Bromide/chloride fractionation in seafloor hydrothermal fluids from 9–10°N East Pacific Rise

Hydrothermal fluids collected from 9–10°N EPR during 5 cruises between 1991 and 1994 were analyzed for Br and Cl. The chlorinity of these fluids ranges between 32.6 and 860 mmol/kg in comparison to local seawater values of 540 mmol/kg. These very low chlorinity fluids, some with temperatures high enough to be within the vapor stability field, were collected following a volcanic eruption in 1991 [1]. Although fluids with chlorinities less than 250 mmol/kg had been sampled once previously, at Axial Volcano [2], the vent fluid chlorinities at 9–10°N EPR were much lower than at Axial Volcano. Fluids with Cl ≥ 250 mmol/kg hadBr/Cl ratios identical to seawater, in agreement with previous studies. In contrast, fluids with Cl < 250 mmol/kg collected from this area, hadBr/Cl ratios as much as 40% less than the seawater value. Bromide depletions observed in the low chlorinity fluids from 9–10°N EPR were probably the result of fractionation of Br from cl during subcritical phase separation. This fractionation process appeared to be limited to fluids with Cl < 250 mmol/kg, and to require open system phase separation. To produce vapors with Cl ≥ 250 mmol/kg at pressure and temperature conditions appropriate for 9–10°N EPR requires conditions that are close to the critical point, where the properties of the vapor and liquid phases are quite similar, resulting in lack of fractionation in theBr/Cl ratio between the two phases.