Stable isotope fractionation of chlorine during the precipitation of single chloride minerals

摘要

In order to better understand chlorine isotopic variations during brine evolution, experiments were designed to determine the changes in the chlorine isotope composition (δ~(37)Cl value) during evaporations of solutions containing NaCl, KCl and MgCl_2·6H_2O at 28 ± 2 ℃. Three evaporation experiments were conducted in a clean environment. The precipitate and brine samples were collected during the evaporation, and the chlorine isotopic ratios of the samples were determined using an improved thermal ionisation mass spectrometry procedure based on Cs_2Cl~+ ion measurement. The results are as follows: the mean fractionation factors of the three solutions are α_(Na) = 1.00055, α_K = 1.00025, and α_(Mg) = 1.00012, respectively, where α_(Na), α_K and α_(Mg) are the fractionation factors between salts (NaCl, KCl and MgCl_2·6H_2O) and saturated solutions. The results showed that the δ~(37)Cl values of precipitate and coexisting brine samples decrease during the precipitation of single chloride minerals. The residual brine was a ~(35)Cl reservoir for different single chloride solutions. New chlorine isotopic evolution curve during seawater evaporation were also calculated. The results indicated that during the primary precipitation stage of halite, δ~(37)Cl decreased continuously, and the most important thing is that this trend continues during the final stages when Mg-salts begin to precipitate.