Sensitivities of seismic velocities to temperature, pressure and composition in the lower mantle

We calculated temperature, pressure and compositional sensitivities of seis mic velocities in the lower mantle using latest mineral physics data. The c ompositional variable refers to the volume proportion of perovskite in a si mplified perovskitemagnesiowustite mantle assemblage. The novelty of our ap proach is the exploration of a reasonable range of input parameters which e nter the lower mantle extrapolations. This leads to realistic error bars on the sensitivities. Temperature variations can be inferred throughout the l ower mantle within a good degree of precision. Contrary to the uppermost ma ntle, modest compositional changes in the lower mantle can be detected by s eismic tomography, with a larger uncertainty though, A likely trade-off bet ween temperature and composition will be largely determined by uncertaintie s in tomography itself. Given current sources of uncertainties on recent da ta, anelastic contributions to the temperature sensitivities (calculated us ing Karate's approach) appear less significant than previously thought. Rec ent seismological determinations of the ratio of relative S to P velocity h eterogeneity can be entirely explain by thermal effects, although isolated spots beneath Africa and the Central Pacific in the lowermost mantle may as k for a compositional origin. (C) 2001 Elsevier Science B.V. All rights res erved.