Coarse-grained lherzolitic peridoties and two-pyroxene gabbros recovered at Deep Sea Drilling Project site 334 beneath a thin layer of fine-grained basalt are primary magmatic cumulates similar to the upper cumulate portion of ophiolite complexes. Mineral chemistry indicates that they are the crystallization product of oceanic tholeiite magma. Long cooling and annealing times indicated by exsolution and equilibration in pyroxenes indicate emplacement near the Mid-Atlantic Ridge and long residence time in oceanic layer 3, prior to tectonic emplacement at shallow crustal levels. Slow depletion of the anorthite component in cumulate plagioclase relative to increase of Fe/Fe+Mg in coexisting mafic phases indicates more efficient fractionation of sodium into residual liquid than implied by fractionation of plagioclase phenocryst found in oceanic magmas. Strong depletion of incompatible minor elements and lack of intercumulate residual phase may indicate the operation of a diffusion mechanism that would allow interaction of pyroxene and magma prior to the crystallization of pyroxene as a liquidus phase.