Mantle temperatures from global seismic models: Uncertainties and limitations

Many geophysical studies require knowledge on the present-day temperature distribution in Earth’s mantle, which can be estimated from seismic velocity perturbations imaged by tomography in combination with thermodynamic models of mantle mineralogy. However, even in the case of (assumed) known chemical composition, both the seismic and the mineralogical information are significantly affected by inherent limitations and different sources of uncertainty. We investigate the theoretical ability to estimate the thermal state of the mantle from tomographic models in a synthetic closed-loop experiment and quantify the interplay of tomographically damped and blurred seismic heterogeneity in combination with different approximations for the mineralogical conversion from seismic velocities to temperature. Our results highlight that, given the limitations of tomography and the incomplete knowledge of mantle mineralogy, magnitudes and spatial scales of a temperature field obtained from global seismic models deviate significantly from the true state. The average deviations from the reference model are on the order of 50–100 K in the upper mantle and can increase with depth to values of up to 200 K, depending on the resolving capabilities of the respective tomography. Furthermore, large systematic errors exist in the vicinity of phase transitions due to the associated mineralogical complexities. When used to constrain buoyancy forces in time-dependent geodynamic simulations, errors in the temperature field might grow non-linearly due to the chaotic nature of mantle flow. This could be particularly problematic in combination with advanced implementations of compressibility, in which densities are extracted from thermodynamic mineralogical models with temperature-dependent phase assemblages. Erroneous temperatures in this case might activate ‘wrong’ phase transitions and potentially flip the sign of the associated Clapeyron slopes, thereby considerably altering the model evolution. Overall, the strategy to estimate the present-day thermodynamic state of the mantle must be selected carefully to minimize the influence of the collective set of uncertainties.