Gabbro and basalt, collected from an area south of the Kane Fracture Zone along the Mid-Atlantic Ridge, have three stages of alteration which record the cooling of a hydrothermal system: (1) Stage 1. Penetration of seawater began between 400¿ and 550¿ C, altering pyroxene to fibrous green amphibole. (2) Stage 2. Propylitic alteration formed along connected fractures between 250¿C and 300¿C. As fracture density increased, the Fe/Mg ratio of chlorite increased, the final result being an Fe chlorite -quartz-sulfide breccia. (3) State 3. Late smectite veinlets formed at low temperatures (≤200¿C) after active circulation ceased. The study focuses on stage 2 calibratin. By assuming local equilibrium between alteration minerals and the hydrothermal fluid, constraints can be placed on the fluid composition responsible for stage 2 alteration, the stage associated with deposition of sulfides. The following activities of species in solution were determined for the system FeO Na2O-CaO-MgO-Al2O3-SiO2-H20 at 350 bars and 250¿C: log a(Ca++)/a2(H+)=8.0, log a(Na+)/a(H+)=5.0, log a(Fe++)/a2(H+)=1.7, log a(Mg++)/a2(H+)=6.0. Log a(SiO2) was set at quartz saturation (-2.3 at 350 bars and 250¿C). Fluid inclusions record the introduction of a low temperature, seawater-salinity fluid during formation of the latest quartz veins associated with stage 2 alteration. Mixing of this and the hydrothermal fluid caused a drop in temperature and increase in oxidation state, resulting in increased precipitation of quartz, pyrite and chalcopyrite. The salinities of fluid inclusions trapped in quartz during stage 2 alteration are as much as 3 times that of seawater. Concentration of a fluid initially of seawater salinity may be the result of boiling at ≤350¿C and ≤3000 m depth.