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doi:10.2204/iodp.proc.309312.201.2008

Introduction

Integrated Ocean Drilling Program (IODP) Hole 1256D (6.736°N, 91.934°W), drilled during Ocean Drilling Program (ODP) Leg 206 and IODP Expedition 309/312 (Wilson et al., 2006) is located on 15 Ma oceanic crust formed at the superfast spreading East Pacific Rise (220 mm/y full spreading rate) (Wilson, 1996) on the Cocos plate in the eastern equatorial Pacific. This site was initially chosen to exploit the inverse relationship between spreading rate and depth to axial low-velocity zones, thought to be magma chambers now frozen as gabbros, observed from seismic experiments (Wilson et al., 2006). Hole 1256D penetrated the entire upper oceanic crust and finally drilled into the uppermost gabbro. Site 1256 is a potential location for a complete penetration of the entire ocean crust in the frame of a “mission,” a new operational tool of IODP (“Mission Moho”) (Christie et al., 2006; Dick et al., 2006). Initial results of drilling at Site 1256, including detailed site maps and information about the geological setting, can be found in the “Expedition 309/312 summary” chapter and Teagle, Wilson, Acton, and Vanko (2007). The first individual scientific results concerning the structure/​composition of the extrusive rocks and details of the hydrothermal alteration within the lavas and dikes are now available (Busigny et al., 2005; Laverne et al., 2006). In this study, 56 samples of the overlying basaltic lava flows and sheeted dikes, drilled during Leg 206 and Expedition 309, were investigated by light microscope and electron microprobe analyses.

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