Fluid inclusions in a suite of gabbro, quartz-breccia, and metabasalt samples recovered from the MARK area on the Mid-Atlantic Ridge are the product of a complex hydrothermal history involving late stage magmatic fluids at temperatures >700 ¿C and penetration by modified seawater at 300--400 ¿C. The evolution of volatiles during the early stages of solidification and cooling of magma bodies near the ridge-transform intersection is marked by exsolution of a CO2 fluid, entrapped within primary inclusions in fluorapatites. Attendant with progressive melt fractionation, residual evolved melts reached water saturation, and locally, supercritical CO2+H2O+NaCl¿Fe brines (>50 wt % NaCl) and cogenetic H2O+CO2-rich vapors (1--2 wt% NaCl) were exsolved as immiscible phases. Concomitant or subsequent fracturing, perhaps in response to volatile exsolution from the melts, allowed migration of these fluids along microfracture networks at >700 ¿C. Trondhjemitic-hosted inclusions, which homogenize by halite dissolution, indicate that the last fluids exsolved from the melts may have been 35--40 wt% brines.

The transition from magmatic to seawater-dominated hydrothermal conditions in the gabbros is marked by initial penetration of lower salinity fluids (1--7 wt% NaCl) at temperatures in excess of 400 ¿C, with the general cessation of fluid flow occurring at minimum temperatures of ≈250 ¿C. The relative enrichment and depletion of NaCl with respect to seawater in these fluids may record supercritical phases separation of seawater or boiling of hydrothermal fluids enriched in NaCl. Migration along microfracture networks of CH4-rich, 350 ¿C fluids, may reflect deeper seated hydrothermal processes involving hydration of underlying mantle material in response to fluid flow along deeply penetrating fault systems. In shallow crustal rocks, circulation of seawater-derived fluids occurred at temperatures up to 400 ¿C, with subsequent collapse of the active hydrothermal system at minimum temperatures of 200--250 ¿C.

In fault-related upflow zones, multiple hydrothermal pulses involving 180--340 ¿C and 3.5--10 wt % NaCl fluids, pervasively altered bounding wall rocks, forming chlorite-rich, pyrite- and chalcopyrite-bearing breccias. At shallow crustal depths, fluids reached temperatures of 150--300 ¿C and contained salinities of 3.8--6.9 wt % NaCl. Following collapse of the axial-related hydrothermal system, the plutonic and shallow crustal rocks were uplifted and emplaced as allocthonous blocks attending formation of the ridge-transform intersection massif. ¿ American Geophysical Union 1993