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doi:10.2204/iodp.sp.335.2010

Introduction

Drilling a complete in situ section of ocean crust has been an unfulfilled ambition of Earth scientists since the inception of ocean drilling. Unfortunately, many of the key questions and primary scientific goals of the IODP Initial Science Plan regarding the formation and evolution of the oceanic crust remain unanswered despite more than 40 years of scientific ocean drilling. This is principally due to cursory sampling of the ocean crust (see Teagle et al., 2004; Wilson, Teagle, Acton, et al., 2003). Fundamental advances in our knowledge of mid-ocean-ridge accretion will be achieved by deepening Hole 1256D a significant distance (i.e., several hundreds of meters) into cumulate gabbro.

Although offset drilling strategies, where deeper parts of the ocean crust are sampled by drilling in tectonic windows, have had notable success at Hess Deep (Gillis, Mével, Allan, et al., 1993), on the Southwest Indian Ridge (Dick, Natland, Miller, et al., 1999), or on the Mid-Atlantic Ridge (Cannat, Karson, Miller, et al., 1995; Kelemen, Kikawa, Miller, et al., 2004; Blackman, Ildefonse, John, Ohara, Miller, MacLeod, et al., 2006), composite sections of ocean crust are not substitutes for long continuous drill holes into intact crust far from fracture zones. Only by drilling intact sections of ocean crust will we be able to estimate the bulk composition of the crust, establish the chemical connections between lavas and melts coming from the mantle, test competing models of lower crustal magmatic accretion, understand the extent and intensity of hydrothermal exchange between the ocean crust and the seawater, calibrate regional seismic measurements, understand the origin of magnetic anomalies, and make estimates of the chemical fluxes returned to the mantle by the subduction of the oceanic lithosphere. Basic observations regarding the architecture of ocean crust, including the rock types; the geochemistry and thicknesses of the volcanic, dike, and plutonic sections; and the nature of the Moho itself, are yet to be made. Sampling through drilling, coupled with detailed geophysical experiments, remain essential to correlate geochemical, seismic, and magnetic imaging of the ocean crust with basic geologic observations.

Although only 20% of modern ridges are moving apart at fast spreading rates (>80 mm/y full rate), ~50% of present-day ocean crust and ~30% of the Earth's surface was produced by this pace of spreading. The great majority of crust subducted back into the mantle during the last 200 m.y. formed at fast spreading ridges. Fast spread ocean crust is believed to be less variable than crust formed at slower spreading rates and closer to the ideal "Penrose" pseudostratigraphy developed from ophiolites (Anonymous, 1972). Hence, understanding accretion processes at a few sites might reasonably be extrapolated to describe a significant portion of the Earth's surface. Importantly, we already have well-developed theoretical models of competing styles of magmatic accretion in fast spreading ridges (e.g., gabbro glaciers versus sheeted sills), and methods have been developed to test these models. However, these hypotheses will be best tested using samples recovered from drilling intact sections of ocean basement.

The "Superfast" campaign has already achieved one of the major unfulfilled goals of ocean drilling—namely, the sampling of a complete section from lavas, through the dikes, and into gabbros. This was accomplished by drilling Hole 1256D in crust that formed at a superfast spreading rate at the East Pacific Rise (EPR) ~15 m.y. ago (Fig. F1). Drilling took place during three scientific ocean drilling cruises: Ocean Drilling Program (ODP) Leg 206 (Wilson, Teagle, Acton, et al., 2003) and IODP Expeditions 309 and 312 (Expedition 309 Scientists, 2005; Expedition 309/312 Scientists, 2006; Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, et al., 2006; Wilson et al., 2006). The base of the hole resides within the dike-gabbro transition zone. In situ cumulate gabbros from intact ocean crust are now within reach of scientific ocean drilling.

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