Neodymium - Hafnium Isotope Systematics in Seawater: Status Quo, Problems, and Promises

Combined Nd and Hf isotope analyses have become widely used in solid Earth geochemistry studies, but their application to the low-temperature environment is still limited. The residence time for both of these elements in the ocean is thought to be similar (500 2000 years) and there is a growing number of studies addressing the sources of Nd and Hf in the ocean and internal cycling processes. In the case of Nd, these studies are accompanied by a number of emerging applications of Nd isotopes as a tracer for past water mass mixing and ocean circulation, since the Nd isotopic composition of various archives in the ocean has been shown to record the Nd isotopic composition of ambient seawater reliably. Due to very low concentrations of Hf in seawater and hydrothermal fluids, and associated analytical difficulties in measuring Hf isotope compositions, our overall understanding about fractionation processes and internal cycling of Hf in the ocean is not as well developed as for Nd. However, recent studies have shown that measurements of seawater and river waters are feasible. Global Nd-Hf isotope systematics show a broad positive correlation with different slopes for mantle and crustal rocks ("terrestrial array"), and authigenic precipitates ("seawater array"). The offset between these two arrays provides evidence for the sources and mechanisms by which these elements are added to ocean water and the favored explanation for the offset is incongruent weathering of continental crust, fractionating Lu and Hf. Simple mixing calculations provide compelling evidence that while hydrothermal contributions of Hf to the ocean are allowed, continental contributions are absolutely required to explain the variations in Hf isotopes in seawater. Furthermore, it can be shown that incongruent weathering of continental sources is sufficient, without any hydrothermal contributions, to create the observed offset of the Nd-Hf isotope array in seawater.