Synthetic seismic signature of thermal mantle plumes

The first seismic images of mantle plumes have been a source of significant debate. To interpret these images, it is useful to have an idea of a plume's expected seismic signature. We determined a set of dynamic thermal whole-mantle plumes, with parameters appropriate for the Earth's mantle and shallow-mantle temperature contrasts compatible with surface observations. We explore the sensitivity of amplitude and width of thermal plume anomalies to model parameters. The conversion of thermal to seismic structure accounts for effects of temperature, pressure, an average mantle composition including phase transitions, and anelasticity. With depth-dependent expansivity and temperature-and depth-dependent viscosity, these relatively weak plumes have lower-mantle diameters of 300-600 kra at one half of the maximum temperature anomaly. To attain the narrow upper-mantle plumes inferred from surface observations and tomography, viscosity reduction by a factor 30-100 is necessary, either as a jump or as a strong gradient. All model plumes had buoyancy fluxes greater than or equal to 4 Mg/s and it seems difficult to generate whole-mantle thermal plumes with fluxes much lower. Due to changing seismic sensitivity to temperature with depth and mineralogy, variations in the plumes' seismic amplitude and width do not coincide with those in their thermal structure. Velocity anomalies of 2-4% are predicted in the uppermost mantle. Reduced sensitivity in the transition zone as well as complex velocity anomalies due to phase boundary topography may hamper imaging continuous whole-mantle plumes. In the lower mantle, our plumes have seismic amplitudes of only 0.5-1%. Unlike seismic velocities, anclasticity reflects thermal structure closely, and yields plume anomalies of 50-100% in dln(1/Q(S)). (C) 2003 Elsevier B.V. All rights reserved.