Heterogeneous lowermost mantle: Compositional constraints and seismological observables

Abstract

Several seismological observations indicate the existence of compositional heterogeneities in the lowermost mantle, in particular, the anticorrelation between bulk sound and shear wave velocity anomalies, and anomalously high values (i.e., >2.7) of the ratio R = dinVs/dlnVf. Constraining the composition of such heterogeneous material is fundamental to determine its origin and its possible role on the dynamical evolution of the Earth's mantle. In this paper we propose a new approach to constrain the composition of chemically denser material in the lower mantle. Using geodynamical and seismological constraints, we show that the denser material has to be enriched in both iron and silica with respect to a pyrolitic lower mantle. The required enrichment is reduced if we consider that at high pressure Al-perovskite decreases the iron-magnesium partition coefficient between magnesiowiistite and perovskite. We then apply the estimated composition to the distribution of chemical heterogeneities calculated by our thermochemical convection model. In the deep mantle we predict broad seismic velocity anomalies and strong lateral velocity variations. Moreover, we find that areas of anticorrelation are associated with upwelling mantle flow, in agreement with tomographic studies. The calculated R ratio varies laterally and may locally have values >2.7, often associated with areas of anticorrelation. Our results compare well with seismic observations and provide a way to reconcile apparent discrepancies between global tomographic models. Finally, we suggest that only an enrichment in iron and silica in the lowermost mantle is required to explain seismological observations.