron X-ray diffraction experiments were performed in the beamline BL04B1 at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2003A0087, 2003B0638, 2004A0368, 2004B0497, 2015A1359, 2015B1196, 2016A1172, 2016A1274, 2016A1434, 2016B1094, 2017A1150, 2018A1071, 2018B1218).

Author information

T.I. conducted most of the experiments, analysed all the samples and data, conducted thermodynamic analysis in the Mg2SiO4–Fe2SiO4 system and wrote the manuscript. T.Katsura directed this project. R.H. and T.Katsura conducted trial runs of preliminary experiments. R.H. and H.F. helped in starting sample preparations. I.K. helped to establish the cell assembly. R.M. conducted thermodynamic calculations to discuss effects of secondary components. F.M., L.Y., Z.L., L.W., D.D., T.Y., S.B., R.F., T.Kawazoe, N.T., E.K., Y.H. and Y.T. operated synchrotron radiation experiments at the beamline BL04B1 at SPring-8. All authors discussed the results and commented on the manuscript.

Correspondence to Takayuki Ishii.

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Supplementary Figs. 1–5.

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