Fluid flow patterns in fast spreading East Pacific Rise crust exposed at Hess Deep

Tectonic exposures of a volcanic sequence and sheeted dike complex over a 4 -km-wide region at Hess Deep (equatorial Pacific) reveal significant spatia l heterogeneity (10-10(3) m) in the extent and nature of hydrothermal alter ation in young, fast spreading East Pacific Rise crust. The volcanic sequen ce is fairly uniformly altered, with only minor oxidation and alteration to clay minerals. Sheeted dikes in the eastern part of the field area are hig hly fractured with narrow intervals of intact dikes that dip up to 60 degre es. Their alteration characteristics show a simple depth trend such that wi th increasing depth the dominant secondary mafic mineral changes from chlor ite to amphibole, clinopyroxene replacement increases (< 20% to > 40%), who le rock delta O-18 values decreases (4.4-5.5 parts per thousand to 3.5-4.5 parts per thousand), and calculated peak metamorphic temperatures increase (similar to 250 degreesC to 450 degrees -700 degreesC). Within the deepest dikes, localized zones up to 400-m-wide are chlorite-rich and have low-delt a O-18 (2.9-4.1 parts per thousand) and low peak metamorphic temperatures ( similar to 345 degreesC). These alteration patterns likely formed within br oad recharge zones whereby the low-delta O-18 zones developed in the region s with the highest fluid flux. In the west, massive, slightly rotated sheet ed dikes near the volcanic-sheeted dike transition are delta O-18 and Cu de pleted and display higher peak temperatures (greater than or equal to 345 d egreesC) than elsewhere in the shallow dikes. These characteristics are con sistent with formation within a high temperature, hydrothermal discharge zo ne. 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.