Kinetic fractionation of Mg isotopes during chemical diffusion in aqueous solutions: A reappraisal

Diffusion is a basic process that could result in isotopic variability in nature, and diffusion-driven Mg isotope fractionation has been widely reported in a variety of magmatic processes. Whether chemical diffusion of Mg aquo ions in aqueous solution is associated with significant isotope fractionation, however, had been contentious. Addressing this issue can advance our understanding of the molecular configuration and movement of cations in aqueous solutions. Here, we modified the recently reported "diffusion cell" method to investigate the isotopic effect of diffusion of aqueous Mg under various conditions, with emphasis on the effects of temperature and anions. The experimental results show that in aqueous solutions, lighter Mg isotopes diffuse faster than the heavier isotopes. The ratio of diffusion coefficients between ²⁶Mg and ²⁴Mg (D_26Mg/D_24Mg, or kinetic isotope fractionation factor α) ranges from 0.99990 ± 0.00003 (1σ) to 0.99982 ± 0.00003 (1σ), depending on the specific experimental conditions. Notably, the magnitude of diffusion-induced Mg isotope fractionation in aqueous solution increases with higher temperature, and is greater for MgCl₂ and Mg(NO₃)₂ than for MgSO₄. Based on these new findings and a comparison with published data on diffusion-driven isotope fractionation for other divalent elements, we suggest that the mass dependency of isotope fractionation during chemical diffusion in aqueous solutions is regulated by the strength of ion-water interactions. Stronger ion-water interactions result in more pronounced hydrodynamic behavior at a molecular level for the diffusing ions, leading to less significant isotope fractionation in aqueous fluids.