An extensive suite of hydrothermally altered gabbros was recovered by Alvin and dredging from the Mid-Atlantic Ridge, in the vicinity of the eastern ridge-transform intersection (RTI) of the Kane Fracture Zone (MARK), where tectonic extension has provided a window into the lower crustal component of hydrothermal cells. Four alteration types are distinguished on the basis of metamorphic assemblage, mineral composition, and deformation textures. A conceptual model is presented that places temperature-fluid/rock relationships, in conjunction with styles of deformation, into a spatial and temporal framework. Exsolution of late stage magmatic fluids in the vicinity of the magma-hydrothermal interface at temperatures >700 ¿C marks the onset of fluid-rock interaction in the MARK gabbros (Kelley et al., this issue), with subsequent alteration involving seawater-derived fluids being strongly influenced by deformation mechanisms. In zones of ductile shear, hydration was initiated at temperatures between 550¿ and 700 ¿C and moderate fluid/rock ratios. Elsewhere in the lower crust, the onset of seawater penetration occurred at temperatures of 400¿--550 ¿C and was facilitated by brittle fracturing. Early vein networks indicate very low initial fluid/rock ratios and are interpreted as being related to the solidification of plutons. Later fractures indicate higher water/rock ratios and may be related to the downward propagation of shallow fault systems. Throughout the crust, fluid-rock interaction ceased at temperatures 180¿--300 ¿C. Subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the RTI massif produced moderate fluid/rock ratios within localized zones. Hydrothermal alteration in the crustal section exposed at MARK was initiated at lower temperatures, and water/rock interaction proceeded to lower temperatures than many other plutonic suites. These differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust. ¿ American Geophysical Union 1993