P. J. Tackley
Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA.
Numerical simulations are used to characterize the volumetric heterogeneity associated with proposed chemical layering in the deep mantle. A globally continuous, undulating layer generates enormous amounts of volumetric heterogeneity, which is caused by lateral variations in the layer boundary depth and by strong hot upwellings arising from the top of the layer, in the mid lower mantle region. In contrast, the heterogeneity in the deepest mantle is very weak. Partial cancellation of the temperature and compositional contributions to seismic velocity could mask the layer itself but the strong signatures of the thermo-chemical boundary layer and induced upwellings remain, even when fields are filtered to approximate the resolution of global seismic tomographic models. This distinctive heterogeneity is not observed in global seismic tomographic models. Dense material that is restricted to discontinuous “piles” generates heterogeneity that is more consistent, although not entirely, with seismic observations, with strong heterogeneity at the core-mantle boundary. Radial correlation functions display a characteristic narrowing in the vicinity of any chemical layer, but are very sensitive to the applied “seismic tomographic” filtering. It is concluded that a deep "HAL" layer is inconsistent with seismic tomography, but that isolated piles of dense material under the large scale "superplumes" are permitted.