The oxygen and hydrogen isotope compositions of vein minerals and δ 18O values of 64 whole rocks from the deeper kilometer of the 1.5 km lower crustal section of ODP Hole 735B in the SW Indian Ocean were analyzed in order to investigate seawater penetration into and reaction with the lower oceanic crust. Vein amphiboles have δD values of -35 to -65?, indicating formation from seawater fluids and hydrothermal fluids containing a component of low-δD magmatic fluid. Crystal-plastic shear zones reacted with 18O-enriched fluids at high temperatures and have δ 18O values of 6.0--7.0?, indistinguishable from surrounding rocks. Reaction with seawater-dominated fluids over a range of temperatures (~350--700¿C) led to local 18O-depleted rocks, but much of the section exhibits 18O enrichment (δ 18O up to 7.6?) as the result of static alteration of plagioclase over a range of temperatures (<250 to ~600¿C) and fluid compositions. Chloritic veins and associated alteration by 18O-enriched fluids occurred at temperatures of 150--250¿C, and late carbonates and Fe-oxyhydroxides formed from cold (~10¿) seawater associated with open fault-related fractures. Bulk rock δ 18O values of the lower 1 km range from 3.7 to 10.2? but are mostly 5.6--7.6?, and the section has a mean bulk δ 18O of 6.5 ¿ 1.0?. In contrast, rocks from the shallower 500 m exhibit net 18O depletion (5.1 ¿ 1.0?) reflecting high-temperature deformation and seawater hydrothermal alteration (Stakes, 1991). The weighted average δ 18O for the entire gabbro section of Hole 735B is 6.0?, exhibiting little or no change compared to unaltered crust. Although tectonism at slow spreading rates may allow exposure of lower crust to greater amounts of low-temperature alteration and 18O enrichment, there is no evidence for a consistent difference in the bulk δ 18O of the plutonic sections of oceanic crust formed at fast versus slow spreading rates.