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This report provides the results of geochemical analyses of basalt and gabbro shipboard samples recovered during Integrated Ocean Drilling Program (IODP) Expedition 312 from Ocean Drilling Program (ODP) Hole 1256D. Hole 1256D is in the oceanic crust of the Cocos plate, which was formed by superfast spreading at the East Pacific Rise (220 mm/y full spreading rate) at 15 Ma. Hole 1256D was deepened to 1507.1 meters below seafloor (mbsf) during three combined expeditions: ODP Leg 206 and IODP Expeditions 309 and 312. These expeditions succeeded in collecting gabbro samples from an upper oceanic crust sequence composed of deep-sea sediment, pillow and lava flows, and sheeted dikes.

During Expedition 312, drilling was started in the sheeted dikes at 1255 mbsf. The lowermost 60 m was partially or completely recrystallized. Because these sheeted dikes are characterized by distinctive granoblastic textures with secondary mineral assemblages of plagioclase, clinopyroxene, orthopyroxene, amphibole, and Fe-Ti oxide, they were classified as the “granoblastic dikes” (Wilson et al., 2006). At 1406.6 mbsf, drilling in Hole 1256D encountered a felsic plutonic rock intruded into the granoblastic dikes. This depth is regarded as the beginning of the “plutonic complex” (Wilson et al., 2006). The plutonic complex includes two gabbroic intervals (52.3 m thick upper Gabbro 1 and 24 m thick lower Gabbro 2) separated by the 24.2 m thick upper dike screen (see “Igneous petrology” in the “Site 1256” chapter). The lower portion of Gabbro 2 consists of a 12.1 m thick gabbronorite of uncertain origin. We hereafter refer to this gabbronorite unit as the lower dike screen, which is assigned to metamorphosed basalt dike screen in the “Site 1256” chapter.

A data set of geochemical analysis of the sheeted dikes–gabbro transition in Hole 1256D gives basic information for understanding the magmatic, metamorphic, and hydrothermal systems beneath the superfast spreading ridge.