Zinc isotope fractionation under vaporization processes and in aqueous solutions  

作　　者：Jixi Zhang Yun Liu 

机构地区：[1]School of Karst Science, Guizhou Normal University/State Engineering Technology Institute for Karst Desertification Control [2]State Key Laboratory of Ore Deposit Geochemistry, Instituteof Geochemistry, Chinese Academy of Sciences

出　　处：《Acta Geochimica》2018年第5期663-675,共13页地球化学学报（英文）

基　　金：support from973 Program Fund(No.2014CB440904);Chinese National Science Fund Projects(Nos.41530210,41490635,41403051)

摘　　要：Equilibrium Zn isotope fractionation was inves- tigated using first-principles quantum chemistry methods at the B3LYP/6-311G level. The volume variable cluster model method was used to calculate isotope fractionation factors of sphalerite, smithsonite, calcite, anorthite, for- sterite, and enstatite. The water-droplet method was used to calculate Zn isotope fractionation factors of Zn^2＋-bearing aqueous species; their reduced partition function ratio factors decreased in the order [Zn（H2O）6]^2＋ 〉 [ZnCl（H2O）5]^ ＋ 〉 [ZnCl2（H2O）4] 〉 [ZnCl3（H20）2]^-〉 ZnCl4]^2-. Gas- eous ZnCl2 was also calculated for vaporization processes. Kinetic isotope fractionation of diffusional processes in a vacuum was directly calculated using formulas provided by Richter and co-workers. Our calculations show that in addition to the kinetic isotope effect of diffusional processes, equilibrium isotope fractionation also contributed nontriv- ially to observed Zn isotope fractionation of vaporization processes. The calculated net Zn isotope fractionation of vaporization processes was 7-7.5‰, with ZnCl2 as the gas- eous species. This matches experimental observations of the range of Zn isotope distribution of lunar samples. Therefore, vaporization processes may be the cause of the large distri- bution of Zn isotope signals found on the Moon. However, we cannot further distinguish the origin of such vaporization processes; it might be due either to igneous rock melting inmeteorite bombardments or to a giant impact event. Fur- thermore, isotope fractionation between Zn-bearing aqueous species and minerals that we have provided helps explain Zn isotope data in the fields of ore deposits and petrology.Equilibrium Zn isotope fractionation was investigated using first-principles quantum chemistry methods at the B3LYP/6-311G* level. The volume variable cluster model method was used to calculate isotope fractionation factors of sphalerite, smithsonite, calcite, anorthite, forsterite, and enstatite. The water-droplet method was used to calculate Zn isotope fractionation factors of Zn^(2+)-bearing aqueous species; their reduced partition function ratio factors decreased in the order [Zn(H_2O)_6]^(2+)>[ZnCl(H_2O)_5]^+>[ZnCl_2(H_2O)_4]>[ZnCl_3(H_2O)_2]^- >[ ZnCl_4]^(2-). Gaseous ZnCl_2 was also calculated for vaporization processes.Kinetic isotope fractionation of diffusional processes in a vacuum was directly calculated using formulas provided by Richter and co-workers. Our calculations show that in addition to the kinetic isotope effect of diffusional processes,equilibrium isotope fractionation also contributed nontrivially to observed Zn isotope fractionation of vaporization processes. The calculated net Zn isotope fractionation of vaporization processes was 7–7.5%, with ZnCl_2 as the gaseous species. This matches experimental observations of the range of Zn isotope distribution of lunar samples. Therefore,vaporization processes may be the cause of the large distribution of Zn isotope signals found on the Moon. However,we cannot further distinguish the origin of such vaporization processes; it might be due either to igneous rock melting inmeteorite bombardments or to a giant impact event. Furthermore, isotope fractionation between Zn-bearing aqueous species and minerals that we have provided helps explain Zn isotope data in the fields of ore deposits and petrology.

关 键 词：Evaporation process Zinc isotope Kineticisotope fractionation Equilibrium fractionation Zincspecies in solution 

分 类 号：P575[天文地球—矿物学]