Highly siderophile elements (HSEs; namely Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) in Earth's mantle require the addition of metals after the formation of Earth's core. Early, large collisions have the potential to deliver metals, but the details of their mixing with Earth's mantle remain unresolved. As a large projectile disrupts and penetrates Earth's mantle, a fraction of its metallic core may directly merge with Earth's core. Ensuing gravitational instabilities remove the remaining projectile's core stranded in Earth's mantle, leaving the latter deprived of HSEs. Here, we propose a framework that can efficiently retain the metallic components during large impacts. The mechanism is based on the ubiquitous presence of a partially molten region in the mantle beneath an impact-generated magma ocean, and it involves rapid three-phase flow with solid silicate, molten silicate, and liquid metal as well as long-term mixing by mantle convection. In addition, large low-shear-velocity provinces in the lower mantle may originate from compositional heterogeneities resulting from the proposed three-phase flow during high-energy collisions.
Keywords: mantle mixing; mantle structure; planetary formation.