Cr-Rich Spinel as a Tracer for Melt Migration and Melt—Wall Rock Interaction in the Mantle: Hess Deep, Leg 147

Abstract

Site 895 of the Ocean Drilling Program Leg 147 cored melt-impregnated peridotite from the southeast flank of the Hess Deep intrarift high, recovering generalized stratigraphic sequences of harzburgite-dunite-troctolite-gabbro-troctolite-dunite- harzburgite, with the gabbros representing fossil melt conduits through preexisting harzburgite. Cr-rich spinel within the Site 895 samples reflects and records the effects of these multiple injections of melt into depleted, residual harzburgites of the East Pacific Rise uppermost mantle. Gabbroic spinel compositions vary widely (TiO2, 0.55%-2.56%; Mg/[Mg + Fe2+], 0.30-0.60), showing that the impregnating melts became locally quite evolved, with variation in melt composition controlled by both crys- tal fractionation and local melt-rock interaction. TiO2 contents in associated dunitic spinel are 10-20 times as high as in harzburgitic spinels (0.5%—1.1% vs. <0.07%, respectively), suggesting that the dunite was produced by melt-wall rock interac- tion associated with melt impregnation. This process produced halos of dunite around residual bodies of gabbroic rock and pos- sible residual dunite "trails" of melt migration through peridotite. Spinel and other phase compositional data are consistent with the injection of depleted, exotic N-MORB melt by both diking and intergranular flow at depths shallower than 10 km and tem- peratures well above 900°C, and most likely approaching about 1230°C. High ZnO (to 0.75%) in the metamorphically altered rims of spinel in gabbro and troctolite indicates that Zn-rich hydrothermal fluids may have been present during post-crystalliza- tion, lower amphibolite to zeolite facies metamorphic alteration of the Hess Deep peridotites, with the gabbroic melt impregna- tions providing possible preferential conduits for fluid flow.