Serpentinized ultramafics and hydrothermal activity at the Mid‐Atlantic Ridge crest near 15°N

The initial occurrence of serpentinized ultramafic rocks at the nontransform intersection of a wall of a rift valley with the wall of a fracture zone is described from a site at the Fifteen Twenty Fracture Zone. The ultramafics crop out in block‐faulted terrain on the upper portion of the eastern intersection between the rift valley and fracture zone walls in water depths between 2910 and 3300 m. They comprise cumulate harzburgites, pyroxenites, Iherzolites, and wehrlites, as well as gabbronorites, olivine gabbronorites, gabbropegmatites, and alteration products including serpentinites, bastite serpentinites, and asbestos. The Ti‐Zr‐Y relations and relatively constant Zr/Ti ratio in basalts recovered with the ultramafic rocks indicate a cogenetic relation from a common magmatic source that has undergone a late stage differentiation in the lower crust. Ongoing hydrothermal activity is indicated by chemical anomalies (δ 3 He, Mn) in the near‐bottom water at the ultramafic outcrop. The upwelling hydrothermal circulation apparently follows crust‐penetrating faults that may have controlled the diapiric ascent of the serpentinites and that continue to tap degassing magma and/or mantle. The observations presented indicate that ultramafic cumulates form beneath the rift valley adjacent to long‐offset (>100 km) ridge‐ridge transform faults, where they are serpentinized by hydrothermal processes within the initial 1 × 10 6 years of generation of lithosphère at a slow spreading axis. The corners formed by the intersections of the walls of a rift valley with both the transform (RT corner) and the nontransform (RN corner) portions of these fracture zones are principal loci of diapiric emplacement of serpentinized ultramafics.