Bradley Peters, Andrea Mundl-Petermeier, Valerie Finlayson 

The existence of resolvable 182W/184W deficits in modern ocean island basalts (OIB) relative to the bulk silicate Earth has raised questions about the relationship of these rocks to Earth’s core. However, because the core is expected to host high abundances of highly siderophile elements (HSE), it would be expected that such heterogeneity is accompanied by correlating variability in HSE abundances among OIB, but this has not been observed. We report instead a relationship between the W isotopic compositions of Hawaiʻi and Iceland OIB, representing two of Earth’s primary mantle plumes, and their Ru/Ir ratios. Previous studies have highlighted the unique behavior of Ru relative to Os and Ir during metal-silicate fractionation, particularly when sulfide is segregated with metal. Using the information from these studies, we construct models predicting the consequences for HSE fractionation of various scenarios in which 182W/184W deficits can be created. It is shown that the observed trends are likely inconsistent with modern, active core-mantle interaction at the CMB in OIB sources, and instead the observed low-Ru/Ir, low-182W/184W OIB are best explained by metal-silicate interaction that happened at significantly lower pressures. Such conditions reflect what is expected for metal-silicate equilibration during core formation itself, meaning that the deep mantle sources of OIB, such as ultra-low velocity zones, may instead reflect preserved relics of core formation. An ancient origin for core-mantle boundary domains is consistent with geophysical and petrological observations, for example that the Mg/Fe ratio of ferropericlase in the D′′ layer is in significant disequilibrium with the modern core. Additional work is required to constrain the behavior of HSE during silicate differentiation processes that may also generate low 182W/184W ratios. However, if modern OIB represent a direct link to the ancient processes of core formation, future geochemical studies may be able to unlock new information about the formation and evolution of the core, as well as the identity and nature of the cosmic building blocks that delivered HSE to Earth during its accretion.