Stable isotope fractionation between liquid and vapour in water–salt systems up to 600°C

Hydrogen and oxygen isotope fractionation factors between liquid and vapour were measured between 350°C and 600°C in the systems H2O–NaCl and H2O–KCl. Fractionation factors are similar for the NaCl- and KCl-bearing systems. At all conditions, D preferentially fractionates into the vapour phase relative to H, 18O into the liquid relative to 16O. Fractionation factors are given in terms of 1000 lnαD/H(L–V) and 1000 lnα18O/16O(L–V), respectively. At constant temperature, fractionations are approximated by linear correlations of the fractionation factor with the salt concentration in the liquid. Isotherms all have similar slopes and start at the respective critical composition where the fractionation is zero. D/H fractionation increases by 0.55‰ per wt.% NaCl(L), 18O/16O fractionation by about 0.05‰ per wt.% NaCl(L). If extrapolated up to the limit of salt-saturation in the system, maximum fractionations of 28‰ in D/H(V–L) and about 2‰ in 18O/16O(L–V) result along the liquid+vapour+halite curve over the temperature range from 350°C to 600°C. Maximum fractionations occur in boiling hydrothermal systems when the water–salt solvus opens up to salt-saturated conditions. This happens most likely in hydrothermal solutions at the roof of shallow intrusions. In this environment, salt-bearing solutions of magmatic, meteoric, or mixed origin intersect the solvus during isothermal or adiabatic decompression. Open-system behaviour may rapidly result in salt saturation of the liquid residue. Vapour leaving the hydrothermal system may have an isotopic signature similar to seawater whereas brine residues may become increasingly depleted in D and slightly enriched in 18O. If the strong fractionation effect inherent in a boiling fluid system is disregarded, one may easily misinterpret the stable isotope ratios of hydrothermal minerals from such systems since boiling may strongly mask the source of the parent fluids.