Revisiting the hydrothermal geochemistry of europium(II/III) in light of new in-situ XAS spectroscopy results

Knowledge of the mobility of Rare Earth Elements (REE) in crustal fluids is important for understanding both the formation of REE deposits and their use as geochemical tracers. This is particularly true for europium (Eu), which exists in both Eu(II) and Eu(III) states in natural fluids, making it a sensitive probe of hydrothermal redox processes. Currently, our understanding of Eu complexation in hydrothermal fluids relies mainly on extrapolations from room-temperature data, as there are no high-temperature experimental data available for Eu(II) species. In this study we conducted in-situ synchrotron X-ray absorption spectroscopy (XAS) experiments to investigate the complexing of Eu(II) and Eu(III) in chloride- and bromide-rich acidic solutions and Eu(III) sulphate complexes, at 35–400 °C, 600 bar. For Eu(III) chloride complexes, the total number of ligands decreased and the ratio of H₂O:Cl ligands in the first coordination shell decreased with increasing temperature. For the Eu(II) system, our data confirm Sverjensky's (1984, EPSL 67, 70–78) theoretical predictions that Eu(II) species become increasingly stable (relative to Eu(III)) at elevated temperatures. However the new data indicate that complexation between Eu(II) and chloride is much weaker than Haas's (1995, GCA 59, 4329–4350) theoretical estimates. The thermodynamic properties of Eu(II) chloride complexes have been reassessed in light of the new data. Thermodynamic calculations reveal that the identity of the Eu(III) aqueous complexes (e.g., chloride or hydroxide species) have a significant impact on the oxidation state of Eu in hydrothermal fluids at elevated temperatures.