EVOLUTION OF A BRINE-SATURATED LAYER AT THE BASE OF A RIDGE-CREST HYDROTHERMAL SYSTEM

Vent fluids with chlorinities significantly greater than seawater are indicative of mixing between hydrothermal seawater and brines that hav e formed as a result of supercritical phase separation. We relate the evolution of a brine-saturated layer to magmatic and tectonic processe s at the base of a ridge crest hydrothermal system. A conductive heat balance indicates that under steady state conditions such a brine-satu rated layer is typically similar to 1-10 m thick and contains 10(7) to 10(8) kg of brine with 6 - 10 times the chlorinity of seawater. Simpl e mixing and fractionation models indicate that the depletion and repl enishment of brine-saturated layers may occur on a timescale of years to centuries. A timescale of years is consistent with temporal changes in chlorinity of fluids at vents in areas of recent, known dike injec tion. In these systems, brines may be undergoing depletion by mixing w ith liquid-phase seawater. Vents stably emitting high-chlorinity fluid s on timescales of decades may represent older systems in which brines are being depleted by diffusion into an overlying liquid-phase hydrot hermal system.