DIRECT OBSERVATION OF THE EVOLUTION OF A SEA-FLOOR BLACK SMOKER FROM VAPOR TO BRINE

A single hydrothermal vent, 'F' vent, occurring on very young crust at 9 degrees 16.8'N, East Pacific Rise, was sampled in 1991 and 1994. In 1991, at the measured temperature of 388 degrees C and seafloor press ure of 258 bar, the fluids from this vent were on the two-phase curve for seawater. These fluids were very low in chlorinity and other disso lved species, and high in gases compared to seawater and most sampled seafloor hydrothermal vent fluids. In 1994, when this vent was next sa mpled, it had cooled to 351 degrees C and was venting fluids similar t o 1.5 times seawater chlorinity. This is the first reported example of a single seafloor hydrothermal vent evolving from vapor to brine. The 1991 and 1994 fluids sampled from this vent are compositionally conju gate pairs to one another. These results support the hypothesis that v apor-phase fluids vent in the early period following a volcanic erupti on, and that the liquid-phase brines are stored within the oceanic cru st, and vent at a later time, in this case 3 years. These results demo nstrate that the venting of brines can occur in the same location, in fact from the same sulfide edifice, where the vapor-phase fluids vente d previously.