Hafnium and neodymium isotopes in seawater and in ferromanganese crusts: The “element perspective”

Hafnium and Nd isotopes are increasingly used as paleoceanographic proxies. Comparing the “mantle–crust array” and the “seawater array” in plots of ε_Hf vs. ε_Nd, it has been observed that for a given ε_Nd value the corresponding ε_Hf value is higher for seawater than it is for terrestrial rocks. While this difference had initially been explained by significant hydrothermal input of mantle Hf into seawater, the currently favoured explanation is incongruent weathering of continental rocks producing radiogenic riverine Hf input.We here address this topic from the perspective of the behaviour of these two elements in seawater and in ferromanganese (Fe–Mn) crusts. We distinguish between a “truly dissolved” and a “dissolved” Hf and Nd pool, the latter being comprised of truly dissolved and colloid-bound (“colloidal”) Hf and Nd. While there exists a hydrothermal pathway for colloid-bound dissolved mantle Hf into the oceans, there is, in marked contrast to Nd, no important riverine pathway for colloidal or truly dissolved continental Hf. Owing to their respective chemical speciation in seawater, there exists truly dissolved Nd in the ocean, while the amount of truly dissolved Hf is insignificant.Neodymium is in exchange equilibrium between local seawater and both, the hydrous Fe and Mn oxides hydrogenetic Fe–Mn crusts are composed of. Due to continuous ad- and desorption there is continuous isotopic re-equilibration and the isotopic composition of Nd in a Fe–Mn crust reflects that of truly dissolved Nd in local ambient seawater. In contrast, Hf is only associated with the hydrous Fe oxides on which it forms surface precipitates that do not exchange with seawater. Due to this lack of isotopic re-equilibration, the isotopic composition of Hf in a Fe–Mn crust is the average of that of all the Hf scavenged during the lifetime of the hydrous Fe oxide particles. Since the Hf-bearing hydrous Fe oxides in a Fe–Mn crust do not form from local ambient seawater at the crust's growth site but are advected as colloids or fine particles, their Hf isotopic composition depends on the origin and migration pathway of these colloids. Hence, while Nd isotopes in Fe–Mn crusts provide reliable information on truly dissolved Nd in local ambient seawater, Hf isotopes rather indicate the origin and pathway of hydrous Fe oxide colloids, and might differ from truly dissolved Hf in local ambient seawater. This may explain the occasional decoupling of Nd and Hf isotopes in Fe–Mn crusts and supports the notion of a significant hydrothermal mantle signal of Hf in seawater.