Deep mantle heterogeneities formed through a basal magma ocean contaminated by core exsolution

Abstract

Earth’s lowermost mantle harbours two large low-velocity provinces with patches of ultralow-velocity zones. These seismic anomalies may retain geochemical signatures distinct from the surrounding mantle. Yet, their origin remains enigmatic. One proposed explanation is the differentiation of an early-formed basal magma ocean. However, the presence of an excessively thick layer of iron-rich ferropericlase in the crystallized basal magma ocean conflicts with seismic tomography models. Here we use combined thermodynamic and geodynamic modelling to investigate the crystallization of a basal magma ocean continuously contaminated by oxide exsolved from the core, termed the basal exsolution contaminated magma ocean. We find suppression of ferropericlase crystallization. Geodynamic modelling demonstrates that the solidified contaminated magma ocean mantle can lead to the formation of deep mantle structures consistent with large low-velocity provinces and ultralow-velocity zones. In addition, diapirs of core exsolution entrained into the solid mantle may cause small-scale scattering. The basal exsolution contaminated magma ocean inherits the silicon, tungsten and helium isotope compositions from the core and exhibits trace element enrichments, suggesting its possible role as a source material for ocean island basalts that may sample the large low-velocity provinces, pointing to a unified mechanism for forming deep mantle heterogeneities.