The stable isotope fractionation of neodymium on ferrihydrite revisited

Ferrihydrite is a ubiquitous scavenger in aqueous environments that removes large amounts of dissolved metals thus it is pivotal to global geochemical cycles. Previous work found contradictory directions of stable isotope fractionation amongst light rare earth elements (REE) between aqueous solutions and ferrihydrite. Here, the isotope fractionation of 146Nd/144Nd on ferrihydrite has been revisited using a double spike that corrects for mass fractionation during sample processing. Surprisingly, contrary to previous observations, Nd on ferrihydrite was found to be enriched in lighter isotopes compared to Nd in aqueous solution by 103ln αSol-Liq = -0.113 ± 0.030. We also report the first data set of stable δ146/144Nd isotopes in seawater samples which are isotopically heavier than Fe-Mn (hydr)oxides by 0.357‰ on average. This is consistent with the scavenging of lighter Nd isotopes by ferrihydrite seen in the adsorption experiments. The fractionation is, however, much larger than observed in the experiments. This can be explained by carbonate complexes being the predominant aqueous complex of Nd in seawater which are isotopically heavier than free Nd by 0.107‰. However, additional data on natural samples is required to elucidate heterogeneities in seawater and Fe-Mn oxides in marine sediments. The enrichment of lighter Nd in the surface complex can be best explained by stiffer bonds in the aqueous complex which is demonstrated by a lower Debye-Waller factor (σ2). This study lays the foundation for understanding the geochemical cycling of Nd and other REE between Earth’s crust and hydrosphere using stable isotope signatures.

Advances in Geochemistry and Cosmochemistry

Advances in Geochemistry and Cosmochemistry

2

2977-1994

N/A

1

19