Determination of the oxidation state of cerium in rocks by Ce L III-edge X-ray absorption near-edge structure spectroscopy

Abstract

Details of the determination of the Ce(IV)/Ce(III) ratio by Ce L III-edge X-ray absorption near-edge structure spectroscopy are given. Cerium(III) shows a single peak in the XANES region while a distinct doublet was observed for Ce(IV) species. Each spectrum can be fitted by a combination of Arctangent and Lorentzian functions. A linear combination of XANES spectra for Ce(III) and Ce(IV) species can simulate XANES spectra of Ce in natural rocks. The peak area ratio of the Lorentzian functions for the Ce(IV) and Ce(III) species is correlated with the mole ratio of Ce(IV) to Ce(III). This relation was used to determine Ce(IV)/Ce(III) ratios in natural rocks and Ce sorbed onto α-Fe 2O 3 and δ-MnO 2 prepared in the laboratory. The determined Ce(IV)/Ce(III) ratios were compared with the degree of Ce anomaly found in rare earth element (REE) patterns, their relative abundance of REE being normalized by appropriate reference material. The observed Ce(IV)/Ce(III) ratios determined by XANES were consistent with the degree of Ce anomalies in REE partitioning patterns between water and α-Fe 2O 3 or δ-MnO 2, suggesting the validity of the present method. The established method was applied to the determination of the Ce(IV)/Ce(III) ratio in the apatite fossil, conodont. The degree of Ce anomaly of conodont has been regarded as providing a record of palaeoredox variation in ancient oceans. It is expected that the Ce anomaly coupled with the Ce(IV)/Ce(III) ratio will provide useful geochemical information such as elucidation of palaeoredox by its application to conodont.