Nestmeyer, Mark, and McCoy-West, Alex J. (2025) Quantifying mass-dependent isotope fractionation and nuclear field shift effects for the light rare Earth elements in hydrous systems. Geochimica et Cosmochimica Acta, 388. pp. 236-252.

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Abstract

The improving sensitivity of mass-spectrometers has opened the potential of using stable isotope signatures of the rare Earth elements (REE) as geochemical tracers. However, thus far only limited studies have utilised REE stable isotopes, despite resolvable variations having been observed in a range of systems. An interesting but poorly explored area remains variations in aqueous environments across a range of temperatures including seawater, marine sediments, ion adsorption deposits or hydrothermal systems. Furthermore, the magnitudes and competing effects of mass-dependent isotope fractionation and mass-independent nuclear field shift effects, which can be significant factor for heavy elements especially at high temperatures, remain poorly understood. To contribute to a holistic understanding of REE isotope signatures, we calculated reduced partition function ratios (as 103lnβ) for mass-dependent and nuclear field shift effects for aqueous La, Ce, and Nd complexes from first principles. Theoretical calculations were compared with experimental data, and this demonstrates that calculations involving a combination of two explicit hydration shells and additional implicit solvation effects are required to accurately describe the coordination and molecular vibrations of aqueous complexes. This data was then combined with speciation modelling of seawater and hydrothermal fluids to assess isotope fractionation in hydrous systems. The predicted magnitude of isotope fractionation is significant in low (T = 25 ◦C) temperature systems (Δ139/138LaMax-Min = 0.20 ‰, Δ142/140CeMax-Min = 0.33 ‰, Δ146/144NdMax-Min = 0.34 ‰) and remains above currently achievable analytical uncertainties even at high (T = 500 ◦C) temperatures (±0.015 ‰ for δ146/144Nd; ±0.040 ‰ for δ142/140Ce). Unless oxidation occurs, which is only relevant for Ce in terrestrial environments, nuclear field shift effects are negligible even at temperatures up to 500 ◦C. The large difference in nuclear charge radius between 140Ce and 142Ce strongly enriches the lighter Ce isotope in the higher oxidation state (103lnβCe(IV)-Ce(III) = 0.45 at 25 ◦C) which almost completely cancels out the opposing mass-dependent effect (103lnβCe(IV)-Ce(III) = +0.46 at 25 ◦C). This means that variations in δ142/140Ce isotope signatures cannot easily be related to oxidation in ancient or recent environments.

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