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

• Solutions of NaCl, KCl, and MgCl2⋅6H2O were evaporated at 28 ± 2 °C.
• The δ37Cl values of samples decreased during precipitation.
• Cl isotope has fractionation features different from predecessor research.
• New Cl isotopic evolution curve of seawater precipitation were calculated.
• The δ37Cl values can be used as an better indicator of brine evolution.

In order to better understand chlorine isotopic variations during brine evolution, experiments were designed to determine the changes in the chlorine isotope composition (δ37Cl value) during evaporations of solutions containing NaCl, KCl and MgCl2⋅6H2O at 28 ± 2 °C. 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 Cs2Cl+ ion measurement. The results are as follows: the mean fractionation factors of the three solutions are αNa = 1.00055, αK = 1.00025, and αMg = 1.00012, respectively, where αNa, αK and αMg are the fractionation factors between salts (NaCl, KCl and MgCl2⋅6H2O) and saturated solutions. The results showed that the δ37Cl values of precipitate and coexisting brine samples decrease during the precipitation of single chloride minerals. The residual brine was a 35Cl 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, δ37Cl decreased continuously, and the most important thing is that this trend continues during the final stages when Mg–salts begin to precipitate.

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