Thermo-chemical interpretation of one-dimensional seismic reference models for the lower mantle

Average seismic structure of the lower mantle is extremely tightly constrained by global travel times. Very smoothly varying velocity-depth gradients between depths of 800 and 2500 km are commonly taken as an indication of a homogeneous lower mantle composition. However, with increasing depth, lower-mantle gradients are less and less well matched by those expected for a thermally well-mixed adiabatic-subadiabatic mantle of a homogeneous pyrolitic composition. We map out thermal and chemical sensitivity of seismic wave velocities as a function of lower mantle pressure and temperature, and the uncertainties in the velocities and derivatives due to uncertainties in the mineral physics parameters and EoS. Taking these uncertainties into account, we cannot fully preclude an adiabatic pyrolitic average lower mantle structure. However, seismic constraints are better matched by a mantle that is (1) superadiabatic plus has an increasingly fast composition with depth, e.g. Si-rich, potentially from an increasing proportion of basaltic components with depth, or (2) becomes increasingly iron-rich (primitive?) with depth, with temperatures similar to those of an adiabat compatible with upper mantle temperature constraints. Additional seismic and/or dynamic constraints, as well as reduction of the mineral physics uncertainties may help to distinguish between the possibilities.