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Geologic background

Drilling in Hole U1309D penetrated >1.4 km with >75% recovery, comparable to recovery from the previous most successful penetration into oceanic lithosphere in Ocean Drilling Program (ODP) Leg 118 Hole 735B on the Southwest Indian Ridge. Despite high seismic velocities interpreted to possibly represent residual mantle at relatively shallow depth, the 1.4 km section appears to be wholly igneous crust with little evidence of ductile deformation and brittle deformation confined to a few intervals less than a couple of meters thick.

Major plutonic lithologies recovered are, in decreasing abundance, gabbro, olivine gabbro, troctolite, oxide-bearing gabbro, oxide gabbro, and oxide- and olivine-bearing gabbro. The gabbroic rocks have compositions that are among the most primitive sampled along the MAR, as reflected in bulk Mg numbers ranging from ~67 to 90 (see the “Expedition 304/305 summary” chapter). At least two generations of sulfide mineralization have been identified during shipboard and postcruise reconnaissance examination. Based on optical properties, the primary sulfide mineral assemblage includes at least pyrrhotite, pentlandite, and chalcopyrite, indicative of relatively oxygenated and sulfur-rich conditions. These phases occur as armored inclusions in silicate phases and as intergranular crystals. Secondary sulfides, which occur in veins and as inclusions in alteration phases, include pyrite and tentatively identified tochilinite and awaruite, indicative of highly oxygen and sulfur depleted conditions. In some cases, both primary and secondary assemblages occur in the same thin section, attesting to the complex history of sulfide mineralization in the section.