Hafnium isotope fractionation during continental weathering: Implications for the generation of the seawater Nd‐Hf isotope relationships

Abstract

AbstractTo investigate the mechanisms of the offset of seawater radiogenic Nd‐Hf isotope compositions from those of the upper continental crust rocks, combined Nd‐Hf isotope compositions of desert and loess samples from northern China (which integrate a wide range of lithologies and ages of continental rocks) are presented in this study. The results show significant and systematic fractionation of Hf isotopes between fine‐grained detritals/leachates (<5 µm) and coarser fractions (<75 µm) of the same samples. A small but systematic difference of Nd isotope compositions between leachates and detrital silicates is also revealed. Overall, the leaching data either plot along or slightly above the Nd‐Hf seawater array, providing strong direct support that the seawater Nd‐Hf isotope relationship is predominantly generated by weathering of upper continental crust. Our study supports the application of dissolved Hf isotopes as a proxy for different modes of weathering regimes rather than for continental source provenances.