Leg 147 of the Ocean Drilling Program recovered sections of the East Pacific Rise lower crust and shallow mantle (~1 Ma), tectonically exposed at the western end of the Cocos-Nazca propagator of the Hess Deep Rift Valley. These rocks record a polyphase history of hydrothermal alteration and provide new constraints on the depth and mechanisms of hydrothermal circulation at fast-spreading ridges. A complex sequence of harzburgite-dunite-troctolite-gabbro recovered at Site 895 is considered to be the result of processes of melt migration and wall-rock reaction close to the mantle/crust boundary. The peridotites are extensively serpentinized (50%-100%) and are cut by multiple generations of fracture-filling veins. In the gabbros, progressive alteration under greenschist to zeolite facies conditions is characterized by tremolite + chlorite + diopside + anorthite ± prehnite assemblages in the least altered samples, and incipient rodingitization to prehnite + hydrogrossular + zeolite + clays as cataclastic deformation and veining increases.

Oxygen isotope ratios of mineral separates from the gabbros and peridotites from Site 895 show a depletion in ¹⁸O relative to mantle values and are consistent with high-temperature exchange with aqueous fluids. Dunite/harzburgite ratios of chlorite, serpentine, and tremolite, together with delta¹³C values of CO₂ extracted from completely serpentinized dunites, suggest at least two, but possibly three, components of the hydrothermal fluids: hydrothermally altered seawater; magmatic volatiles; and H₂ released during serpentinization. These data combined with structural data imply that penetration of seawater and high-temperature hydrothermal alteration produced a low ¹⁸O shallow mantle sequence at some distance off-axis of the East Pacific Rise, but at an early stage in the propagation of the Cocos-Nazca rift and formation of the Hess Deep Rift Valley. Mineral assemblages in the gabbroic rocks and the presence of antigorite at Hess Deep, combined with oxygen isotope ratios, suggest that faulting associated with the Cocos-Nazca propagator enhanced seawater penetration and hydrothermal alteration at temperatures above 350°C in this segment of the East Pacific Rise oceanic lithosphere. The results of this study suggest that seawater-peridotite interactions and high-temperature serpentinization processes may be an important contribution to the overall ¹⁸O-budget in the oceanic lithosphere nd may represent a significant sink for mantle CO₂ and source of H₂.

Date of initial receipt: 3 August 1994
Date of acceptance: 27 April 1995

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