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A doubling of the post-perovskite phase boundary and structure of the Earth's lowermost mantle

Nature volume 434, pages 882–886 (2005)Cite this article

Abstract

The thermal structure of the Earth's lowermost mantle—the D″ layer spanning depths of ∼2,600–2,900 kilometres1—is key to understanding the dynamical state and history of our planet. Earth's temperature profile (the geotherm) is mostly constrained by phase transitions, such as freezing at the inner-core boundary or changes in crystal structure within the solid mantle, that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory. A recently discovered phase transition at pressures of the D″ layer2,3,4 is ideally situated to reveal the thermal structure of the lowermost mantle, where no phase transitions were previously known to exist. Here we show that a pair of seismic discontinuities observed in some regions of D″ can be explained by the same phase transition as the result of a double-crossing of the phase boundary by the geotherm at two different depths. This simple model can also explain why a seismic discontinuity is not observed in some other regions, and provides new constraints for the magnitude of temperature variations within D″.

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Figure 1: The relationship between the geotherm and pPv phase boundary for different core temperatures (a), and corresponding seismic shear-wave profiles (b).
Figure 2: The relationship between three schematic geotherms and the pPv phase boundary (a), corresponding vS profiles (b), and a sketch of possible lower-mantle structures (c).
Figure 3: Comparison of data (a) with synthetic shear-wave seismograms calculated for the cold (b), warm (c), and hot (d) vS profiles shown in Fig. 2b.
Figure 4: Summary of the migration results for the synthetic cold-mantle (left column) and warm-mantle (centre column) vS profiles along with real data (right column) using the traces shown in Fig. 3.

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Acknowledgements

This collaboration was facilitated by the Meeting of Young Researchers in the Earth Sciences (MYRES) held in La Jolla, California, in August 2004. This work was supported by a grant from IGPP Los Alamos.

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Authors and Affiliations

  1. Department of Earth and Space Sciences,

    John W. Hernlund & Paul J. Tackley

  2. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California, 90095-1567, USA

    Paul J. Tackley

  3. Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3GP, UK

    Christine Thomas

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  1. John W. Hernlund
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Correspondence to John W. Hernlund.

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Hernlund, J., Thomas, C. & Tackley, P. A doubling of the post-perovskite phase boundary and structure of the Earth's lowermost mantle. Nature 434, 882–886 (2005). https://doi.org/10.1038/nature03472

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