Internal structure of ultralow-velocity zones consistent with origin from a basal magma ocean

Seismological observations reveal patches of low-velocity anomalies at the core–mantle boundary known as ultralow-velocity zones. Despite recent advances, their origin and dynamic link to the lowermost mantle remain unclear. Here we employ seismic data analysis and high-resolution geodynamic modelling to study the origin of ultralow-velocity zones beneath the Coral Sea between Australia and New Zealand. The analysis of core-reflected waveforms with rigorous estimation of Bayesian uncertainties shows strong evidence of stratified density increases (~30%) and shear-wave velocity decreases (~50%) within the ultralow-velocity zones. These zones thin on two sides and occur at the edge of the Pacific large low-shear-velocity province. Geodynamic modelling demonstrates that these features are consistent with the presence of compositional heterogeneities within the ultralow-velocity zones that may be caused by the remnants of Earth’s early differentiation. We conclude that small-scale structures that are compositionally distinct from their surroundings reside at the bottom of the mantle without full homogenization, throughout Earth’s history.