BROMIDE CHLORIDE FRACTIONATION IN SEA-FLOOR HYDROTHERMAL FLUIDS FROM 9-10-DEGREES-N EAST PACIFIC RISE/

Hydrothermal fluids collected from 9-10 degrees 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 loca l seawater values of 540 mmol/kg. These very low chlorinity fluids, so me with temperatures high enough to be within the vapor stability fiel d, were collected following a volcanic eruption in 1991 [1]. Although fluids with chlorinities less than 250 mmol/kg had been sampled once p reviously, at Axial Volcano [2], the vent fluid chlorinities at 9-10 d egrees N EPR were much lower than at Axial Volcano. Fluids with Cl gre ater than or equal to 250 mmol/kg had Br/Cl ratios identical to seawat er, in agreement with previous studies. In contrast, fluids with Cl <2 50 mmol/kg collected from this area, had Br/Cl ratios as much as 40% l ess than the seawater value. Bromide depletions observed in the low ch lorinity fluids from 9-10 degrees N EPR were probably the result of fr actionation of Br from Cl during subcritical phase separation. This fr actionation process appeared to be limited to fluids with Cl <250 mmol /kg, and to require open system phase separation. To produce vapors wi th Cl greater than or equal to 250 mmol/kg at pressure and temperature conditions appropriate for 9-10 degrees N EPR requires conditions tha t are close to the critical point, where the properties of the vapor a nd liquid phases are quite similar, resulting in lack of fractionation in the Br/Cl ratio between the two phases.