New Insights into Antimony Isotopic Fractionation Mechanisms from Experimental Investigations

Abstract Numerous mechanisms have been theorized to account for the natural fractionation of antimony (Sb). However, empirical evidence to substantiate and elucidate the intricacies of these processes remain scarce. Herein, we conduct a series of laboratory experiments aimed to better constrain direction and extent of fractionation induced by these geologically prominent mechanisms. In the weathering experiment of stibnite, neither ultrapure water (MQ) nor 0.5N HNO3 induced distinguishable Sb isotopic fractionation beyond analytical uncertainty. By contrast, weathering with 0.4N HCl resulted in a significant enrichment of heavier Sb isotopes in leachate. Follow-up leaching experiments with HCl across a range of pH conditions confirmed that for native Sb, lower pH enhances fractionation, while for stibnite, the fractionation is largely independent of pH. These collective results reveal that the direction and magnitude of Sb isotopic fractionation are significantly influenced by ligand species in solution, pH conditions and specific Sb-bearing mineral species during leaching (or weathering) process. Notably, the chloric ligand exhibits a greater capacity to dissolve, transport and fractionate Sb than nitric ligand or ultrapure water, which lead to leachates observably enrich in heavier Sb isotopes relative to starting minerals with ∆123SbLeaching solution–Starting mineral ≤ 0.52‰. Regarding the evaporation and precipitation experiments, while no discernible Sb isotopic fractionation was observed, combined with previous related research, several noteworthy insights were nonetheless gleaned. In detail, the valence state of antimony ions, species of anion in solution, temperature and heating duration all contribute to effecting Sb isotopic fractionation during evaporation process. Meanwhile, key factors that influence redox-controlled Sb isotopic fractionation include the valence states of antimony, chemical bond types, biotic or abiotic environment, medium species. Importantly, the findings of this study will deepen our understanding of the mechanisms behind Sb isotopic fractionation, and thereby aiding in the interpretation of Sb isotopic system in geological research.