Origin of W-182 Anomalies in Ocean Island Basalts

Ocean island basalts (OIB) show variable W-182 deficits that have been attributed to either early differentiation of the mantle or core-mantle interaction. However, W-182 variations may also reflect nucleosynthetic isotope heterogeneity inherited from Earth's building material, which would be evident from correlated W-182 and W-183 anomalies. Some datasets for OIB indeed show hints for such correlated variations, meaning that a nucleosynthetic origin of W isotope anomalies in OIB cannot be excluded. We report high-precision W isotope data for OIB from Samoa and Hawaii, which confirm previously reported W-182 deficits for these samples, but also demonstrate that none of these samples have resolvable W-183 anomalies. These data therefore rule out a nucleosynthetic origin of the W-182 deficits in OIB, which most likely reflect the entrainment of either core material or an overabundance of late-accreted materials within OIB mantle sources. If these processes occurred over Earth's history, they may have also been responsible for shifting the W-182 composition of the bulk mantle to its modern-day value. We also report Mo isotope data for some Hawaiian OIB, which reveal no resolved nucleosynthetic Mo isotopic anomalies. This is consistent with inheritance of W-182 deficits in OIB from the addition of either core or late-accreted material, but only if these materials have a non-carbonaceous (NC) meteorite-like heritage. As such, these data rule out significant contributions of carbonaceous chondrite (CC)-like materials to either Earth's core or late accretion.