The close association of harzburgite, dunite, troctolites, olivine gabbro, and gabbro in the core recovered from the mantle exposed at Hess Deep suggests that these rocks belong to a suite of tectonized residual mantle peridotites through which a mafic melt was transported. Textures of this suite of rocks are interpreted to result from complex interactions between various igneous processes, including partial melting, multiple episodes of melt injection, wall-rock interactions and olivine crystallization. The undeformed nature of intergranular plagioclase, pyroxene and spinel in dunite containing deformed olivine grains suggests that crystallization of an interstitial mafic melt post-dates the high temperature deformation of olivine. Dislocation densities in olivine grains of troctolites are higher than those of olivine within harzburgites and dunites; thus the introduction of dislocations in the troctolites appears to correspond to a localized late-stage higher stress deformation that did not affect the harzburgites. Dislocation geometries and identification of the [100] Burgers vector for dislocations in olivine grains from the troctolites suggest that the introduction of dislocations occurred at relatively high temperatures; dislocations with the [100] Burgers vector are not activated under experimental conditions in olivine at temperatures below 800°C. We suggest that the introduction of mafic melt within the dunites was responsible for the local high-stress event recorded by olivine grains, in contrast to the low stress deformation that may be expected for slow melt migration by percolation through porous dunites. Two rapid mechanisms of melt impregnation may have been responsible for this late-stage deformation: (1) the rapid introduction of melt by propagation of dikes in which the wall rock deforms in a visco-elastic manner, rather than by purely brittle processes, and (2) the rapid introduction of melt within dissolution channels. Crosscutting relationships of gabbros indicate that melt injection was episodic; this implies that troctolites continued to serve as pathways for melt which was extracted from below. The olivine gabbros possess two foliations: an early formed foliation may have resulted from the flow of partial melt while the latter foliation resulted from cataclastic deformation. The cataclastic deformation observed in the gabbros is probably due to rifting of the East Pacific Rise (EPR) lithosphere associated with the Cocos-Nazca propagator. Brittle deformation during rifting is highly localized within the mantle gabbros, suggesting that the gabbros were mechanically weaker than the surrounding peridotites.

Date of initial receipt: 1 August 1994
Date of acceptance: 27 April 1995

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