The Blanco Trough (Juan de Fuca Ridge) exposes a continuous section of upper oceanic crust (basalts and diabases) that has been hydrothermally altered in a transform fault environment. Microthermometric measurements on fluid inclusions and stable oxygen isotope compositions of whole rocks and secondary minerals from pillow lava and diabase units were performed to determine the evolution of hydrothermal fluids. A few low Δ18O values (down to +4.8?) for diabase, and temperatures from 280 to 350 ¿C obtained from fluid inclusions in quartz, attest to an early high temperature stage of hydrothermal circulation close to the ridge axis. A dense fracture network has maintained moderate temperature hydrothermal circulation (about 200 ¿C) rooted down to the top of the diabase unit. Oxygen isotope profiles in two reference sections reveal that the upper part of the oceanic crust exposed in the Blanco Trough is significantly 18O-enriched (+6.6?) relative to its mantle source (5.7¿0.2?). We propose that extensive off-axis tectonic activity linked to the transform zone favored fluid circulation in crust at maximum temperatures of about 200 ¿C. The low-temperature water-rock interactions (T<200 ¿C) were more pervasive in the Blanco crust, despite being 6 times younger than the crust at Hole 504B, which evolved far from any influence of transform fault activity. Transform faults, which are common in slow-spreading ridge environments, may therefore significantly affect the hydrothermal activity, which contributes to the regulation of the chemical and isotope composition of seawater. ¿ 1999 American Geophysical Union