Magma plumbing beneath the Mid-Atlantic Ridge

A suite of fresh, ∼0.75 Ma old, gabbroic samples from ODP Hole 923A on the Mid-Atlantic ridge just south of the Kane Fracture Zone, 23°N, have been studied texturally and by electron probe and ion probe. This hole penetrated ∼70 mbsf recovering 40 m of core from the basal 55 m of the hole. The core is dominated by two interlayed lithologies: poikilitic olivine gabbros and brown-pyroxene gabbros, plus volumetrically minor microgabbros, oxide gabbros and leucocratic veins.;Poikilitic olivine gabbros are plagioclase +/- olivine cumulates with relatively primitive mineral composition. Horizons of poikilitic olivine gabbros in the core represent magma chamber replenishment in which the replenishing magma ponded at the base of the chamber and fractionated. Variations in plagioclase trace element compositions (e.g. La/Nd and Ba/K) in part reflect compositional variation in their parental melts derived from the mantle. Calculated equilibrium melt compositions are highly LREE depleted (Lan/Ndn as low as 0.25).;Brown-pyroxene gabbros are olivine - plagioclase - clinopyroxene cumulates which have more evolved mineral compositions than poikilitic olivine gabbros. A series of major and trace element characteristics of the rocks, and textural phenomena, indicate that post-cumulus processes were important in their formation. In particular, extreme variations in incompatible element abundances, and fractionation of Zr and Y (and REE's), from the core to rim of clinopyroxene crystals occurs. It is proposed that the fractionation of the interstitial melt was caused by reaction between migration interstitial melt and the crystal assemblage (magmatic metasomatism). This led to the interstitial melt becoming highly evolved in terms of incompatible elements whilst the major elements were efficiently buffered by the crystals..