From the earliest seismic refraction studies in the ocean basins, the ocean crust has been known to have a surprisingly uncomplicated and uniform layered seismic structure (Hill, 1957; Raitt, 1963; Christensen and Salisbury, 1975). Beneath a first layer of low velocity sediments, there is a second layer, roughly 1 to 2 km thick, with an average seismic velocity of 5.1 km/s. Beneath this, following a short intervening gradient in velocity, there is a third 4- to 6-km-thick layer having an average seismic velocity of 6.8 km/s. These layers overlie material with an average velocity of 8.1 km/s at an abrupt seismic transition known as the Mohorovicic discontinuity or Moho. Earth scientists have long equated this 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 at or near the Moho. Accretion of the lower crust is seen as resulting from crystallization from some form of near steady-state magma chamber, or crystal mush zone, where magmas accumulate beneath a sheeted dike­pillow lava sequence over the ascending mantle. Internally, a simple stratigraphy of primitive layered gabbros was believed to be overlain by more evolved isotropic gabbros, all of which were equated to seismic Layer 3. This simple hypothesis has been modified somewhat over the last 20 years. Provisions have been made for thinner crust in areas near large transforms (e.g., Mutter and Detrick, 1984; Dick, 1989) and in areas where half-spreading rates are significantly below 10 mm/yr (Reid and Jackson, 1981; Bown and White, 1994). At the same time, models of the internal stratigraphy of the lower crust have become increasingly complex, with the introduction of narrow accumulation zones (e.g., Bloomer and Meyer, 1992), ephemeral magma chambers, and a host of complexities arising from variations in thermal structure and spreading rate (e.g., Sinton and Detrick, 1992), including deep faulting and hydrothermal activity beneath slow spreading ridges (e.g., Dick et al., 1992).

The remarkably uniform seismic structure of the ocean crust, together with a knowledge of the rate at which new seafloor forms from plate-tectonic theory, has been used to estimate 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 as to the internal structure, composition, and origin of the ocean crust. 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 inferred 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) proposed that the Moho was an alteration front in the mantle, and that seismic Layer 3 was made up mostly of partially serpentinized peridotite. In his model, the Moho was produced by auto-metasomatism of peridotite by primary water contained in the mantle as the mantle cooled into the stability field of serpentine during solid-state flow to the base of the crust. Later investigators, however, found it 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, though, that the ocean crust has a complex, three-dimensional structure that is highly dependent on magma supply and spreading rates, and that it does not contain large steady-state magma chambers (e.g., Whitehead et al., 1984; Dick, 1989; Detrick et al., 1990; Sinton and Detrick, 1992; Carbotte and Macdonald, 1992). Compilations of dredge results and seismic data have suggested that a continuous gabbroic layer does not exist at slow-spreading ridges (Whitehead et al., 1984; Mutter et al., 1985; McCarthy et al., 1988; Dick, 1989; Cannat, 1993; Tucholke and Lin, 1994), and that the internal stratigraphy of slow-spread ocean crust 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; Thompson and Melson, 1972; Dick, 1989; Cannat, 1993) raises the serious possibility that serpentinite is a major component of seismic Layer 3. Therefore, 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 sea water down cracks into the Earth's interior.

These factors — and an increasing awareness that spreading rate, ridge geometry, and proximity to mantle hot spots also 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 Ocean Drilling Program (ODP) have directly sampled in situ ocean crust in a variety of spreading environments, once even drilling as deep as 2 km. This has confirmed many inferences from ophiolites as to its shallow structure and composition. The results at ODP Hole 504B, however, 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, no true representative section of seismic Layer 3 had ever been obtained in situ from the oceans, leaving its composition, state of alteration, and internal structure almost entirely a matter of inference.

With the completion of drilling on ODP Leg 176, however, Hole 735B comes close to this goal, representing a 1500-m section of coarsely crystalline gabbroic rock drilled in a tectonically exposed lower crustal section on a wave-cut platform flanking the Atlantis II Fracture Zone on the slow spreading Southwest Indian Ridge. The Hole 735B cores radically change our perception of the lower ocean crust at slow-spreading ridges. They indicate that the crust formed by a complex interaction of magmatic, tectonic, and hydrothermal processes. This presents a systematic variation in igneous petrology, structure, and alteration with depth quite unlike that associated with large magma chambers, thought to exist beneath fast-spreading ridges, or presently documented in ophiolites. Given the typical 4- to 6-km thicknesses of seismic Layer 3, this 1500-m section does not adequately characterize the lower crust for all the ocean basins, but, if the lower crust at ultra-slow-spreading ridges is only about 2 km thick as predicted by modeling and seismic refraction experiments, then it is a beginning. Because no other place has had the excellent drilling conditions, high recovery, ease of guide-base placement, and superb shallow exposures of lower crust as Hole 735B, this is the one location where we know we can drill a fully representative section of the lower ocean crust, and test the nature of the Moho and the crust/mantle boundary.

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