Two major stages of high-temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the Hess Deep Rift Valley at the East Pacific Rise (2 ¿N, 101 ¿W). The 154 m of plutonic rocks cored at site 894G show mineralogies and textures that suggest that they represent the roof of an approximate 1 Myr magma chamber. A late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite-upper greenschist facies conditions. The hydration of the gabbros occurred locally through the development of well-crystallized green amphiboles. Significant chemical fluxes were associated with the fluid flow and precipitation-dissolution mechanisms. Oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the Hess Deep gabbros at temperatures above 500 ¿C. Hydration was limited with only minute amphibole lamellae in pyroxenes that trapped Cl, Na, and minor K from seawater. Ca-plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns. We propose that this high-temperature event lowered the Δ18O of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion. Although it is unclear whether the 18O depletion of Hole 894G gabbros is representative of the entire lower oceanic crust, it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans. During this stage, water rock ratios calculated using Sr isotopes are underestimated (0.1--0.5) because of insignificant diffusion of this cation in plagioclases. Better estimates of water rock ratios are provided by oxygen isotopes (0.2--1) which are important in unraveling the cooling history of the oceanic crust. ¿ American Geophysical Union 1996