INTRODUCTION

Accretion of oceanic crust is a major means of heat loss from the Earth's interior and is a fundamental component of the plate tectonic processes responsible for formation and evolution of our planet's surface. Hydrothermal interactions at mid-ocean spreading centers and on ridge flanks influence the chemistry of the oceans and, through subduction, the composition of the upper mantle. Despite the role the ocean crust has played in the evolution of our planet, our sampling of in situ oceanic basement remains rudimentary. Samples of basalts, dikes, gabbros, and peridotites have been retrieved by dredging and shallow drill holes from most of the ocean basins, but the geological context of these samples is rarely well established. As such, the nature and variability of the composition and structure of the ocean crust away from transform faults and other tectonic windows remains poorly known.

Drilling a complete crustal section has always been a major goal of ocean drilling (Bascom, 1961; Shor, 1985), but this goal has been impeded by technical difficulties and the time investments required. The distribution of drill holes in intact oceanic crust of different ages and formed at different spreading rates is extremely sparse (Fig. F1). Hole 504B, on the southern flank of the Costa Rica Rift, remains the only hole to penetrate extrusive lavas and most of the way through the sheeted dike complex. The dike/gabbro boundary has never been drilled, and the nature of the plutonic rocks directly underlying the sheeted dike complex is not well established, despite this zone being perhaps the most influential in determining the mechanisms of crustal accretion and the geometry of magmatic and hydrothermal interactions. Importantly, there are few significant penetrations (>100 m) of crust generated at a fast or superfast spreading ridge and, before Leg 206, only one (Hole 1224F) in relatively young ocean crust (<50 Ma) formed at a fast spreading rate. Our poor sampling of ocean crust at different spreading rates and crustal ages and absence of information on crustal variability compromises our ability to extrapolate observations from specific sites to global descriptions of magmatic accretion processes and hydrothermal exchange in the ocean crust.

Oceanic crust formation and evolution is one of the primary themes for investigation in the Initial Science Plan for the Integrated Ocean Drilling Program (International Working Group, 2001) and other major science priority submissions (e.g., Conference on Multiple Platform Exploration of the Ocean [COMPLEX]: Pisias and Delaney, 1999; Ocean Drilling Program [ODP] Geochemistry Futures Workshop: Murray et al., 2002). These documents and others specifically related to the study of the oceanic lithosphere (Second Conference on Scientific Ocean Drilling [COSOD II]; ODP Long Range Plan, 1996; ODP–International Cooperation in Ridge-Crest Studies [InterRIDGE]–International Association of Volcanology and Chemistry of the Earth's Interior [IAVCEI] workshop; 4D-Architecture of the Ocean Crust Program Planning Group) reemphasize deep drilling to obtain complete sections of the ocean crust as a priority and note that the deep drilling capabilities of riserless technology have yet to be fully utilized. Offset drilling strategies, where deeper portions of the ocean crust are sampled by drilling in tectonic windows, have recently been high priorities for ocean drilling (COSOD II, 1987; ODP Long Range Plan, 1996). Drilling at several sites has provided a wealth of new data and understanding of gabbros and peridotites from the lower crust and upper mantle (e.g., Hess Deep: Gillis, Mével, Allan, et al., 1993; Kane Fracture Zone area [MARK]: Cannat, Karson, Miller, et al., 1995; Southwest Indian Ridge [SWIR]: Dick, Natland, Miller, et al., 1999; 14°–16°N Mid-Atlantic Ridge [MAR]: Kelemen, Kikawa, Miller, et al., 2004; Atlantis Massif: Expedition Scientific Party, 2005a, 2005b). However, serious problems still exist in drilling tectonized rocks with little sediment blanket or without erosional removal of fractured material, and it is also commonly difficult to relate drilled sections to regional geology. Composite sections are not substitutes for deep, in situ penetrations, and drilling deep holes to obtain complete upper crustal sections continues to be a primary challenge for scientific ocean drilling (Dick and Mevel, 1996; Murray et al., 2002).

Unfortunately, there are no on-land alternatives to drilling in the oceans. Although ophiolites, ancient slices of ocean crust now preserved on land, provided much of the early inspiration for ocean crust studies, the classic outcrops of Semail ophiolite (Oman) and Troodos massif (Cyprus) formed in suprasubduction zone settings and their different magma and volatile chemistries compromise their applicability to understanding processes in the major ocean basins. Macquarie Island (Varne et al., 2000), uplifted along the Australian/Pacific plate boundary ~1000 km south of New Zealand, may be the only outcrop of subaerially exposed ocean crust formed at a mid-ocean ridge, but the island is complexly faulted and is an environmentally sensitive United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage site from which drilling, even for scientific purposes, is prohibited.

IODP Expeditions 309 and 312 will deepen ODP Hole 1256D (Fig. F2, F3), which already extends 502 m into basement, to core through the sheeted dike complex and into plutonic rocks in a section of crust generated at a superfast spreading rate in the eastern Pacific to