Letter

Nature 435, 666-669 (2 June 2005) | doi:10.1038/nature03620; Received 2 February 2005; Accepted 6 April 2005

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

Sebastian Rost1, Edward J. Garnero1, Quentin Williams2 & Michael Manga3

  1. Department of Geological Sciences, Arizona State University, Box 871404, Tempe , Arizona 85287-1404, USA
  2. Department of Earth Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
  3. Department of Earth and Planetary Science, University of California Berkeley, 307 McCone Hall, Berkeley, California 94720-4767, USA

Correspondence to: Sebastian Rost1 Correspondence and requests for materials should be addressed to S.R. (Email: srost@asu.edu).

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 approx50-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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