Veins in Hole 735B record the cooling history and paths of fluid circulation in a segment of lower ocean crust formed at a very slow spreading ridge. Vein assemblages span the entire range of temperature from late magmatic (+800°C) to ambient seawater and record the tectonic evolution of the crust as it formed at the spreading axis, migrated across the rift valley, and was uplifted into the transverse ridge along the edge of the fracture zone. Felsic and plagioclase-rich veins, particularly those with myrmekitic textures, are clearly magmatic. Although most of these veins have a significant hydrothermal overprint that obscures their origin, their textures, mineralogies, and chemical compositions argue strongly for an original late magmatic origin. The monomineralic amphibole veins in the highly deformed sequence at the top of the section formed as seawater gained access to the hot crust during brittle-ductile deformation associated with a major detachment fault that unroofed the lower crust and presumably caused rapid cooling. As the crustal section migrated away from the ridge axis and across the rift valley floor, the original magmatic veins acted as conduits for the hydrothermal fluids, and new veins also developed as brittle deformation continued. Most of the hydrothermal veins record declining fluid temperatures as indicated by precipitation of albite on plagioclase rims, deposition of late-stage quartz in the groundmass, and alteration of original minerals to greenschist assemblages. A number of veins also have discrete fractures that are filled with smectite ± zeolite ± carbonate. Both groundmass alteration of the host rocks and vein intensity decrease markedly downhole, indicating little penetration of fluids below ~1000 mbsf.
Chemical maps indicate very little interaction between vein fluids and the host rock, and most groundmass alteration appears to be associated with intense shearing in zones of crystal-plastic deformation. Detachment faulting played a major role not only in the tectonic evolution of this crustal segment but also in the timing and extent of alteration. Compared to gabbros formed at the fast-spreading East Pacific Rise, sampled in ODP Holes 894F and 894G, the Hole 735B section is more highly deformed and has a wider variety of igneous and hydrothermal veins.