Determination of cerium isotope ratios in geochemical samples using oxidative extraction technique with chelating resin

Abstract

The cerium isotope ratio (¹³⁸Ce/¹⁴²Ce) is used for dating and as a chemical tracer in geochemistry and cosmochemistry. Selective chemical separation techniques are required prior to precise Ce isotopic analysis because other rare earth elements and barium interfere with the analyses. The separation of Ce from other rare earth elements has not been well established because of their similar chemical properties. Here we report a precise determination method of Ce isotopes using a highly selective extraction technique of Ce from other interfering elements with a chelating resin. During the extraction procedure, Ce is oxidized from Ce(III) to Ce(IV) using KBrO₃ in nitric acid; it is then separated easily from other rare earth elements and Ba. The oxidative extraction procedure excluded the interfering elements, such as Ba, La, Pr and Nd from Ce to undetectable levels (<0.1 ng). This extraction method was applied to rock and seawater samples. Interference by isobars was negligibly small in all cases, with the result that Ce isotopic data were obtained more precisely than by conventional separation using α-hydroxy-isobutyric acid. The ε_Ce values for typical continental rock, island arc basalt, and ferromanganese nodule samples were, respectively, +1.34 (2σ_m = ±0.25), −1.56 (±0.22), and −0.88 (±0.19). For a surface seawater sample from the western North Pacific Ocean, both new and conventional methods were applied, giving a consistent ε_Ce value. This new method is beneficial for various geochemical studies of Ce isotopes.