Measurement of the hydrogen and oxygen isotopic compositions of concentrated chloride brines and brines from fluid inclusions in halite

Solutions of 1–5 m NaCl, KCl, CaCl₂ and MgCl₂, and synthetic brines were analyzed for hydrogen and oxygen isotopic compositions by vacuum distillation-microequilibration techniques. No effect on the hydrogen isotopic composition was observed for any of the solutions within the precision of the technique of ±5‰ determined from analyses of distilled water. Oxygen isotopic compositions of solutions measured by vacuum distillation-microequilibration indicate that oxygen isotopes are not fractionated during analysis in NaCl, KCl and CaCl₂ solutions. Vacuum distillation-microequilibration analyses of MgCl₂ solutions show depletions in ¹⁸O relative to pure water that can be related to the concentration of Mg²⁺ in the solution. The greatest depletion in ¹⁸O occurs in the 4 m MgCl₂ solutions which have δ¹⁸O-values 6‰ lower than that measured for pure water. Corrections based on the measurement of the δ¹⁸O-values of the pure MgCl₂ solutions can also be applied to mixed chloride brines. Both melting and crushing of halite grown in solutions of known isotopic composition followed by microequilibration to determine the stable isotopic composition of fluid inclusions give results within the precision determined from analyses of distilled water.