Towards a lower mantle reference temperature and composition

We aim to constrain the lower mantle geotherm and average composition from 1D seismic models and experimental mineralogy data, explicitly accounting for possible sources of uncertainty. We employ an isentropic third-order Birch–Murnaghan equation of state, which is in excellent agreement with recent ab initio calculations of density and bulk modulus for Mg-perovskite. Furthermore, ab initio and experimental data are reasonably consistent with each other. Modelling the shear modulus is not as straightforward, but is needed because density and the bulk modulus alone do not sufficiently constrain temperature and composition. To correctly predict ab initio calculations for the shear modulus of Mg-perovskite, we needed to prescribe a cross-derivative at zero pressure, which we determined by trial and errors. Unless this ad hoc cross-derivative is confirmed by further experimental results, there seems to be an inconsistency between ab initio and experimental data. Purely experimental data most likely require a non-adiabatic temperature profile, but it is difficult to infer the number and location(s) of the non-adiabatic increase(s). If ab initio data are used, at least one thermal boundary layer seems reasonable, but its location depends on the modelling of the iron content. A strong chemical density contrast in the mid-mantle (≥2%) is not supported by ab initio data, but is possible with experimental data. Other major sources of uncertainty are the trade-off between thermal and compositional effects, the possible influence of aluminium perovskite, and poorly understood frequency effects.