From the earliest studies, analyses of seismic refraction profiles have suggested that the structure of the oceanic crust is surprisingly uncomplicated and uniform (Christensen and Salisbury, 1975; Hill, 1957; Raitt, 1963). Earth scientists have long equated the seismic structure to a simple layer cake sequence of sediment, pillow basalt and diabase, and a thick gabbro section overlying the earth's mantle, with the igneous crust/mantle boundary corresponding to the Mohorovicic seismic discontinuity (MOHO). Accretion of the lower crust was seen as a result of crystallization from some form of near steady-state magma chamber or crystal mush zone where magmas accumulated beneath a sheeted dike/pillow lava sequence over the ascending mantle. It was believed that a simple stratigraphy of primitive layered gabbros was overlain by more evolved isotropic gabbros‹all of which were equated to seismic layer 3. This simple hypothesis has been modified somewhat over the past 20 years. Provisions have been made for thinner crust areas near large transforms (e.g., Dick, 1989; Mutter and Detrick, 1984) and for areas with spreading rates that are significantly below 10 mm/yr (i.e., half-rate) (Bown and White, 1994; Reid and Jackson, 1981). At the same time, models of the internal stratigraphy of the lower crust have become increasingly complex with the introduction of narrow accumulation zones, ephemeral magma chambers, and a host of complexities arising from variations in thermal structure and spreading rate (e.g., Sinton and Detrick, 1992) such as provision for tectonism and hydrothermal alteration in the accretion zone at the slow-spreading rates (e.g., Dick et al., 1992)

Interpretations derived from plate tectonic theory about the rate of formation of new ocean crust over the last few hundred million years, and the remarkably uniform seismic structure of the ocean crust and the geologic model used to explain it, have been taken to provide an estimate of the transfer of heat and mass from the earth's deep interior to the crust, oceans, and atmosphere. Fossil sections of ocean crust, termed ophiolite complexes, preserved on land in tectonic collision zones at continental margins and island arcs, have been used both to support this hypothesis and to allow more direct inference into the internal structure and composition of the ocean crust and the igneous, hydrothermal, and tectonic processes by which it was created. Nonetheless, it has long been known that significant differences exist between these fossil sections and what is known of the in situ ocean crust‹both in the details of their rock chemistries and in the inferred thickness of many ophiolites, which are often much thinner than commonly observed in the modern ocean (Coleman and Irwin, 1974). Thus, ophiolites have an inherently ambiguous provenance, with most attributed to a supra-subduction zone environment atypical of the world's oceans.

Hess (1962) originally proposed that the MOHO was an alteration front in the mantle and that seismic layer 3 was largely partially serpentinized peridotite. In his model, the MOHO was produced by auto-metasomatism of peridotite by deuteric water as the mantle cooled into the stability field of serpentine after being emplaced by solid-state flow to the base of the crust. Later investigators, however, found that it was difficult to match laboratory observations of both seismic P- and S-wave velocities for partially serpentinized peridotite to those of layer 3 (Christensen, 1972; Christensen and Salisbury, 1975) and rejected Hess's model in favor of a gabbroic lower crust. Recent studies now suggest that the ocean crust has a complex, three-dimensional structure that is highly dependent on magma supply and spreading rates without large steady-state magma chambers (e.g., Whitehead et al., 1984; Detrick et al., 1990; Sinton and Detrick, 1992; Barth, 1994; Carbotte and MacDonald, 1994). Compilations of dredge results and seismic data have indicated that a continuous gabbroic layer does not exist at slow-spreading ridges (Mutter et al., 1985; McCarthy et al., 1988; Dick, 1989; Cannat et al., 1992; Tucholke, unpubl. data), and that its internal stratigraphy is governed by dynamic processes of alteration and tectonism as much as by igneous processes. The exceptional abundance of serpentinized peridotite in dredge hauls from the walls of rift valleys in fracture zones and in the rift mountains away from fracture zones (Aumento and Loubat, 1971; Cannat et al., 1992; Dick, 1989; Thompson and Melson, 1972) raises the serious possibility that serpentinite is a major component of seismic layer 3. In these scenarios, the MOHO does not correspond everywhere in the oceans to the boundary between igneous crust and the mantle, but may be an alteration front, corresponding locally to the depth of circulation of seawater down cracks into the earth's interior.

These factors, and an increasing awareness that spreading rate, ridge geometry, and proximity to mantle hot spots have major impacts on ocean crust thickness and lithostratigraphy, make in situ observation of the lower ocean crust by drilling a necessity if the processes of ocean crust accretion and the nature of the ocean crust are ever to be understood. The Deep Sea Drilling Project (DSDP), and its successor the Ocean Drilling Program (ODP), have directly sampled in situ ocean crust down to 2 km in a variety of spreading environments, confirming many inferences from ophiolites as to its shallow-depth structure and composition. The results show that the seismic layer 2/layer 3 boundary may be an alteration front rather than simply the boundary between diabase and gabbro, thus raising questions about the nature of the MOHO as well. Despite the recovery of short sections of lower ocean crust and mantle by several ODP legs, however, no true representative section of seismic layer 3 has ever been obtained in situ from the oceans, leaving its composition, state of alteration, and internal structure largely a matter of inference.

ODP Hole 735B came closest to this goal, recovering a 500-m section of coarsely crystalline gabbroic rock drilled in a tectonically exposed lower crustal section on a wave-cut platform that flanks the Atlantis II Fracture Zone on the slow-spreading SW Indian Ridge (Fig. 1). Hole 735B cores radically changed our perception of the lower ocean crust at slow-spreading ridges. The data indicate the crust formed by a complex interaction of magmatic, tectonic, and hydrothermal processes (e.g., Dick et al., 1992), quite unlike the simple large magma chamber once envisioned as the primary driver of crustal accretion (e.g., Cann, 1974). Given the typical thicknesses of seismic layer 3, this section is not long enough to adequately characterize the lower crust, even at a very slow-spreading ridge. No other known place, however, offers the excellent drilling conditions, high recovery, ease of guide-base placement, and superb shallow exposures of lower crust. It is, therefore, the ideal place to go to begin drilling representative sections of the lower ocean crust to test the nature of the MOHO and the crust.

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