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The model presented is based on the interpretation of marine geophysical data, studies of dredged rocks, theoretical modeling, geologic investigations of ophiolite complexes on the continents, and results of deep-sea crustal drilling by JOIDES/IPOD.
Along the axis of the midoceanic ridge system a zone of upwelling asthenosphere extends from the base of the lithosphere at 50 to 70 km to the base of the oceanic crust. Within this prism, which narrows upward, adiabatic decompression of asthenospheric material results in partial melting, forming basaltic melt. The basaltic liquid coalesces into pockets of magma at shallow depths, forming magma chambers typically located a few kilometers beneath seafloor and centered beneath the axis of the ridge crest. Crystal fractionation takes place within these chambers, but generally never evolves too far because of the periodic addition of fresh magma from below and loss of magma to the seafloor. Profound complications exist, however, because several primitive magma types have been clearly defi ed which cannot be related to each other by crystal fractionation in shallow, crustal magma chambers, but must reflect different mantle compositions and/or melting processes. Either several zones of melting and magma ascent in the asthenosphere or a compositionally heterogeneous mantle is implied. Furthermore, drilling results demonstrate that distinct magma types occur in units of variable thickness (50 to 200 m), implying generation and fractionation of distinct batches of magma. This suggests that magma generation and emplacement is an episodic rather than a steady-state process, and argues for the coexistence of several magma chambers of restricted size, rather than a single, large, continuous magma chamber. In time, cooling of the magma chamber leads to a lower oceanic crust compose of gabbroic rocks and cumulates. The plutonic foundation of the oceanic crust is overlain by an assemblage of sheeted dikes which are capped by a chaotic extrusive carapace of pillow basalts, massive and thin flows, sills, and intercalated sediments. Seawater percolates down through the brittle carapace of the oceanic crust along permeable pathways, reacts with the hot rock at depth, and leads to metamorphism of the lower crust. Furthermore, the high thermal gradients at the ridge crest lead to the development of convective circulation of seawater through the shallow intrusive and extrusive lid of the crust, causing widespread low-temperature alteration. The water is heated and leaches material from the rocks; these dissolved constituents are either deposited along voids within the crus or are deposited on the seafloor as metallic sulfides, manganese and iron oxides, or metal-enriched sediments.
This model is still a working hypothesis, and much of it is based on circumstantial evidence. The model will change as a function of the evolving, accreting-plate-margin mosaic.
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