Seismic and mineralogical structures of the lower mantle from probabilistic tomography

Abstract

One of the most powerful approaches for understanding the 3-D thermo-chemical structure of the lower mantle is to link tomographic models with mineral physics data. This is not straightforward because of strong trade-offs between thermal and chemical structures and their influence on seismic structures. They can be reduced by mapping simultaneously perturbations of wave speeds and density anomalies and by the quantitative assessment of the accuracy and uniqueness of seismic and mineralogical data. Here, we present new tomographic maps of low order even-degree seismic structures which are an improvement on earlier models. They satisfy constraints from body wave, surface wave and normal mode data simultaneously, thereby enhancing the spatial resolution. Furthermore, the seismic structure at a given location is represented by a probability density function (pdf) which takes into account the uncertainty and non-uniqueness of the solution due to modeling and data restrictions. Following a robust statistical procedure, we fit heterogeneity of wave speeds and density from hypothetical thermo-chemical models to those of our tomographic maps. We thereby constrain lateral variations of temperature as well as iron, silica and post-perovskite concentration in terms of pdfs. Our work shows that large scale chemical variations are likely everywhere in the lower mantle. In most of the D″ region post-perovskite is most abundant in the Circum-Pacific belt, but near the core its lateral variation is more complex. Furthermore, post-perovskite concentration trades off with the amplitudes of temperature and silicate variations, but not with their lateral distribution. This might be the reason why temperature and silicate variations appear not constrained by our data in the lowermost few hundred km of the mantle