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The post-spinel transformation in Mg2SiO4 and its relation to the 660-km seismic discontinuity

Nature volume 411, pages 571–574 (2001)Cite this article

Abstract

The 660-km seismic discontinuity in the Earth's mantle has long been identified with the transformation of (Mg,Fe)2SiO4 from γ-spinel (ringwoodite) to (Mg,Fe)SiO3-perovskite and (Mg,Fe)O-magnesiowüstite. This has been based on experimental studies of materials quenched from high pressure and temperature1,2,3, which have shown that the transformation is consistent with the seismically observed sharpness and the depth of the discontinuity at expected mantle temperatures4. But the first in situ examination of this phase transformation in Mg2SiO4 using a multi-anvil press5 indicated that the transformation occurs at a pressure about 2 GPa lower than previously thought (equivalent to ∼600 km depth) and hence that it may not be associated with the 660-km discontinuity. Here we report the results of an in situ study of Mg2SiO4 at pressures of 20–36 GPa using a combination of double-sided laser-heating and synchrotron X-ray diffraction in a diamond-anvil cell. The phase transformation from γ-Mg2SiO4 to MgSiO3-perovskite and MgO (periclase) is readily observed in both the forward and reverse directions. In contrast to the in situ multi-anvil-press study5, we find that the pressure and temperature of the post-spinel transformation in Mg2SiO4 is consistent with seismic observations4,6 for the 660-km discontinuity.

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Figure 1: a–e, Representative X-ray diffraction patterns at the indicated P–T conditions.
Figure 2: Phase diagram of Mg2SiO4.
Figure 3: Pressure difference between two equations of state for periclase16,20 at different P–T conditions.
Figure 4: Seismic velocity profiles6 and phase boundaries of mantle constituents near the 660-km discontinuity.

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Acknowledgements

We thank S. Speziale for experimental assistance, and F. Dahlen, T. Irifune, Y. Fei, O. Anderson and I. Jackson for discussions. This work was supported by the NSF.

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Authors and Affiliations

  1. Department of Geosciences, Princeton University, Princeton, 08544, New Jersey, USA

    Sang-Heon Shim & Thomas S. Duffy

  2. CARS, University of Chicago, Chicago, 60637, Illinois, USA

    Guoyin Shen

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Correspondence to Sang-Heon Shim.

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Shim, SH., Duffy, T. & Shen, G. The post-spinel transformation in Mg2SiO4 and its relation to the 660-km seismic discontinuity. Nature 411, 571–574 (2001). https://doi.org/10.1038/35079053

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