Theoretical calculations of Cd isotope fractionation in hydrothermal fluids

• Cadmium isotope fractionations in hydrothermal fluid were calculated.
• The density functional theory was employed for Cd isotope fractionation.
• Cd hydroxides and Cd hydrosulfide may produce the largest fractionation at 25 °C.
• CdHS+ has the lightest equilibrium Cd-isotopic composition.

This study reports a systematic approach of quantum chemical calculations (density functional theory) of cadmium isotope reduced partition function ratios to understand the fractionation properties of Cd species in hydrothermal fluids, including cadmium hydrate, chloride, hydroxide, nitrate, hydrosulfide complexes. The optimized structures and the calculated vibrational frequencies of the selected Cd complexes by B3LYP method are consistent with the experimental data. The reduced partition function ratios 103ln (β114–110) of the Cd complexes were calculated with varying temperatures from 0 °C to 400 °C or 500 °C, which are from 0.680‰ to 2.098‰ at 50 °C. The isotope fractionation between different Cd species seems to be small, and there exists a trend of heavy Cd isotope enrichment as cadmium hydroxides > cadmium nitrates > cadmium hydrates > cadmium chlorides > cadmium hydrosulfide, according to the calculated reduced partition function ratios 103ln (β114–110) by this study. This suggests that Cd hydroxide complexes and hydrosulfide complexes may produce larger fractionation than others, which even reaches + 1.641‰ between Cd(OH)Cl and CdHS+ at 25 °C. As the major Cd-bearing species in hydrothermal conditions, CdOH(H2O)5+ and CdCl(H2O)5+, the fractionation factor α is found to be + 0.120‰ at 150 °C. These theoretical calculations provide insight into Cd isotope behavior in solution conditions, especially in hydrothermal fluids.