Nuclear volume isotope fractionation of europium and other lanthanide elements

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The nuclear volume component of equilibrium field shift isotope fractionations in europium and other lanthanide elements is estimated using Mössbauer spectroscopy and electronic structure calculations. This effect goes in the opposite direction from equilibrium mass-dependent fractionation, and in the case of europium is predicted to dominate over mass dependent fractionation for most materials. Including both effects, Eu²⁺-bearing species will have approximately 0.4–1‰ higher ¹⁵³Eu/¹⁵¹Eu than Eu³⁺-bearing species at 298 K (25°C), and about 0.3‰ higher ¹⁵³Eu/¹⁵¹Eu at 973 K (700°C). Field shift fractionation mainly depends on oxidation state; differences in coordination structure without changes in oxidation state appear to have much weaker associated fractionations. Nuclear volume isotope fractionation will become even more dominant over mass dependent fractionation at higher temperatures because nuclear volume effects scale with 1/T (K), vs. 1/T² for mass-dependent fractionation. Fractionation favoring high ¹⁵³Eu/¹⁵¹Eu in minerals that preferentially incorporate Eu²⁺, such as plagioclase, is consistent with recent measurements on igneous rocks showing low ¹⁵³Eu/¹⁵¹Eu in samples with large negative europium anomalies (Lee and Tanaka, 2021). The present results agree with the recent conclusion that equilibrium fractionation cannot explain cosmochemical REE fractionations in primitive meteoritical materials (Hu et al., 2021), because the net fractionation is too small (~0.2‰ or less) at temperatures >1200 K where vapor-phase REE species are relevant.

The field shift effect will also tend to drive ¹⁴²Ce/^xxxCe ratios higher in reduced species (+2 > +3 > +4), for instance by 0.4‰ for ¹⁴²Ce/¹⁴⁰Ce in Ce³⁺ vs. Ce⁴⁺ at 298 K, approximately canceling the mass-dependent fractionation. The nuclear volume component of fractionation in cerium isotope pairs not involving ¹⁴²Ce will be smaller (<0.1‰) because the other nuclei have very similar volumes. As a result, mass-dependent fractionation will drive ¹³⁸Ce/¹⁴⁰Ce and ¹³⁶Ce/¹⁴⁰Ce lower in Ce⁴⁺ species while ¹⁴²Ce/¹⁴⁰Ce hardly fractionates at equilibrium. This will yield an atypical mass vs. fractionation pattern in redox pairs.