Results of diabase alteration experiments at elevated temperatures and pressures have been combined with theoretical calculations to define the metasomatic processes reflected by the chemistry of hot-spring fluids and the chemistry and mineralogy of metabasalts dredged from mid-ocean ridges and observed in ophiolite outcrops on land. These data demonstrate that Mg-, Ca- and Na-fixation reactions are affected differently by different alteration conditions, and thus it is likely that each dominates a specific region within the submarine geothermal system. Experimental models of ridge-crest hydrothermal processes suggest that epidote and plagioclase solid solutions control the chemistry and pH of hot-spring fluids. Temperatures and pressures of approximately 385-400°C and 300-400 bars characterize the reaction zone of fluids discharging from black-smoker vents at 21°N, East Pacific Rise. Reaction-zone conditions (T,P) can be estimated for virtually all hot-spring fluids provided that salinity effects and retrograde processes caused by conductive cooling and/or subseafloor mixing are unambiguously accounted for.