Metamorphism Of Oceanic Crust

Many of the rocks sampled from the ocean basins show evidence of metamorphism, including abundant green-schist facies assemblages and alkali metasomatism: In close proximity to such rocks, however, are found completely unaltered species.

It is probable that this metamorphism is accomplished by the hydrothermal circulation of seawater within the oceanic crust. There is much evidence for the existence of such circulation, such as the presence of metalliferous deposits which probably formed by the leaching and concentration of minerals by seawater, observations of active hydrothermal vents on ocean ridges (Section 6.5), and the observed metamorphism within ophiolite sequences.

Hydrothermal circulation takes place by convective flow, probably through the whole of the oceanic crust (Fyfe & Lonsdale, 1981), and is of great significance. It influences models of heat production, as it has been estimated that approximately 25% of the heat escaping from the Earth's surface is vented at the mid-ocean ridges. The circulation must modify the chemistry of the ocean crust, and consequently will affect the chemical relationship of lithosphere and asthenosphere over geologic time because of the recycling of lithosphere that occurs at subduction zones. It is also responsible for the formation of certain economically important ore deposits, particularly massive sulfides.

These hydrothermal processes are most conveniently studied in the metamorphic assemblages of ophiolite complexes, and the model described below has been derived by Elthon (1981).

Hydrothermal metamorphism of pillow lavas and other extrusives gives rise to low-temperature (<230°C)

Hydrothermal Metamorphism
Figure 2.20 Schematic model for hydrothermal metamorphism of the oceanic crust at a spreading center (redrawn from Elthon, 1981).

and greenschist facies assemblages (Fig. 2.20). The distribution of alteration is highly irregular, and is controlled by the localized fissuring of the extrusive rocks. Higher temperature metamorphism is widespread within the sheeted dike complex, producing assemblages typical of the actinolite facies, although pockets of unaltered rock do occur. The highest metamorphic temperatures are achieved at the base of the sheeted dike complex and the upper part of the gabbroic section. Rarely, retrograde rocks of the greenschist facies occur at this level. Alteration decreases to only about 10% within the top kilometer of the gabbroic section and thereafter metamorphism is restricted to the locality of fissures and dikes, although metamor-phism does not completely terminate at depth. According to this model, seawater circulation occurs extensively in the upper 3 km of the crust, producing the metamor-phic assemblages and cooling the crust. High-temperature metamorphism only occurs near the spreading center. At depth the circulation becomes diminished as secondary minerals are deposited within the flow channels.

As the ridge spreads continuously, oceanic lithosphere is moved laterally from the heat source and undergoes retrograde metamorphism. This depends upon an adequate water supply, as water distribution is the major control of metamorphic grade. The absence of sufficient water allows the preservation of relict high temperature assemblages. The heterogeneous nature of the distribution of metamorphic facies is consequently explained by a similarly heterogeneous distribution ofcirculating fluids rather than extreme temperature variations. As indicated in Sections 2.4.7 and 2.5, parts of the oceanic crust consist of serpentinite, that is, hydrated ultramafic rock. The ultramafic rock may be formed by magmatic differentiation within the gabbro layer, or derived directly from the mantle.

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