Ultramafic rocks are a major component of the oceanic lithosphere and are commonly exposed near and along slow-and ultra-spreading ridges and in other tectonically active environments. The serpentinization of mantle material is a fundamental process that has significant geophysical, geochemical and biological importance for the global marine system and for subduction zone environments. Mineral assemblages and textures are typically complex and reflect multiple phases ofalteration, deformation and veining during emplacement, hydrothermal alteration, and weathering. In this paper, we review mineralogical and geochemical consequences of serpentinization processes in oceanic upper mantle sequences in different tectonic environments and discuss the relationship between serpentinization and fluid chemistry. We present phase equilibria that provide models for interpreting mineral-fluid relationships in oceanic serpentinitesand allow the simultaneous evaluation ofthe conditions for redox, hydration and carbonation processes. These models predict that serpentinization reactions are sensitive to Si content of ultramafic rocks and that serpentine phases have an upper stability limit of~450°C, where H 2 0-rich fluids will be dominant. More pervasive serpentinization commences with olivine breakdown reactions below~425°C and leads to progressively more reduced fluids with decreasing temperature. Our calculations indicate that carbonates may have extensive stability fields in CH 4 -rich fluids in Si-deficient systems and that they may be significant in generating reducing conditions. Ifmethane formation driven by serpentinization is common, its contribution to the carbon cycle in submarine biogeochemical systems may be substantial. Serpentinization may thus be an important process in sustaining diverse microbial communities in subsurface and near-vent environments and has consequences for the existence of a deep biosphere.

Leg 147 of the Ocean Drilling Program recovered sections of the East Pacific Rise lower crust and shallow mantle ( l 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 + anorthjte ± 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 I8O relative to mantle values and are consistent with high-temperature exchange with aqueous fluids. Dunite/harzburgite ratios of chlorite, serpentine, and tremolite, together with δ' 3 C values of CO 2 extracted from completely serpentinized dunites, suggest at least two, but possibly three, components of the hydrothermal fluids: hydrothermally altered seawater; magmatic volatiles; and H 2 released during serpentinization. These data combined with structural data imply that penetration of seawater and high-temperature hydrothermal alteration produced a low 18 O 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 seawaterperidotite interactions and high-temperature serpentinization processes may be an important contribution to the overall 18 Obudget in the oceanic lithosphere nd may represent a significant sink for mantle CO 2 and source of H 2 .

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Leg 147 of the Ocean Drilling Program recovered sections of the East Pacific Rise lower crust and shallow mantle ( l 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 (δ l8 θ between 3.0%o and 6.3%o); and a group with δ 18 θ between 8.3%o 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 (δD O‰) and a mixed magmatic-derived/altered seawater component (δD -30%c to -20‰). These data, combined with microstructural data, suggest that penetration of seawater at high temperatures (>500°C), possibly mixing with CO 2 -rich magmatic volatiles, resulted in a low 18 O 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.

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