Short sections of mantle rocks were drilled at Site 895 during Ocean Drilling Program Leg 147 at Hess Deep, where tectonism has exposed lower crust and mantle generated at the equatorial East Pacific Rise. These mantle sections consist of harzburgites and dunites that are impregnated by mafic liquids and crosscut by gabbroic and diabase dikes. The mantle rocks are thought to represent the Moho transition zone. This suite of samples has been extensively altered by interaction with seawater. Ultramafics are largely serpentinized. Gabbroic rocks are strongly hydrated and recrystallized and partly rodingitized. Most recrystallization occurred under static conditions. Secondary assemblages in the gabbroic rocks provide evidence for at least three successive stages of recrystallization. High-temperature amphiboles are scarce and likely represent an early episode that also crystallized secondary clinopyroxene in veins. The major episode of alteration occurred under greenschist facies conditions and is characterized by coronas of serpentine, tremolite, chlorite, and prehnite. The coronas are contemporaneous with the bulk of serpentinization. A later alteration episode produced thomsonite and potassic micas after plagioclase, and late carbonate veins. Comparison of temperatures determined from mineral assemblages with thermal models of the oceanic lithosphere suggests that peak alteration took place at some distance from the ridge axis. Massive penetration of seawater in the mantle is therefore interpreted to result from the rifting of the crust generated at the East Pacific Rise by the Cocos-Nazca propagator.

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

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