Earth's hot lowermost mantle revealed by Stoneley mode splitting observations

Abstract:

Advances in our understanding of Earth's thermal evolution and the style of mantle convection rely on robust constraints on the distribution of radiogenic heating in the mantle. If the Earth were derived from chondritic meteorites, its mantle must have an isolated, chemically distinct reservoir enriched in heat-producing elements, presumably present at the base of the mantle. Direct assessment of mantle radioactivity through the observations of geoneutrinos has the potential to detect this missing reservoir in the deep mantle.

One potential location of this reservoir would be the two Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle beneath Africa and the Pacific, which stand out in every global map of the Earth's lower mantle. Whole Earth oscillations, or normal modes, are key to determining the large scale density variations in the mantle and to test models of the origin, composition and longevity of the LLSVPs. Here, we show that anomalous splitting of Stoneley modes, a unique class of free oscillations that are perturbed primarily by velocity and density variations at the core-mantle boundary, are optimally fit when the density of the LLSVPs is lower than the surrounding material. We hypothesise that these low-velocity, low-density structures in the lower mantle are extremely hot due to the high concentration of heat-producing elements. Ensuing geoneutrino research will test this hypothesis by mapping the regional distribution of U and Th in the deep mantle, especially in the oceans.