HR: 11:05h
AN: S11G-04    [PDF]
TI: Robust Compositional Heterogeneity Throughout the Mantle Inferred from Probabilistic Tomography
AU: * Trampert, J
EM: jeannot@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, PO BOX 80021, Utrecht, 3508 TA Netherlands
AU: Deschamps, F
EM: deschamp@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, PO BOX 80021, Utrecht, 3508 TA Netherlands
AU: Resovsky, J
EM: resosvky@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, PO BOX 80021, Utrecht, 3508 TA Netherlands
AU: Yuen, D
EM: davey@krissy.geo.umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 United States
AB: To date, the most compelling evidence for compositional heterogeneity comes from several observations of regions with high ratios of relative S to P velocity heterogeneity and a general anti-correlation between bulk-sound and shear wave speed in the lowermost mantle. No clear picture has yet emerged, since the inferences are clearly study dependent, most likely due to the inherent non-uniqueness of the associated inverse problem. To identify all possible models compatible with seismic data (fundamental mode and overtone phase velocity data and normal mode splitting functions), we employed a full model search technique to the tomographic problem. We used Sambridge's Neighbourhood Algorithm. There is no need to employ ad hoc damping parameters, the algorithm is easy to tune, and most importantly, it converts the solutions into probability density functions for long wavelength models (spherical harmonic degree 2, 4 and 6) of bulk-sound and shear wave speed, density and boundary topography in the mantle. Using appropriate sensitivities (which take into account our ignorance on the thermodynamic reference state and the published range of mineral physics data), we convert the results from probabilistic tomography into likelihoods of variations in temperature, perovskite and iron content throughout the lower mantle. Several robust features emerge which shed a new light on the nature of the lower mantle. Throughout the mantle temperature variations are much weaker than classically inferred from shear wave speed alone. Compositional variations are essential to explain the seismic data. In most places, the inferences are robust, i.e. the amplitudes of chemical variations are much larger than the uncertainties inferred from the width of the likelihoods. Below 2000 km, the correlation between relative shear wave speed variations and temperature is quite low. In particular, we find that the much debated superplumes beneath the Pacific and Africa are due to an enrichment in perovskite and iron. This makes these features denser than the surrounding mantle and hence not buoyant. Slabs can be thought of as sinking cold oceanic lithosphere, in the lower mantle they should then correspond to regions colder than average and depleted in perovskite. Robust mapping of places of lower temperatures and perovskite depletion do not generally correspond to where we would expect slabs.
DE: 1025 Composition of the mantle
DE: 3260 Inverse theory
DE: 7207 Core and mantle
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8180 Tomography
SC: Seismology [S]
MN: 2003 Fall Meeting