Stable isotope fractionation of chlorine during evaporation of brine from a saline lake

Abstract

Brine from the saline Qarhan Lake was evaporated at 28±2°C in a clean environment. Two groups of experiments were conducted; one with complete separation of precipitate and brine at different stages of evaporation, and the other with continuous precipitation during the evaporation. Seventy-nine precipitate and brine samples were collected during the experiments, and the δ 37Cl values were determined using an improved thermal ionization mass spectrometry procedure for precise measurement of chlorine isotopes based on Cs2Cl+ ions. Based on the concentrations of Na+, K+, and Mg2+, evaporation was divided into three main precipitation stages as follows: halite dominant, carnallite dominant, and bischofite dominant. The δ 37Clsolid and δ 37Clliquid values of the precipitate and coexisting brine samples at different stages showed the following characteristics. The precipitates were enriched with 37Cl relative to the coexisting brine samples, and the δ 37Cl of both the precipitate and brine samples decreased gradually during evaporation. The fractionation factors (α h) between halite and brine were the highest, followed by that (α c) between carnallite and brine, and then that (α b) between bischofite and brine. The α c and α b values of less than one, which indicate the precipitate is enriched in 35Cl, were found when the evaporation process entered a new stage. However, the δ 37Cl values of carnallite, bischofite, and the coexisting brine samples decreased during evaporation. The residual brine is a 35Cl reservoir. The experimental phenomena were consistent with the δ 37Cl values in saline deposits in the literature. δ 37Cl can be used as an indicator of brine evaporation processes, which is important in the exploration of sylvinite deposits. © 2012 The Author(s).