Petrologic and Stable Isotope Constraints on Hydrothermal Alteration and Serpentinization of the EPR Shallow Mantle at Hess Deep (Site 895)

Abstract

Leg 147 of the Ocean Drilling Program recovered sections of the EastPacific Rise lower crust and shallow mantle (~1 Ma), tectonicallyexposed at the western end of the Cocos-Nazca propagator of the HessDeep Rift Valley. These rocks record a polyphase history of hydrothermalalteration and provide new constraints on the depth and mechanismsof hydrothermal circulation at fast-spreading ridges. A complex sequenceof harzburgite-dunite-troctolite-gabbro recovered at Site 895 isconsidered to be the result of processes of melt migration and wall-rockreaction close to the mantle/crust boundary. The peridotites areextensively serpentinized (50%-100%) and are cut by multiple generationsof fracture-filling veins. In the gabbros, progressive alterationunder greenschist to zeolite facies conditions is characterized bytremolite + chlorite + diopside + anorthite ± prehnite assemblagesin the least altered samples, and incipient rodingitization to prehnite+ hydrogrossular + zeolite + clays as cataclastic deformation andveining increases. Oxygen isotope ratios of mineral separates fromthe gabbros and peridotites from Site 895 show a depletion in 18Orelative to mantle values and are consistent with high-temperatureexchange with aqueous fluids. Dunite/harzburgite ratios of chlorite,serpentine, and tremolite, together with delta13C values of CO2 extractedfrom completely serpentinized dunites, suggest at least two, butpossibly three, components of the hydrothermal fluids: hydrothermallyaltered seawater; magmatic volatiles; and H2 released during serpentinization.These data combined with structural data imply that penetration ofseawater and high-temperature hydrothermal alteration produced alow 18O shallow mantle sequence at some distance off-axis of theEast Pacific Rise, but at an early stage in the propagation of theCocos-Nazca rift and formation of the Hess Deep Rift Valley. Mineralassemblages in the gabbroic rocks and the presence of antigoriteat Hess Deep, combined with oxygen isotope ratios, suggest that faultingassociated with the Cocos-Nazca propagator enhanced seawater penetrationand hydrothermal alteration at temperatures above 350°C in this segmentof the East Pacific Rise oceanic lithosphere. The results of thisstudy suggest that seawater-peridotite interactions and high-temperatureserpentinization processes may be an important contribution to theoverall 18O-budget in the oceanic lithosphere nd may represent asignificant sink for mantle CO2 and source of H2