Combined Lithophile-Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

Abstract

Detection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long-term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short-lived 146Sm-142Nd and 182Hf-182W systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between 142Nd/144Nd and 182W/184W ratios among Réunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of 182Hf. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long-lived 143Nd-176Hf isotopic systematics mirror classical models invoking mixing of recycled trace-element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources. If the core provides a detectible contribution to the tungsten element budget of the silicate Earth, this represents a critical component to planetary-scale tungsten mass balance. A basic model is explored that reconciles the W abundance and isotopic composition of the bulk silicate Earth resulting from both late accretion and core-mantle interaction. The veracity of core-mantle interaction as proposed here would have many implications for long-term thermochemical cycling.