The distribution of small-scale heterogeneity at the core-mantle boundary (Invited)

Since the early 1980s tomographic imaging has been very successful in resolving the large-scale velocity structure of the Earth's interior giving essential insight into the dynamics, structure and composition of our planet. Other seismic imaging techniques give evidence for structures on shorter scale lengths that can be expected in a dynamically mixing environment such as the Earth's mantle. The fine-scale structure (on the scales of 10's of kilometers) is typically inferred from the analysis of scattered seismic waves observed in the short-period seismic wavefield. Although most of the scattered seismic wavefield is evident in the codas trailing main arrivals, certain seismic raypaths allow scattered seismic raypaths to arrive as precursors. Here we exploit back-scattering from the core mantle boundary region (CMB) related to the seismic phase PKKP (PK*KP) to elucidate the mineralogy, dynamics and chemical constitution of the contact region between mantle and core. We use a global dataset of almost 700 earthquakes recorded at the dense arrays of the International Monitoring System. The study allows unprecedented sampling of the CMB with a single scattering probe. We find a strongly varying distribution of heterogeneity along the CMB and areas where heterogeneity reaches up several tens of kilometers into the mantle from the CMB indicating dynamic entrainment through mantle flow. Due to the high thermal diffusivity of the mantle material the imaged heterogeneity is most likely compositional rather than thermal. The distribution of heterogeneities shows evidence for mechanical mixing and partial melting likely related to the large scale structure and flow of mantle convection.