Equilibrium Mg and K isotope fractionation between carnallite and saturated brine: Calibrations and applications

• Published in: Geochimica et cosmochimica acta 2024-04, Vol.371, p.173-188
• Main Authors: Xia, Zhiguang, Lin, Yongjie, Li, Dongdong, Reuning, Lars, Hu, Zhongya, Liu, Chuan, Mu, Jun, Li, Weiqiang
• Format: Article
• Language: English
• Subjects: Ancient seawater; Evaporation experiment; Evaporite; Mg and K isotope fractionation; Synthesis experiment
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2024.02.001

Evaporites are important sedimentary records of seawater history, and their Mg and K isotopic signatures could be used to constrain the evolution of surficial cycles of Mg and K on Earth. However, the lack of detailed understanding of the Mg and K isotopic behaviors during the precipitation of key evaporite minerals has impeded the applications of Mg and K isotopes in evaporites. In this study, we systematically investigated Mg and K isotope fractionation factors between carnallite (KCl·MgCl2·6H2O) and saturated brine through mineral synthesis experiments, laboratory simulation of brine evaporation, and analysis of field samples. Carnallite synthesis experiments were performed at three different temperatures, and the results show that carnallite preferentially incorporates heavy Mg isotopes and light K isotopes over brines. Specifically, the Mg isotope fractionation factors (Δ26Mgcar-sol) at 5, 25, and 50 °C are 0.83 ± 0.09 ‰, 0.84 ± 0.09 ‰, and 0.57 ± 0.08 ‰, respectively, whereas the measured K isotope fractionation factors (Δ41Kcar-sol) at 5, 25, and 50 °C are −0.49 ± 0.08 ‰, −0.43 ± 0.11 ‰, and −0.41 ± 0.08 ‰, respectively. The Mg and K isotope fractionation factors calibrated by the synthesis experiment were verified by brine evaporation experiments and field samples. Based on a set of laboratory brine evaporation experiment, the instantaneous isotope fractionation factors for carnallite are 0.82 ‰ ∼ 0.83 ‰ (Δ26Mgcar-sol) and −0.38 ‰ ∼ −0.43 ‰ (Δ41Kcar-sol) after correction based on Li contents and a Rayleigh fractionation model. Additionally, based on analyses of brine and carnallite samples from the salt pan in the Qarhan Salt Lake region, Δ26Mgcar-sol and Δ41Kcar-sol are determined to be 0.85 ± 0.05 ‰ and −0.45 ± 0.09 ‰, respectively. Using the obtained Mg and K fractionation factors for evaporite minerals and thermodynamic calculations, we predicted how the Mg and K isotopes of brine would change in response to the evaporation of seawater with varying chemistries. The modeling results show that different initial seawater/brine chemistry (e.g., Ca-rich, sulfate-rich, and “Ca-SO4 crossover”) would result in different evolutionary trends of δ41K and δ26Mg in the brines during evaporation, primarily driven by carnallite precipitation. Therefore, Mg and K isotope signatures in evaporites or brines can provide key constraints for brine evolution in enclosed basins and ancient seawater chemistry.