Br/Cl signature of hydrothermal fluids: liquid–vapour fractionation of bromine revisited

Br/Cl ratios of hydrothermal fluids are widely used as geochemical tracers in marine hydrothermal systems to prove fluid phase separation processes. However, previous results of the liquid–vapour fractionation of bromine are ambiguous. Here we report new experimental results of the liquid–vapour fractionation of bromine in the system H 2 O–NaCl–NaBr at 380–450°C and 22.9–41.7 MPa. Our data indicate that bromine is generally more enriched than chlorine in the liquid phase. Calculated exchange coefficients K D(Br‐Cl) liquid‐vapour for the reaction Br vapour  + Cl liquid  = Br liquid  + Cl vapour are between 0.94 ± 0.08 and 1.66 ± 0.14 within the investigated P–T range. They correlate positively with D Cl liquid‐vapour and suggest increasing bromine–chlorine fractionation with increasing opening of the liquid–vapour solvus, i.e. increasing distance to the critical curve in the H 2 O–NaCl system. An empirical fit of the form K D(Br‐Cl) liquid‐vapour  =  a *ln[ b *( D Cl liquid‐vapour −1) + e 1/a ] yields a  = 0.349 and b  = 1.697. Based on this empirical fit and the well‐constrained phase relations in the H 2 O–NaCl system we calculated the effect of fluid phase separation on the Br/Cl signature of a hydrothermal fluid with initial seawater composition for closed and open adiabatic ascents along the 4.5 and 4.8 J g −1  K −1 isentropes. The calculations indicate that fluid phase separation can significantly alter the Br/Cl ratio in hydrothermal fluids. The predicted Br/Cl evolutions are in accord with the Br/Cl signatures in low‐salinity vent fluids from the 9 to 10°N East Pacific Rise.