Layer 3 is the main component of the oceanic crust and represents its plutonic foundation (Fox & Stroup, 1981). Some workers have subdivided it into sublayer 3A, with a velocity range of 6.5-6.8 km s-1, and a higher velocity lower sublayer 3B (7.0-7.7 km s-1) (Christensen & Salisbury, 1972), although the majority of seismic data can be explained in terms of a layer with a slight positive velocity gradient (Spudich & Orcutt, 1980).
Hess (1962) suggested that layer 3 was formed from upper mantle material whose olivine had reacted with water to varying degrees to produce serpentinized peri-dotite, and, indeed, 20-60% serpentinization can explain the observed range of P wave velocities. However for oceanic crust of normal thickness (6-7 km) this notion can now be discounted, as the value of Poisson's ratio for layer 3A, which can be estimated directly from a knowledge of both P and S wave velocities, is much lower than would be expected for serpentinized perido-tite. In fact, Poisson's ratio for layer 3A is more in accord with a gabbroic composition, which also provides seismic velocities in the observed range. It is possible, however, that all or at least part of layer 3B, where recognized, consists of serpentinized ultramafic material.
The concept of a predominantly gabbroic layer 3 is in accord with models suggested for the origin of oceanic lithosphere (Section 6.10). These propose that layer 3 forms by the crystallization of a magma chamber or magma chambers, with an upper layer, possibly corresponding to sublayer 3A, of isotropic gabbro and a lower layer, possibly corresponding to 3B, consisting of cumulate gabbro and ultramafic rocks formed by crystal settling. This layering has been confirmed by direct observation and sampling by submersible on the Vema Fracture Zone in the North Atlantic (Auzende et al., 1989).
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