Metamorphism at Great Depths - EclogitesEclogites are high-grade metamorphic rocks that form due to

Metamorphism at Great Depths - EclogitesEclogites are high-grade metamorphic rocks that form due to extremely high pressures and temperatures deep within the Earth’s crust. As shown in the picture, eclogites have a very distinctive red and green colouring, and have been nicknamed ‘Christmas rocks’ by geologists. This colouring is due to their primary mineral composition of pyrope garnet (red) and omphacite pyroxene (green). Eclogite can also contain kyanite, zoisite, rutile, various amphiboles, and quartz / coesite (a high-pressure polymorph of quartz). The formation of eclogite from its basaltic protolith can be shown by this idealized mineral reaction: plagioclase + olivine + diopside = omphacite + garnet. One of the distinguishing factors of an eclogite is the lack of plagioclase, which becomes unstable and breaks down at very high pressures and temperatures.Eclogites form at temperatures exceeding 500°C and pressures over 12 kbar. These high pressures can only be attained once a rock has been buried to a minimum of 50 km below the surface of the Earth. Eclogites most commonly form in subduction zones, where oceanic crust is forced deep into the upper mantle at convergent boundaries between two tectonic plates. The downgoing oceanic crust is referred to as a ‘slab’. As Eclogites form at extremely high pressures, they are much denser than their protoliths with densities around 3.4 – 3.5 g/cm^3. This densification of the slab material as it descends, a process known as ‘slab pull’, is the primary force pulling the slab into the mantle and one of the driving forces of plate tectonics.While geologists believe eclogites to be common in subducting slabs, they are a rare find on the surface of the Earth. The vast majority of eclogites never return to the surface, but are buried ever deeper into the mantle until they melt due to the high temperatures. It takes a very uncommon series of events to exhume a subducted oceanic slab from 50+ km below and expose it on the surface. Several notable eclogite localities around the world are in Greenland, Norway, Italy, China, Newfoundland, and France.Eclogites are of interest to geologists as they provide clues to the nature of subduction and the physical characteristics of slab material. An interesting example of this comes from a 2012 paper published in Geology (http://bit.ly/2q3QqxM- paywall), about eclogites found in the Italian Alps. These eclogites, which were buried to 80 km below the surface, show evidence for multiple generations of fracturing and ductile deformation. The authors suggest that the fracturing results from earthquakes occurring at depths of 70 – 80 km, within the subducting slabs. The presence of deep, intra-slab earthquakes have been detected by seismic networks, but this locality gives researchers one of the first physical representations as to the nature of these events.-CDSourceshttp://www.sandatlas.org/eclogite/http://bit.ly/2roxpKFBest, M.G. 2012. Igneous and Metamorphic Petrology 2nd Ed. Blackwell Publishing.Image Credithttp://bit.ly/2rClbv7 -- source link

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