Lower mantle structure and composition: insights from generalized inversions of radial seismic profiles

A large number of studies have already been dedicated to the interpretation of 1-D radial seismological profiles of seismic wave velocities, density, and elastic properties in terms of mantle mineralogy and thermal structure. No clear consensus has, however, emerged from these studies. We show results for lower mantle structure and bulk composition obtained by using a generalized inverse technique together with the most recent and accurate values of physical properties (including, of particular importance, shear moduli) of relevant lower mantle minerals. We have designed an iterative numerical code that allows us to precisely evaluate the a posteriori uncertainties, correlations and resolution of best matching chemical composition and geotherm. We discuss the effect of various sources of uncertainties and study the effect of experimental uncertainties in the physical properties, of various averaging schemes for the composite mantle assemblages, and of different formulations of equation of state. We compare the results obtained from inversions of density and bulk sound velocity to those obtained from density and the seismic velocities Vp and Vs. We show that including the shear properties significantly reduces the a posteriori uncertainties on the resulting geotherm models, and we evaluate the correlations between the thermal structure and averaged depth-dependent bulk composition. We also test the compatibility of radial seismic profiles with the hypothesis of a chemical discontinuity in the deep lower mantle. We show that although the a posteriori uncertainties do not allow us to identify a unique compositional-thermal model, several robust conclusions can be drawn. (1) Average lower mantle compositional models that are not pyrolitic-like in composition are consistent with seismological observations. (2) There are large trade-off's between thermal structure and bulk composition. (3) Regardless of the origin of the 660-km interface, other (chemical) discontinuities may be located at greater depth in the lower mantle. The upper-part of the lower mantle presents a lithospheric signature while the underlying layer is consistent with silica enrichment. (4) The role of minor elements (such as aliminium, calcium, water) is difficult to constrain but not remains significant.