Comparing lower mantle compositions and temperatures inverted from different seismic observations

Most constraints on lower mantle composition and temperature come from comparison of seismic profiles with their equivalents deduced from mineral physics. For seismologists, the parameters directly observable are the velocities V_P and V_S, whereas the most readily determined properties experimentally are density ρ and bulk modulus K_S. In this study, we apply a generalized inverse method and high-quality experimental datasets to infer the lower mantle composition and temperature profile from seismic observations. Inversion from density and bulk sound velocity highlights the following points: (1) equally satisfactory fits to seismic profiles can be obtained either for pyrolite-type composition with a cool geotherm, or for perovskite-rich composition with a hot geotherm, (2) consistent features in all inversions are a total iron content of 0.1 and a subadiabatic temperature gradient, with a peculiar correlated behavior of these two parameters below the 660 km discontinuity (3) the results of inversions are unaffected by the partitioning of iron between perovskite and magnesiowüstite (4) the inversion does not constrain the Al₂O₃ and CaO contents of the lower mantle. Using the compressional and shear velocities as additional constraints decreases the a posteriori uncertainties on the inverted parameter and removes the dependence of the final results on the a priori model. However, the inverted composition and temperature profiles drastically depend on the shear properties of lower mantle minerals, particularly of silicate perovskite. The experimental uncertainties yield a wide range of compositional and thermal profiles ranging from a uniform pyrolite-type lower mantle with closely adiabatic geotherm to a lower mantle whose composition gradually increases from pyrolite at 660 km depth to pure perovskite at the CMB with a strongly superadiabatic profile. A better understanding of lower mantle composition and temperature requires better constraints on shear properties, especially their pressure and temperature dependence.