Peridotites and related mafic plutonic rocks from Hess Deep in the equatorial Pacific (ODP Leg 147 Site 895) were examined in order to understand deep magmatic processes prevailing beneath the East Pacific Rise, a fast-spreading ridge system. Mantle harzburgite from Hess Deep is transitional to gabbro via dunite, troctolite, and olivine gabbro, in terms of petrography. The dunite-troctolite-olivine gabbro association can be interpreted as an interaction product between injected and stagnated melt and the harzburgite wall in the shallowest mantle. Troctolite and olivine gabbro are highly heterogeneous, for both their relative mineral ratio and mineral chemistry. Olivine-rich (i.e., plagioclase-poor) troctolite has the most Ti-enriched mineralogy, whereas the plagioclase-poor dunite associated with harzburgite has low-Ti mineralogy; the mineral chemical change possibly depends on the degree of melt stagnation. Chromian spinel is concentrated in some dunite and troctolite; a small chromitite pod was discovered. Primary hydrous minerals, K- and Na-phlogopites and pargasite, are usually present as inclusions in spinel from troctolite and chromitite, as commonly found in spinels from chromitites of both alpine and layered types. This indicates that the primary hydrous minerals can form in the oceanic mantle without slab-derived fluids.

The rocks examined are similar to constituents of the transition zone of some ophiolites, especially the Oman ophiolite, except for the podiform chromitites. The podiform chromitites of the ophiolites usually have high-Cr# (Cr/[Cr + Al] atomic ratio, 0.7 to 0.8) spinel, which has not been reported from ocean-floor rocks. The podiform chromitites could have formed in oceanic mantle conditions, but the typical podiform chromitites in ophiolites could not have been derived from the oceanic mantle.

Melt-mantle interaction possibly occurs in the mantle/crust boundary beneath the mid-ocean ridge axis, especially beneath a fast-spreading ridge axis with a high geothermal gradient. The interaction could form a spinel-olivine (Cr#-Fo) compositional trend similar to that for relatively primitive MORB. The interaction between the high-pressure MORB and the wall harzburgite at the shallow upper mantle may contribute to the evolution of the MORB erupted on the ocean floor.

Date of initial receipt: 2 August 1994
Date of acceptance: 5 January 1995

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