Stable neodymium isotopic fractionation during chemical weathering

Abstract

Stable Nd isotope ratio is a potentially useful tool to provide new constraints on the geological reservoir formation processes, complementing information from the radiogenic Nd isotope system. Here for the first time, we present the behavior of stable Nd isotopes during low-temperature weathering via a well-studied basaltic soil profile on Hainan Island, South China. The results showed that δ146/144Nd values spanned a range of approximately 0.21‰, from +0.058‰ in the lower profile to −0.152‰ in the upper soil profiles, while the τTh,Nd (%) values decreased from +387.7% to −68.2%, indicating that significant fractionation of stable Nd isotopes and migration of Nd elements occur during chemical weathering. Sequential extraction experiments suggest that the majority of Nd in this profile was hosted in the exchangeable and residual phases. The exchangeable phase was enriched with the heavy Nd isotope (146Nd) due to the adsorption of Nd onto gibbsite and the adsorption/precipitation of Fe-oxides from a heavier dissolved Nd pool in soils. The evidence for adsorption of Nd onto gibbsite is the positive correlation between the abundance of gibbsite and the proportion of Nd in the exchangeable phase, as well as the δ146/144Nd values. The evidence for the adsorption/precipitation of Nd on Fe-oxides comes from the positive correlations between Fe and Nd concentrations and δ146/144Nd values in the exchangeable fractions. The residual phases acquire depleted 146Nd signatures in the upper soil horizon, which could be caused by preferential desorption of heavy Nd isotopes from clay minerals or preferential incorporation of light Nd isotopes during kaolinite formation. These findings elucidate the mechanisms affecting stable Nd isotopic fractionation in low-temperature geochemistry and indicate that the stable Nd isotope ratio is a highly useful indicator for chemical weathering. This, combined with the radiogenic 143Nd/144Nd ratio, will be a powerful geochemical tracer for understanding global Nd cycles.