Seismological constraints on a possible plume root at the core–mantle boundary

Abstract

Recent seismological discoveries have indicated that the Earth's core–mantle boundary is far more complex than a simple boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earth's crust1. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent2,3,4,5,6,7, which appear not to be global in extent7,8,9. Here we present robust evidence for an 8.5-km-thick and 50-km-wide pocket of dense, partially molten material at the core–mantle boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.

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Figure 1: Array beams of ScP and precursors.
Figure 2: ULVZ detections at ScP core-reflection locations.
Figure 3: Synthetic waveform modelling of ScP precursors.
Figure 4: Preferred model of dense partially molten ULVZ.

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Acknowledgements

We thank R. van der Hilst and S. Grand for supplying tomographic models, S. Grand for a Futterman t* code, and the Seismological group of MoD at Blacknest for the WRA data set. This research was supported by NSF.

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Correspondence to Sebastian Rost.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Materials

Additional tests and methods, and legends for the two Supplementary Figures. This file also contains Supplementary Table S1, which details earthquake location and origin time information for events showing ScP precursory energy (DOC 54 kb)

Supplementary Figure S1

Map view of the best fit t* values. The best-fit t* operators where found by convolving The P-wavelet with the t* operator in the frequency domain and comparing the resultant waveform to the ScP wavelet. (PDF 210 kb)

Supplementary Figure S2a and b.

Subfigures a and b of Supplementary Figure S2 a) Synthetic waveforms for varying density change (p) and fixed P-wave velocity change: α(-10%), S-wave velocity change: β(-10%) and ULVZ thickness: D (8.5km). b) Synthetic waveforms for varying density and fixed α(-10%), β(-25%) and D (8.5km). (PDF 300 kb)

Supplementary Figure S2c

Part c of Supplementary Figure S2 c) Synthetic waveforms for varying βand fixed α(-10%), p(+10%) and D (8.5km). (PDF 242 kb)

Supplementary Figure S2d and e

Contains: Parts d and e of Supplementary Figure S2 c) Synthetic waveforms for varying α and fixed p(0%), β(-25%) and D (8.5km). d) Synthetic waveforms for varying α and fixed p(+10%), β(-25%) and D (8.5km). (PDF 254 kb)

Supplementary Figure S2f and g.

Parts f and g of Supplementary Figure S2 f) Synthetic waveforms for fixed α:β= 1:1 (absolute values vary) and fixed p(+10%), and D (8.5km). g) Synthetic waveforms for fixed α:β= 1:3 (absolute values vary) and fixed p(+10%), and D (8.5km). (PDF 221 kb)

Supplementary Figure S2h and i

Parts h and i of Supplemental Figure 2 h) Synthetic waveforms for varying thickness and fixed α(-10%), β(-25%) and p(0%) i) Synthetic waveforms for varying thickness and fixed α(-10%), β(-25%) and p(+10%). (PDF 257 kb)

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Rost, S., Garnero, E., Williams, Q. et al. Seismological constraints on a possible plume root at the core–mantle boundary. Nature 435, 666–669 (2005). https://doi.org/10.1038/nature03620

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