Gerd Steinle-Neumann

Thermal versus elastic heterogeneity in high‐resolution mantle circulation models with pyrolite composition: High plume excess temperatures in the lowermost mantle

It is now widely recognised that there is a substantial heat flux through the CMB from the core into the mantle, up to 12 TW and significantly more than what was inferred earlier from observations of dynamic topography over mantle hotspots. A strong thermal gradient across the CMB is likely a source of large thermal anomalies in the form of hot upwelling plumes and correspondingly low seismic velocities in the lowermost mantle. Here we focus on temperature as a key parameter to constrain large-scale mantle structure and dynamics. We explore high-resolution mantle circulation models and predict their corresponding elastic heterogeneity. Absolute temperatures of our models are converted to seismic velocities using published thermodynamically self-consistent models of mantle mineralogy for a pyrolite composition. A grid spacing of about 25 km globally allows us to simulate mantle flow at earth-like convective vigor so that modelled temperature variations are consistent with the underlying mineralogy. We concentrate on isochemical convection and the relative importance of internal and bottom heating in order to isolate the thermal effects on elasticity. Importantly, models having a high temperature contrast on the order of 1000 K across the CMB produce elastic structures that are in excellent agreement with tomography for a number of quantitative measures: These include spectral power and histograms of heterogeneity as well as radial profiles of root-mean-square amplitudes. High plume excess temperatures of +1000‐1500 K in the lowermost mantle are particularly important in understanding the strong velocity reductions mapped by seismic tomography in lowvelocity bodies of the deep mantle, as they lead to significant negative anomalies of shear wave velocity of up to 4%. We note that our results do not account for the curious observation of seismic anti-correlation, which appears difficult to explain in any case and will require further improvements in the ability to map seismic heterogeneity to thermal and compositional variations. Our results underline the need to include mineral physics information in the geodynamic interpretation of tomographic models to obtain a better understanding of seismic images of mantle plumes.

A mineralogical model for density and elasticity of the Earth's mantle

[1]xa0We present a thermodynamic model of high-pressure mineralogy that allows the evaluation of phase stability and physical properties for the Earths mantle. The thermodynamic model is built from previous assessments and experiments in the five-component CFMAS system (CaO-FeO-MgO-Al2O3-SiO2), including mineral phases that occur close to typical chemical models of the mantle and reasonable mantle temperatures. In this system we have performed a system Gibbs free energy minimization, including pure end-member phases and a nonideal formulation for solid solutions. Solid solutions were subdivided into discrete pseudocompounds and treated as stoichiometric phases during computation of chemical equilibrium by the simplex method. We have complemented the thermodynamic model with a model of shear wave properties to obtain a full description of aggregate elastic properties (density, bulk, and shear moduli) that provide a useful basis for the consideration of seismic and geodynamic models of the Earths mantle. The thermodynamic model described here is made available for research and training purposes through a Web interface (http://www.earthmodel.org). We examine its validity in light of experiments from mineral physics and briefly discuss inferences for mantle structure.