The accretion of the oceanic crust is one of the major means of heat loss from the Earth's interior and is a fundamental component of the plate tectonic processes responsible for the formation and evolution of our planet's surface. Hydrothermal interactions at mid-ocean spreading centers and on the 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 basalt, dikes, gabbros, and peridotites have been retrieved by dredging and from shallow drill holes from most of the ocean basins, but the geological context of these samples is rarely 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 deep ocean drilling (Shor, 1985), but this has been impeded by technical difficulties and the time investments required (Table T1; Figs. F1, F2). The distribution of drill holes into in situ basement formed at mid-ocean ridges is described in detail in the next section, but the present sparse sampling is instantly striking. Hole 504B, on the southern flank of the Costa Rica Rift, remains our only complete section of in situ upper crust and the only hole to penetrate the 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 has never been established. There are few significant penetrations (>100 m) of crust generated at fast or superfast spreading ridges and, before Leg 206, only one (Hole 1224F) in relatively young ocean crust (<50 Ma) (Table T1). Our poor sampling of ocean crust at different spreading rates and crustal ages and the absence of information on crustal variability compromises our ability to extrapolate observations from specific sites to global descriptions of the magmatic accretion processes and hydrothermal exchange in the ocean crust.
Deep drilling is perhaps the only approach able to address many fundamental questions about the formation, composition, and evolution of the oceanic lithosphere. Unfortunately, deep drilling of in situ ocean basement or the sampling of deeper crustal rocks in tectonic windows (e.g., Hess Deep) remains poor. As such, many of the primary questions that first motivated the inception of scientific ocean drilling remain unanswered.
Table T1 is a compilation of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drill holes that have sampled more than 50 m of in situ ocean basement cored from the top of the lava sequences. We have not included drill holes into plutonic rocks, such as the highly successful drilling in Hole 735B on the Southwest Indian Ridge, or shallow penetrations at Hess Deep or the Kane Fracture Zone. Holes selected in this compilation were formed at normal, bare-rock, ocean spreading ridges away from the influence of hotspots and have approximately normal mid-ocean-ridge (N-MORB) chemistry. Holes into arc crust (e.g., Holes 786B and 834B) have not been included, although arcs are another poorly sampled crustal domain.
Figure F1 shows the global distribution of deep drill holes into ocean basement. With a few exceptions most effort in basement drilling has been concentrated on the northern Mid-Atlantic Ridge and in the eastern Pacific Basin. There are only three holes south of the equator in the Pacific Ocean (Holes 319A, 595B, and 597C) and only a handful of penetrations into Indian Ocean crust. There are no holes into abyssal basement in the South Atlantic. Even though the Pacific Ocean is the largest ocean basin with the longest record of ocean spreading (0–180 Ma), there are only eight holes deeper than 50 m in crust older than 15 Ma, with Hole 801C being the only one drilled deeper than 400 meters subbasement (msb).
The penetration and age distribution of drill holes at all spreading rates and from all oceans is shown in Figure F2. It is clear from this figure that there is only one truly deep hole (>600 msb) into in situ crust. Hole 504B on south flank of the intermediate–spreading rate Costa Rica Rift is the only hole that samples a complete section of extrusive lavas and penetrates into the sheeted dikes. Samples of the sheeted dike complex or the boundary between lavas and dikes have not been recovered by drilling from either slow- or fast-spreading crust. Although Hole 504B is the only hole where this important transition has been recovered, it remains difficult to evaluate how representative the recovered sequence is relative to ocean crust overall. Presently, there are no samples of the uppermost gabbros and the thermal and chemical nature of the boundary between dikes and gabbros remains poorly known.
Seawater interaction with basalt continues for many millions of years on the ridge flanks, and a departure from the theoretical heat flow is discernible on average out to ~65 Ma (Stein and Stein, 1994), indicating the hydrothermal advection of heat. In order to evaluate the role that seawater-basement interactions play in buffering the chemistry of the oceans as well as the upper mantle, it is important to quantify the processes and extent of chemical and isotopic exchange with crustal age (e.g., Alt and Teagle, 1999). As can be seen in Figure F2, in addition to Hole 504B, there are a number of moderate-depth holes (600 msb) drilled into young crust (<20 Ma). There are also a number of moderately deep samples of relatively old ocean crust, in particular the sampling of Cretaceous crust in the western Atlantic at Sites 417/418 and the recently deepened Hole 801C in Jurassic crust (~170 Ma) in the far western Pacific. However, there are very few samples of even the uppermost ocean crust and no holes of moderate depth into ocean crust in the age range of 20–110 Ma, despite this being the critical age range. The average age of the ocean crust is ~61 Ma (Pollack et al., 1993), and the average age of oceanic basement presently being subducted is ~77 Ma. Knowledge of the extent of chemical and isotopic exchange in the time interval between 40 and 90 Ma is essential to constrain global hydrothermal budgets and the geochemistry of the upper mantle, but our present sampling of the ocean crust is woefully inadequate to address these issues.
The crisis in our sampling of the oceanic lithosphere is even more acute when the current suite of drill cores is subdivided by spreading rates or ocean basin (Figs. F3, F4). Clearly, an extensive drilling program into in situ crust is required to thoroughly characterize the oceanic basement at a range of spreading rates and in all oceans. Such information is imperative to select a suitable site for the realization of the long-term goal of achieving a full penetration of ocean crust before the end of the first decade of the Integrated Ocean Drilling Program.
The transition from sheeted dikes to gabbros has never been drilled, and this remains an important objective in achieving a complete or even composite crustal section by either offset or deep drilling strategies. The dike–gabbro transition and the uppermost plutonic rocks are the frozen axial melt lens and the fossil thermal boundary layer between magma chambers and vigorous hydrothermal circulation (Fig. F5). Detailed knowledge of this transition zone is critical to our understanding of the mechanisms of crustal accretion and hydrothermal cooling of the ocean crust. The uppermost gabbros and the overlying sheeted dikes and extrusive lavas provide a time-integrated record of the processes of hydrothermal exchange and the associated fluid and chemical fluxes. The geochemistry of the frozen melt lens when compared with the overlying dikes, lavas, and, if possible, lower crustal cumulate rocks, will place important controls on plutonic accretion processes. Drilling this interval will constrain magma chamber geometry and provide a geological context to geophysical observations of low-velocity zones.
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; Ocean Drilling Program, 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, Kane Fracture Zone [MARK], and Southwest Indian Ridge). However, serious technical problems still exist with drilling tectonized rocks with little sediment blanket or without erosional removal of fractured material. It is also often difficult to relate drilled sections to the regional geology. Furthermore, at sites that are tectonized at very young ages, doubts will remain as to whether the same factors that cause tectonic exposures also perturb the ridge axis from the normal state. Composite sections are not substitutes for deep in situ penetrations, and the drilling of deep holes to obtain complete upper crustal sections continues to be a primary challenge for scientific ocean drilling (Dick and Mével, 1996; Ocean Drilling Program, 1996; Murray et al., 2002).
Even some basic observations regarding the architecture of ocean crust, including the lithology, geochemistry, and thicknesses of the volcanic and sheeted dike sections and the nature of the transition from dikes to gabbros, have yet to be made. Further progress is still required in order to correlate and calibrate geochemical, seismic, and magnetic imaging of the structure of the crust with basic geological observations. For example, how do velocity changes within seismic Layer 2 and the Layer 2–Layer 3 transition relate to the physical, structural, or alteration variations in the volcanics and dikes and to the dike–gabbro transition? At Site 504 in crust generated at an intermediate-rate spreading ridge, the Layer 2–Layer 3 transition lies within the 1-km-thick sheeted dike complex and coincides with a metamorphic change (Detrick et al., 1994; Alt et al., 1996), but is this representative of ocean crust and for crust generated at different spreading rates? Is the depth to gabbros shallower in crust generated at a superfast spreading rate, as predicted, and which of the volcanic or dike sections is thinner compared with crust constructed at slow or intermediate spreading rates? Francheteau et al. (1992) estimated a lava thickness of ~200 m at Hess Deep vs. >500 m at Site 504 and in the Atlantic; measurements of the thickness of seismic Layer 2A suggest 500–600 m for the East Pacific Rise (e.g., Kent et al., 1994).
A second objective is to understand the interactions between magmatic and alteration processes, including the relationships between extrusive volcanics, the feeder sheeted dikes, and the underlying gabbroic rocks from the melt lens and subjacent sills/intrusions. Little information presently exists on the heterogeneity of hydrothermal alteration in the upper crust or the variability of the associated thermal, fluid, and chemical fluxes. How these phenomena vary at similar and different spreading rates is poorly known. Metamorphic assemblages and analyses of secondary minerals in material recovered by deep drilling can provide limits on the amount of heat removed by hydrothemal systems and place important constraints on the geometry of magmatic accretion and the thermal history of both the upper and lower crust. Fluid flow paths, the extent of alteration, and the nature of the subsurface reaction and mixing zones are all critical components of our understanding of hydrothermal processes that can only be addressed through drilling. These questions can be answered by examining the "stratigraphy" and relative chronology of alteration within the extrusive lavas and dikes; by determining whether disseminated sulfide mineralization, fluid mixing, and a large step in thermal conditions is present at the volcanic–dike transition (as in Hole 504B and many ophiolites); and from the grade and intensity of alteration in the lower dikes and upper gabbros. The lowermost dikes and upper gabbros have been identified as both the conductive boundary layer between the magma chambers and the axial high-temperature hydrothermal systems and the subsurface reaction zone where downwelling fluids acquire black-smoker chemistries (Alt et al., 1996; Vanko and Laverne, 1998). However, extensive regions of this style of alteration or zones of focused discharge have yet to be recognized and information from ophiolites may not be appropriate to in situ ocean crust (Richardson et al., 1987; Schiffman and Smith, 1988; Bickle and Teagle, 1992; Gillis and Roberts, 1999). Drilling down to the boundary between the lower dikes and upper gabbros will allow tracing of fluid compositions (e.g., Teagle et al., 1998) and the integration of the thermal requirements of hydrothermal alteration in sheeted dikes and underlying gabbros with the magmatic processes in the melt lens.
Marine magnetic anomalies are one of the key observations that led to the development of plate tectonic theory, through the recognition that the ocean crust records the changing polarity of the Earth's magnetic field through time (Vine and Matthews, 1963). It is generally assumed that micrometer-sized grains of titanomagnetite within the erupted basalt are the principal recorders of the marine magnetic anomalies, but recent studies of tectonically exhumed lower crustal rocks and serpentinized upper mantle indicate that these deeper rocks may also be a significant source of the magnetic stripes. Whether these deeper rocks have a significant influence on the magnetic field in undisrupted crust is unknown, as is the extent of secondary magnetite growth in gabbros and mantle assemblages away from transform faults. Sampling the plutonic layers of the crust could refine the Vine-Matthews hypothesis by characterizing the magnetic properties of gabbros and peridotites through drilling normal ocean crust, on a well defined magnetic stripe, away from transform faults.
Although there are several questions that can be answered well with shallow holes in tectonic windows such as Hess Deep, where deep and mid-crustal rocks crop out, other questions on topics from in situ permeability to alteration history require sampling of intact normal oceanic crust. A suite of drill holes into in situ basement from a range of spreading rates and ages is required to thoroughly understand and characterize the variability of the processes occurring during the accretion and evolution of the ocean crust.