User:Catherinepeshek/Eclogite

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Eclogite is a metamorphic rock formed when mafic igneous rock, such as basalt, is subjected to high pressure. Eclogite forms at pressures greater than those typical of Earth's crust. An unusually dense rock, eclogite can play an important role in driving convection within the solid Earth.

The fresh rock is bi-mineralic containing the primary phases of 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 mineral chemistry of primary and accessory phases is used to classify three types of eclogite (A, B, and C). The broad range of eclogitic rocks has led a longstanding debate on the origin of eclogite xenoliths and their origin as altered oceanic crust.

Origins
Eclogites typically result from high to ultrahigh pressure metamorphism of mafic rock at low thermal gradients of <10 C/km as it is subducted to the lower crust to upper mantle depths in a subduction zone. They are generally formed from precursor mineral assemblages typical of blueschist-facies metamorphism.

Prospective addition:

Eclogites are defined as bi-mineralic, broadly basaltic rocks which have been classified into Groups A, B and C based on the chemistry of their primary mineral phases, garnet and clinopyroxene. The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet. The rocks are gradationally less mafic (as defined by SiO2 and MgO) from group A to C, where the least mafic Group C contains higher alkali contents. The transitional nature correlates with their mode of emplacement at the surface. Group A derive from cratonic regions of earth's crust, brought to the surface as xenoliths in deep-seated kimberlite eruptions. Group B show strong compositional overlap with Group A, but are found as lenses or pods surrounded by peridotitic mantle material. Group C are commonly found between layers of mica or glaucophane schist, primarily exemplified by the isolated New Caledonia tectonic block off the coast of California.

The broad range in composition has led a longstanding debate on the origin of eclogite xenoliths as either mantle or surface derived, where the latter associated the gabbro to eclogite transition as a major driving force for subduction.

Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to metasomatic overprinting of their original composition. Models proposing a primary surface origin as seafloor protoliths strongly rely on the oxygen isotope composition which overlaps with obducted oceanic crust, such as the Ibra section of the Samail ophiolite. Alteration of basalt on the seafloor is attributed to both low- and high-temperature alteration which imparts a wide range in oxygen isotope space relative to the canonical mantle value typical of mid ocean ridge basalt glasses.

Final addition:

Origins
Eclogites typically result from high to ultrahigh pressure metamorphism of mafic rock at low thermal gradients of <10 C/km as it is subducted to the lower crust to upper mantle depths in a subduction zone.

Classification
Eclogites are defined as bi-mineralic, broadly basaltic rocks which have been classified into Groups A, B and C based on the chemistry of their primary mineral phases, garnet and clinopyroxene. The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet. The rocks are gradationally less mafic (as defined by SiO2 and MgO) from group A to C, where the least mafic Group C contains higher alkali contents.

The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface. Group A derive from cratonic 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 peridotitic mantle material. Group C are commonly found between layers of mica or glaucophane schist, primarily exemplified by the New Caledonia tectonic block off the coast of California.

Surface versus mantle origin
The broad range in composition has led a longstanding debate on the origin of eclogite xenoliths as either mantle or surface derived, where the latter is associated with the gabbro to eclogite transition as a major driving force for subduction.

Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to metasomatic overprinting of their original composition. Models proposing a primary surface origin as seafloor protoliths strongly rely on the wide range in oxygen isotope composition, which overlaps with obducted oceanic crust, such as the Ibra section of the Samail 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.