Vent fluids with chlorinities significantly greater than seawater are indicative of mixing between hydrothermal seawater and brines that have formed as a result of supercritical phase separation. We relate the evolution of a brine-saturated layer to magmatic and tectonic processes at the base of a ridge crest hydrothermal system. A conductive heat balance indicates that under steady state conditions such a brine-saturated layer is typically ~1--10 m thick and contains 107 to 108 kg of brine with 6--10 times the chlorinity of seawater. Simple mixing and fractionation models indicate that the depletion and replenishment 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 injection. In these systems, brines may be undergoing depletion by mixing with liquid-phase seawater. Vents stably emitting high-chlorinity fluids on timescales of decades may represent older systems in which brines are being depleted by diffusion into an overlying liquid-phase hydrothermal system.¿ 1997 American Geophysical Union