ABSTRACT

Equilibrium cerium isotope fractionations in cerium-bearing minerals and aqueous species are estimated using electronic structure calculations that include both nuclear volume and mass dependent effects. As with europium and uranium, the nuclear volume effect in redox reactions goes in the opposite direction from equilibrium mass-dependent fractionation for ¹⁴²Ce/¹⁴⁰Ce because of the larger nuclear charge radius of the ¹⁴²Ce nucleus. Mass-dependent effects dominate ¹³⁶Ce/¹⁴⁰Ce and ¹³⁸Ce/¹⁴⁰Ce fractionations because ¹³⁶Ce, ¹³⁸Ce, and ¹⁴⁰Ce share very similar nuclear charge radii. ¹⁴²Ce/¹⁴⁰Ce is predicted to be lower in most Ce(IV)-bearing species than in coexisting Ce(III)-bearing species, particularly at high temperatures. However, species with Ce:Zr substitution, such as zirconium-rich compositions along the stetindite-zircon (CeSiO₄-ZrSiO₄) solid solution series and a model Ce-subsituted baddeleyite (CeZr₃O₈), may show higher ¹⁴²Ce/¹⁴⁰Ce at ambient to low-T metamorphic temperatures because of higher effective force constants acting on the smaller, more snug substitution sites. Ce(III)P(V) charge-coupled substitution into zircon is likewise associated with high ¹⁴²Ce/¹⁴⁰Ce relative to other Ce(III) species. ¹³⁶Ce/¹⁴⁰Ce and ¹³⁸Ce/¹⁴⁰Ce fractionations will tend to favor more massive isotopes in Ce(IV)-bearing species, by 0.1-1.0‰ at 25ºC and 0.01-0.1‰ at 727ºC depending on the species present. The models predict ~0.3‰ higher ¹⁴²Ce/¹⁴⁰Ce in Ce(III)-bearing solution than coexisting Ce(IV)-solids at ambient temperatures, roughly agreeing with measurements. Zircon in equilibrium with typical silicate melts is predicted to be slightly enriched in ¹⁴⁰Ce relative to ¹⁴²Ce, ¹³⁸Ce, and ¹³⁶Ce. Supplementary calculations based on ¹⁴¹Pr-Mössbauer spectroscopy literature suggest somewhat (~1/3) smaller nuclear volume fractionation effects than the electronic structure models.