Hydrothermal vent fluids collected with the DSRV Alvin from the southern limb of the Juan de Fuca Ridge are chemically unique, having chloride concentrations ~2 times ambient seawater. The same fluids contain noble gases in relative abundances like 2°C air-saturated seawater, the expected recharge composition. However, the absolute noble gas abundances are depleted by ~30% relative to seawater. The combination of very high chloride and moderately depleted noble gases appears to require the formation of a Cl-rich, gas-free brine by phase separation. This brine is mixed with recharge seawater at temperatures in excess of ~340°C and, therefore, deep in the hydrothermal system.