Sn (tin) has ten stable isotopes and is a newly emerging metal stable isotope tracing tool. It has broad application prospects due to its wide participation in the evolution of magma, hydrothermal fluid, and planets. In recent years, Sn isotope geochemistry has become a hot issue of international research. At present, Dr. Yao Junming from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, and Dr. Wang Da from China University of Geosciences (Beijing) have carried out Sn isotope research. They finished their Sn isotope analysis abroad in cooperation with Professor Ryan Mathur from Juniata College. No laboratory in China that have established independent Sn isotope analysis methods before. The study of granite and tin deposits in South China is the preponderant research direction of Nanjing University. After returning to China as the "Overseas High-level Talents Program" of the Central Organization Department, Professor Li Weiqiang started to establish this Sn isotope method at the State Key Laboratory for Mineral Deposits Research of Nanjing University, hoping to provide a new perspective for tin mineralization and related granite research. The research team combines the sample standard bracketing and the Sb element doping method and establish a high-precision Sn isotope method at Nanjing University. The long-term external accuracy of 122Sn/116Sn is 0.09‰, which is comparable to other international laboratories.
The prerequisite for the application of Sn isotopes as a tracing tool for geological processes is clarifying the fractionation mechanism of Sn isotopes in related processes. The gaseous transport of metal elements is of great significance in the geological process. Previous studies have shown that tin can be transported in gaseous form during the geological process. The gas-liquid and gas-solid separation process may be accompanied by isotopic fractionation. The Sn isotope fractionation caused by the gas-liquid separation of tin in the hydrothermal process and volatile condensation in the evolution of celestial bodies can reveal the relevant reaction mechanism and help us to understand the corresponding geological processes. Computational simulation is a powerful method to study the microscopic reaction mechanism, and its application in geochemical research is increasingly popular. Professor Liu Xiandong of the School of Earth Science and Engineering and his research team have long been devoted to the research of computational mineralogy and computational chemistry. Quantum molecular dynamics and other computational simulation methods have been used at the research of mineral-fluid interface microstructure, acidity, metal ion adsorption mechanism, and the character of hydrothermal fluid such as the type, structure, and thermodynamic stability of metal ion complexes in fluids. Professor Liu Xiandong has made systematic achievements and his research team has reached the advanced level. For the gas-liquid separation of tin in the hydrothermal process, the team of Professor Li Weiqiang and the team of Professor Liu Xiandong carried out in-depth cooperation and combined experimental observations, computational simulations, and theoretical models, revealing the fractionation mechanism of Sn isotopes in the gas-liquid separation process at a micro-level.
In this study, the volatilization mechanism of Sn4+ species in acidic aqueous solution at 96 ℃ and the corresponding Sn isotope fractionation were discussed. The experimental results show that species containing Sn light isotopes are preferentially partitioned to the vapor phase, and there is an overall Sn isotope fractionation of -0.36‰ in this process. Quantum chemical calculations have revealed the Sn species in the gas and liquid phases respectively and the equilibrium Sn isotope fractionation factor between those dominant species in these two phases. The results show that SnCl4 is the only stable species in the gas phase, and the equilibrium Sn isotope fractionation factor of δ122/116Sn between SnCl4 and the liquid species is between 0.35‰ and 1.24‰. Based on this study and the recent experimental results of Sn isotope gas-liquid fractionation at 150 ℃ reported by Wang Da et al., (2019), we conclude that the boiling of the solution has a great influence on the evaporation mechanism of Sn.
This study demonstrates that the Sn isotope fractionation due to volatilization during the sample preparation process cannot be ignored and needs to be avoided or corrected. The tin ore deposits formed at different depths may have different Sn isotope fractionation mechanisms due to gas-liquid separation in closed or open systems. Besides, Sn is a moderate volatile element in cosmochemistry such that the evaporation and condensation mechanism of Sn isotopes may play an important role in the interpretation of Sn isotope data of cosmochemistry.
The above-mentioned achievements were recently published in the first-class geochemical journal "Geochimica et Cosmochimica Acta" entitled "Sn isotope fractionation during volatilization of Sn(IV) chloride: Laboratory experiments and quantum mechanical calculations". Ph.D. students She Jiaxin and Wang Tianhua are the Co-first authors, Professor Li Weiqiang and Professor Liu Xiandong are co-corresponding authors. This research was supported by the National Natural Science Foundation of China, the National Key R&D Program of China, and the State Key Laboratory for Mineral Deposits Research of Nanjing University, and the High Performance Computing Center of Nanjing University.