Description: Dolerite samples, recovered in the dike complex between 1589 and 1991 meters below seafloor in Hole 504B during Legs 137/140, have been altered by hydrothermal fluids as shown by the intense replacement of igneous minerals by secondary amphiboles, chlorites, albite, and anorthite. These rocks display a negative correlation between alteration extent and density. This is interpreted in terms of the development of secondary light minerals and an increase in porosity. For hydrogen, secondary hydroxyl groups (amphibole) dominate over igneous volatiles and control the dD(whole-rock) (-53 per mil to -40 per mil mean = -44 per mil), the D/H fractionation between amphibole (actinolite) and water being about -44 per mil ± 3 per mil at 350°C assuming the fluid has a sea water D/H ratio (dD(fluid) = 0 per mil). The d18O(whole-rock) values, ranging from 4.7 per mil to 2.7 per mil (mean = 4.1 per mil), are negatively correlated with the abundance of secondary amphiboles, chlorites, and albite (alteration percent) determined by point counting. The comparison of halos related to veins or patches with less altered adjacent parts shows that the process of alteration produces an increase in the amphibole content related to a decrease in the d18O(halo/patch). The halos/patches from the deepest samples in Hole 504B have the lowest d18O(whole-rock) values. This feature can be explained by the progressively decreasing proportion of albite with depth in the dike section. According to Taylor's model, the depletion in 18O of these dolerites (relative to fresh magmatic values: d18O(whole-rock) ~5.7 per mil) is explained by seawater-rock interaction in which the seawater/rock ratio is higher than 0.15 in atomic proportion (~0.3 in mass). Consequently, we deduce that large quantities of seawater-derived fluids have circulated deep in the lower part of the sheeted dike complex of Hole 504B. The d18O profile in Hole 504B shows a net disequilibrium in the 18O exchange between seawater and the oceanic crust in favor of enrichment in 18O in the seawater. This can be explained by a particular strong expression of high-temperature hydrothermal interaction with seawater-derived fluids at shallow depths (i.e., in Layer 2).