Hydrothermal Alteration of the Upper-Mantle Section at Hess Deep

Abstract

Short sections of mantle rocks were drilled at Site 895 during OceanDrilling Program Leg 147 at Hess Deep, where tectonism has exposedlower crust and mantle generated at the equatorial East Pacific Rise.These mantle sections consist of harzburgites and dunites that areimpregnated by mafic liquids and crosscut by gabbroic and diabasedikes. The mantle rocks are thought to represent the Moho transitionzone. This suite of samples has been extensively altered by interactionwith seawater. Ultramafics are largely serpentinized. Gabbroic rocksare strongly hydrated and recrystallized and partly rodingitized.Most recrystallization occurred under static conditions. Secondaryassemblages in the gabbroic rocks provide evidence for at least threesuccessive stages of recrystallization. High-temperature amphibolesare scarce and likely represent an early episode that also crystallizedsecondary clinopyroxene in veins. The major episode of alterationoccurred under greenschist facies conditions and is characterizedby coronas of serpentine, tremolite, chlorite, and prehnite. Thecoronas are contemporaneous with the bulk of serpentinization. Alater alteration episode produced thomsonite and potassic micas afterplagioclase, and late carbonate veins. Comparison of temperaturesdetermined from mineral assemblages with thermal models of the oceaniclithosphere suggests that peak alteration took place at some distancefrom the ridge axis. Massive penetration of seawater in the mantleis therefore interpreted to result from the rifting of the crustgenerated at the East Pacific Rise by the Cocos-Nazca propagator