The ongoing homogenization of primordial and late-accreted components in ocean island basalt mantle sources

The formation of a segregated metallic core is viewed as an inevitable consequence of the growth of larger protoplanets. However, the effect of this process on the distribution of siderophile elements is hugely variable depending on the physiochemical nature of the protoplanet and the pressure and temperature at which metal-silicate equilibration occurs. On Earth, study of this process can be complicated by the overprinting effect of late accretion, which delivered additional siderophile element mass to the Earth. Along with Precambrian-aged mantle-derived rocks, ocean island basalts (OIB) are now recognized as an important source of information about the early siderophile evolution of the deep Earth. We demonstrate that the combined W isotopic and highly siderophile element (HSE) characteristics of major global hotspots (Hawaiʻi, Iceland, Réunion) preserve geochemical signatures secondary to Hadean metal-silicate equilibration that have not been overprinted by late accretion. Further, some OIB preserve Ru/Ir ratios that are higher than expected, either for chondritic material delivered by late accretion or for the more highly processed primitive mantle. These elevated Ru/Ir signatures are not always easily explained by partial melting and/or magma differentiation processes and must in part reflect elevated Ru/Ir ratios in the deep mantle sources of OIB. Ruthenium has previously been investigated for its unique behavior among HSE during metal-silicate equilibration and heterogeneous, pre-late accretion Ru isotopic signatures have been recognized in some Archean-aged mantle-derived rocks. This implies that OIB may be an untapped source of information about the state of Earth’s interior during and after core formation.