both low- and high-temperature seawater exchange, resulting in large fractionations in oxygen isotope space relative to the upper mantle value typical of mid ocean ridge basalt glasses. Other mechanisms proposed for the origin of Group A eclogite xenoliths rely on a cumulate model, where garnet and clinopyroxene bulk compositions derive from residues of partial melting within the mantle. Support of this process is result of metasomatic overprinting of the original oxygen isotope composition, driving them back towards the mantle range. Eclogite facies Eclogites containing lawsonite (a hydrous calcium-aluminium silicate) are rarely exposed at Earth's surface, although they are predicted from experiments and thermal models to form during normal subduction of oceanic crust at depths between about . Importance of eclogite Formation of igneous rocks from eclogite Partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts. Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted. The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt. Distribution Occurrences exist in western North America, including the southwest and the Franciscan Formation of the California Coast Ranges. Transitional granulite-eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya. The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively. References External links Mantle eclogites Eclogite sample Eclogite photo gallery Metamorphic rocks Metamorphic petrology