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 at the Hess Deep Rift Valley. At Site 894, variably metamorphosed, isotropic gabbros and gabbronorites from the upper part of the plutonic section were recovered. In this study, we present petrologic and stable isotope data that document a complex polyphase history of fluid infiltration, metamorphism, and deformation from late magmatic activity through upper amphibolite facies to zeolite facies conditions. Alteration occurred through several stages of fracturing and fluid infiltration during progressive transport of the oceanic crust away from the axis of the East Pacific Rise and ultimate intersection with the Cocos-Nazca propagator. Alteration was controlled by fracture permeabilities, grain geometries, and the chemical composition of progressive pulses of hydrothermal fluids. Early, near-axis high temperature (>500°C) fluid infiltration in microveins and along grain boundaries produced amphibolite-facies mineral assemblages. Subsequent off-axis cooling and Cocos-Nazca-related uplift and faulting enhanced fluid penetration, resulting in variable overprinting under greenschist to zeolite facies conditions in association with discrete veins and local cataclastic shear zones.
Oxygen isotope ratios of amphibole separates are depleted in 18O relative to unaltered gabbro compositions and are consistent with high temperature exchange with aqueous fluids. Plagioclase compositions form two groups: a high temperature altered group (delta18O between 3.0‰ and 6.3‰); and a group with delta18O between 8.3‰ and 10.2‰, indicative of local lower temperature (150°–250°C) overprinting associated with discrete veining. D/H ratios of amphibole suggest two components in the early hydrothermal fluids: an altered seawater component (deltaD = ~0‰) and a mixed magmatic-derived/altered seawater component (deltaD -30‰ to -20‰). These data, combined with microstructural data, suggest that penetration of seawater at high temperatures (>500°C), possibly mixing with CO2-rich magmatic volatiles, resulted in a low 18O sequence of upper-amphibolite facies oceanic lithosphere at an early stage in the spreading history of the EPR. Fluid mixing at magma chamber/oceanic crust boundaries and fracture-controlled, high-temperature metamorphism may be characteristic of alteration processes at this fast-spreading ridge environment.
Date of initial receipt: 3 August 1994
Date of acceptance: 27 April 1995
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