Hydrothermal alteration processes at mid-ocean ridges: constraints from diabase alteration experiments, hot-spring fluids and composition of the oceanic crust

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.