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The influence of a chemical boundary layer on the fixity, spacing and lifetime of mantle plumes

Abstract

Seismological observations provide evidence that the lowermost mantle contains superposed thermal and compositional boundary layers1 that are laterally heterogeneous2,3. Whereas the thermal boundary layer forms as a consequence of the heat flux from the Earth's outer core, the origin of an (intrinsically dense) chemical boundary layer remains uncertain4. Observed zones of ‘ultra-low’ seismic velocity5 suggest that this dense layer may contain metals6,7 or partial melt8, and thus it is reasonable to expect the dense layer to have a relatively low viscosity. Also, it is thought that instabilities in the thermal boundary layer could lead to the intermittent formation and rise of mantle plumes. Flow into ascending plumes can deform the dense layer, leading, in turn, to its gradual entrainment9,10,11,12,13,14. Here we use analogue experiments to show that the presence of a dense layer at the bottom of the mantle induces lateral variations in temperature and viscosity that, in turn, determine the location and dynamics of mantle plumes. A dense layer causes mantle plumes to become spatially fixed, and the entrainment of low-viscosity fluid enables plumes to persist within the Earth for hundreds of millions of years.

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Figure 1: A schematic illustration of the apparatus used for the experiments shown in Figs 2 and 4 (ref. 26).
Figure 2: Photographs showing an oblique view of the temporal evolution of a dense, low-viscosity layer of soybean oil (coloured red) emplaced at the base of a vigorously convecting layer of polybutene oil (colourless).
Figure 3: Plot summarizing the conditions leading either to spatially fixed (filled symbols) or not-fixed (open symbols) plumes.
Figure 4: Illustration of the spatial and temporal structure of plumes.

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Acknowledgements

We thank A. Davaille for comments on the previous version of the manuscript, and L. H. Kellogg, R. C. Kerr, M. A. Richards and N. Sleep for comments on earlier versions. This work was supported by the National Science Foundation and The Miller Institute for Basic Research in Science.

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Jellinek, A., Manga, M. The influence of a chemical boundary layer on the fixity, spacing and lifetime of mantle plumes. Nature 418, 760–763 (2002). https://doi.org/10.1038/nature00979

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