Experimental evaporation of hyperacid brines: Effects on chemical composition and chlorine isotope fractionation

Hyperacid brines from active volcanic lakes are some of the chemically most complex aqueous solutions on Earth. Their compositions provide valuable insights into processes of elemental transfer from a magma body to the surface and interactions with solid rocks and the atmosphere. This paper describes changes in chemical and δ³⁷Cl signatures observed in a 1750 h isothermal evaporation experiment on hyperacid (pH 0.1) sulphate-chloride brine water from the active lake of Kawah Ijen volcano (Indonesia). Although gypsum was the only evaporite mineral identified in the evolving brine, decreasing Si concentrations may ultimately result in amorphous silica precipitation. Geochemical simulations predict the additional formation of elemental sulphur at lower water activities (a_H2O ≤ 0.65) that were not reached in the experiment. Absence of other sulphates and halides despite the high load of dissolved elements (initial TDS ca. 100 g/kg) can be attributed to increased solubility of metals, promoted by extensive formation of complexes between the variety of cations and the major anions (HSO₄ ⁻, Cl⁻, F⁻) present. Chlorine deviations from a conservative behaviour point to losses of gaseous hydrogen chloride (HCl_(g)) and consequently an increase in Br/Cl ratios. Chlorine isotope fractionation that accompanied the escape of HCl_(g) showed a marked change in sign and magnitude in the course of progressive evaporation of the brine. The calculated factor of fractionation between HCl_(g) and dissolved Cl for the initial interval (before 500 h) is positive (1000lnα_{HCl(g)-Cldiss.} =+1.55±0.49‰to+3.37±1.11‰), indicating that, at first, the escaping HCl_(g) was isotopically heavier than the dissolved Cl remaining in the brine. Conversely, fractionation shifted to the opposite direction in the subsequent interval (1000lnα_{HCl(g)-Cldiss.} =5.67±0.17‰to-5.64±0.08‰), in agreement with values reported in literature. It is proposed that Cl isotopic fractionation in highly acidic brines is controlled by the distribution of dissolved chlorine species, which changes from Cl⁻ to HCl^o dominance with the progressive pH decline. The Kawah Ijen lake acquired its extreme composition through influx of sulphur and halogen-rich magmatic gas components and extensive rock dissolution. If hyperacid brines with comparable chemical composition existed on Mars, evaporation processes up to the extent reported here (a_H2O=0.85), were likely accompanied by losses of gaseous HCl. The resulting changes in Cl isotope compositions, Br/Cl, S/Cl and other ratios in the residual brine might be potentially recorded in assemblages of halogen-bearing secondary evaporation minerals. Also, volcanic-hydrothermal brines as these would extend the stability of liquid water on the Martian surface down to a temperature of −90 °C.