Nuclear volume isotope fractionation of europium and other lanthanide elements

Abstract

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, Eu 2+ -bearing species will have approximately 0.4-1‰ higher 153 Eu/ 151 Eu than Eu 3+ -bearing species at 25ºC (298 K), and about 0.3‰ higher 153 Eu/ 151 Eu at 700ºC (973 K). 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/T 2 for mass-dependent fractionation.Fractionation favoring high 153 Eu/ 151 Eu in minerals that preferentially incorporate Eu 2+ , such as plagioclase, is consistent with recent measurements on igneous rocks showing low 153 Eu/ 151 Eu 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 142 Ce/ xxx Ce ratios higher in reduced species (+2 > +3 > +4), for instance by 0.4‰ for 142 Ce/ 140 Ce in Ce 3+ vs. Ce 4+ at 25ºC, approximately canceling the mass-dependent fractionation. The nuclear volume component of fractionation in cerium isotope pairs not involving 142 Ce will be smaller (< 0.1‰) because the other nuclei have very similar volumes. As a result, mass-dependent fractionation will drive 138 Ce/ 140 Ce and 136 Ce/ 140 Ce lower in Ce 4+ species while 142 Ce/ 140 Ce hardly fractionates at equilibrium. This will yield an atypical mass vs. fractionation pattern in redox pairs.

Determination of Rare Earth Element Isotopic Compositions Using Sample-Standard Bracketing and Double-Spike Approaches

“…As mentioned in Section , the isotopic fractionation reported using 136 Ce and 138 Ce DS is calculated from 142/140 Ce (also see ref ), while all the isotope analyses using SSB also only reported 142/140 Ce due to the extremely low natural abundances of 136 Ce and 138 Ce. ,− In the discussion below, we consider only Ce isotope fractionation reported as 142 Ce/ 140 Ce. As the NFS scales with 1/ T and equilibrium MDF scales with 1/ T 2 , the maximum equilibrium isotope fractionation of 142/140 Ce is achieved at medium temperature (∼550 K; Figure 3 in ref ). In terms of natural samples, Ce shows stable isotope fractionation with a range over 0.3‰/amu in ferromanganese oxy/hydroxide. , As soluble Ce 3+ is oxidized to Ce 4+ and adsorbed onto manganese oxide/hydroxide (potentially iron oxide/hydroxide), 142 Ce is expected to be enriched in Ce 4+ relative to 140 Ce, which is consistent with the adsorption experiments .…”

“…There has been a growing interest in stable isotope analyses of rare earth elements (REEs), including Ce, ,,,,− ,− Nd, ,,− ,,,− ,− Sm, ,, Eu, ,,− Gd, Dy, Er, , and Yb. , …”

“…Extrapolation based on lanthanide ionic radii also suggests limited isotope fractionations for other REEs in magmatic processes. However, Schauble predicted that equilibrium isotope fractionation induced by NFS dominates over equilibrium MDF for Eu. , As the isotopic fractionation induced by NFS occurs in the redox reaction and goes in the opposite direction of equilibrium MDF, light isotopes tend to be enriched in Eu 3+ compared to Eu 2+ . During its crystallization, plagioclase tends to be enriched in Eu 2+ and heavy isotopes, while the parental melt is depleted in Eu 2+ and enriched in light isotopes, as is observed (Figure 3 in ref ).…”

“…The range of ε 138 Ce in the terrestrial samples is ∼2 epsilon (0.2‰) . Schauble pointed out that 142 Ce has a nuclear charge radius that stands out relative to 136 Ce, 138 Ce, and 140 Ce, giving rise to anomalous (nonmass dependent) isotopic fractionation of 142 Ce/ 140 Ce compared to 136 Ce/ 140 Ce and 138 Ce/ 140 Ce. The majority of 136 Ce (98.97%) and 138 Ce (97.97%) comes from the spike in the optimal spike-sample mixture.…”

“…Europium stands out among other REEs as the nuclear field shift (NFS) effect seems to dominate equilibrium isotope fractionation associated with redox processes . Compared to MDF induced by the lattice vibration (written below as MDF for simplicity), NFS scales as the reciprocal of temperature in K (1/ T ) rather than 1/ T 2 , meaning that equilibrium Eu isotopic fractionation could remain significant at igneous temperatures .…”