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Reconciling dynamic and seismic models of Earth's lower mantle: the dominant role of thermal heterogeneity

By David Rhodri Davies, S. Goes, John Huw Davies, B. S. A. Schuberth, H.-P. Bunge and J. Ritsema

Abstract

Two large regions of low shear wave velocity in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense `piles'. These `piles' are thought to have remained isolated from mantle circulation for several hundred million years, influencing heat transfer within Earth's interior and the mantle's geochemical evolution. The interpretation that these regions are chemically distinct largely hinges on three seismic observations: (i) their shear wave velocity anomalies are considered too large for a purely thermal explanation; (ii) shear wave velocity gradients at their edges are considered too sharp for an isochemical feature; and (iii) their shear and bulk-sound velocity anomalies appear to be anti-correlated, which is not expected for purely thermal structures. However, using compressible global spherical mantle circulation models driven by 300 million years of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale compositional heterogeneity. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, models that include dense chemical `piles' at the base of Earth's mantle, in which `piles' correspond to only 3% of the mantle's volume, produce shear wave velocity anomalies that are too strong to be compatible with tomographic observations, even after filtering results to account for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, the ratio between shear and compressional wave-speed anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth. This is also true for the correlation between shear and bulk-sound velocity anomalies: in all models examined, an anti-correlation only occurs within the post-perovskite stability field. Taken together, this implies that compositional heterogeneity, an unavoidable consequence of plate tectonics, must be widely-distributed, occurring on length-scales that do not strongly influence lower mantle dynamics and the lower mantle's long-wavelength seismic structure

Topics: QE
Publisher: Elsevier
Year: 2012
DOI identifier: 10.1016/j.epsl.2012.08.016
OAI identifier: oai:http://orca.cf.ac.uk:37548
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