Chlorine stable isotope fractionation in evaporites

Abstract

Chlorine isotope fractionation ( 37 Cl 35 Cl ) between NaCl, KCI, and MgCl 2·6H 2O and their saturated solutions was determined in laboratory experiments at 22 ± 2°C. The results are as follows: 10 3 ln α( NaCl—solution) = +0.26 ± 0.07 (1δ) 10 3 ln α( KCl—solution) = −0.09 ± 0.09 (1δ) 10 3 ln α( MgCl 2·6H 2O—solution ) = −0.06 ± 0.10 (1δ) where fractionation factor a is defined as: α = − ( 37 Cl/ 35 Cl) precipitate ( 37 Cl/ 35 Cl) solution These data were used to approximate the isotope fractionation factors of chloride between the saturated solution and halite, kainite, carnallite, and bischofite. From the results, the stable chlorine isotope fractionation during the formation of evaporite was calculated using a Rayleigh fractionation model. The model predicts that δ 37Cl of the precipitate decreases systematically during the main phase of halite crystallization but increases again at the latest stage of evaporation. The chlorine isotope fractionation model was tested on a core from the upper Zechstein III salt formation. The salt core contains layers dominated by either halite or KMg salts. The KMg salts, which are formed during the final stages of evaporation, contain up to 75% carnallite (KMgCl 3·6H 2O) and bischofite (MgCl 2·6H 2O). The observed chlorine isotope fractionation in the salt core is in general agreement with the Rayleigh fractionation model. During the main crystallization phase of halite, δ 37Cl decreases continuously, but this trend reverses during the final stages when Mg-salts begin to crystallize. It is concluded that δ 37Cl can be used as an indicator of evaporation cycles. In addition, it provides quantitative information on the proportion of salt that has been deposited on the input of fresh seawater and on the disturbance by postdepositional processes.