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The oceanic crust is created at mid-ocean ridges, modified by hydrothermal alteration, and returned to the mantle in subduction zones, where phase changes release water into the subducted crust and overlying mantle wedge, enabling large earthquakes and producing melts that are eventually erupted at the surface. Thus, our knowledge of the structure, composition and evolution of the oceanic crust are critically important to understanding fundamental geological and geochemical processes. However, composition of the lower oceanic crust is poorly understood because no complete sampling of the crust has been achieved. The crust is therefore best known by its seismic velocity structure; the problem is to interpret the seismic structure of the lower oceanic crust in terms of its mineral composition and water content. |
Seismic velocities in lower oceanic crust (seismic layer 3) are thought to be controlled by of increasing effective pressure and petrology, e.g., decreasing chlorite or increasing olivine content. Though these correlations are widely cited and seemingly plausible, they are not supported by the available evidence. There is, however, evidence that seismic velocities decrease with increasing alteration of pyroxene to amphibole, and with bound water content. Variations of seismic velocity in the lower oceanic crust may thus reflect the degree of in situ hydrothermal alteration and bound water content of the rocks. This research, therefore, involves a comprehensive and systematic study of the relationships between seismic velocities and mineral content in diabase and gabbro samples recovered from samples retrieved during drilling of the ocean floor by the Ocean Drilling Project. |
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