Stacked Gabbro Units Convert Intervening Mantle to Troctolite in Hole U1309D, IODP Expedition 304/305

IODP Hole U1309D (Exp. 304/305) penetrated 1415 m into the seafloor of the Atlantis Massif, an oceanic core complex at 30N, Mid-Atlantic Ridge. More than 96% of all recovered rocks are gabbroic. Based on petrographic and mineral chemical data, we suggest that between <800 and 1100 meters below seafloor (mbsf) a magmatic unit occurs, ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part. Below 1235 mbsf, massive gabbronorites/oxide gabbros were drilled, which may represent the roof of a separate underlying magmatic unit. The focus of this study is on the zone where these units interact and screens of a microstructurally distinct, 50 m thick olivine-rich troctolite occur. We argue that the olivine-rich troctolite is a former mantle rock which was converted to a crust-mantle transition zone dunite at the base of the upper magmatic unit. Later, as melts derived from the lower magmatic unit percolated through it, it was equilibrated to a more evolved chemistry and transformed to a fine grained, olivine-rich troctolite. Our main arguments against a possible cumulate nature of the olivine-rich troctolite are the lack of any systematic down-hole trend in compatible elements within the olivine-rich troctolite, its distinctly fine-grained microstructure, the high Cr content of cpx, and its Ni-rich olivine composition. The high NiO for a given Mg/(Mg+Fe) in the olivine-rich troctolite can be modeled by simple equilibration of relict mantle olivine with a somewhat evolved melt. Evidence for the percolation of evolved melts through the olivine-rich troctolites are Ti-rich, interstitial pyroxenes, highly evolved amphiboles and orthopyroxenes as inclusions in Cr-spinel plus the occurrence of mm-scale noritic veins. The percolation by evolved melts would also be the major difference to otherwise similar rocks from ophiolitic crust-mantle transition zones.