Tectonic exposures of a volcanic sequence and sheeted dike complex over a 4-km-wide region at Hess Deep (equatorial Pacific) reveal significant spatial heterogeneity (10--103 m) in the extent and nature of hydrothermal alteration in young, fast spreading East Pacific Rise crust. The volcanic sequence is fairly uniformly altered, with only minor oxidation and alteration to clay minerals. Sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60¿. Their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole, clinopyroxene replacement increases (40%), whole rock Δ18O values decreases (4.4--5.5? to 3.5--4.5?), and calculated peak metamorphic temperatures increase (~250 ¿C to 450 ¿C--700 ¿C). Within the deepest dikes, localized zones up to 400-m-wide are chlorite-rich and have low-Δ18O (2.9--4.1?) and low peak metamorphic temperatures (~345 ¿C). These alteration patterns likely formed within broad recharge zones whereby the low-Δ18O zones developed in the regions with the highest fluid flux. In the west, massive, slightly rotated sheeted dikes near the volcanic-sheeted dike transition are Δ18O and Cu depleted and display higher peak temperatures (≥345 ¿C) than elsewhere in the shallow dikes. These characteristics are consistent with formation within a high temperature, hydrothermal discharge zone. We propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes, similar to those preserved in many ophiolites. ¿ 2001 American Geophysical Union