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3. Divergent Metamorphism

Oceanic-ridge metamorphism

Oceanic-ridge metamorphism is affecting the oceanic crust at ocean-ridge spreading centers. Interaction between cold seawater and hot mafic-ultramafic rocks in the 65,000-km-long oceanic spreading-ridge system is likely the largest active metamorphic system on the planet. The metamorphic minerals in ocean-floor rocks represent a wide range of temperatures at relatively low pressure. The metamorphic rocks exhibit considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na in most cases. These changes can be correlated with exchange between basalt and hot seawater. The intensity of metamorphism varies extensively on the local scale, and probably relates to the distribution of pervasive fractures that act as fluid conduits. Direct evidence for such processes are the hot springs and blacks smokers at mid-ocean ridges. The fracture systems ranging from the major fracture zones, spaced kilometers apart, through meter-spaced fractures to centimeter-spaced cracks are attributed to cooling. Seawater penetrates down these fracture systems, where it becomes heated, and leached metals and silica from the hot basalts. The hot water circulates convectively back upward, exchanging components with the rocks with which it comes in contact. Metamorphism- alteration processes occur quickly, most of it relatively close to the ridge where magmatism and subsequently heat is concentrated. If so, this type of metamorphism, although regional in the sense that it affects virtually the entire oceanic crust, is more localized, because the process may be restricted largely to the axial regions of the ridges.

Schematic cross section of an oceanic ridge. (a) Isotherms morphology: In the ridge axis isotherms are banded upwards due to the magma ascending from the mantle. (b) Distribution of different metamorphic facies on the oceanic crust near the ridge. With increasing depth, Buchan-type low-pressure facies are found. Zeolites are metamorphized to greenschist facies, and these to amphibolites and, eventually, to granulite facies near to the magmatic chamber.

It is possible that the whole oceanic crust could be metamorphized to amphibolites and granulites facies near to its base. To transform the basalts and gabbros of the oceanic crust into greenschist or amphibolite facies, is necessary to add water.

Source: MBG, redrawn after Spear (1993).