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Eclogite () is a metamorphic rock containing garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum and, rarely, diamond. The chemistry of primary and accessory minerals is used to classify three types of eclogite (A, B, and C). The broad range of eclogitic compositions has led to a longstanding debate on the origin of eclogite as subducted, altered oceanic crust.
The name eclogite is derived from the Ancient Greek word for 'choice' (, ), meaning 'chosen rock' on account of its perceived beauty. It was first named by René Just Haüy in 1822 in the second edition of his work Traité de minéralogie.
The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface. Group A derive from regions of Earth's crust, brought to the surface as xenoliths from depths greater than 150 km during kimberlite eruptions. Group B show strong compositional overlap with Group A, but are found as lenses or pods surrounded by Peridotite mantle material. Group C are commonly found between layers of mica or glaucophane schist, primarily exemplified by the New Caledonia tectonic block and off the coast of California.
Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to Metasomatism overprinting of their original composition. Models proposing a primary surface origin as seafloor strongly rely on the wide range in oxygen isotope composition, which overlaps with obducted oceanic crust, such as the Ibra section of the Semail Ophiolite. The variation found in some eclogite xenoliths at the Roberts Victor kimberlite pipe are a result of hydrothermal alteration of basalt on the seafloor. This process is attributed to 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.
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 .
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