Peridotite
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Image:PeridotiteUSGOV.jpg Peridotite is a dense, coarse-grained rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic and ultrabasic, as the rock contains less than 45% silica and is high in magnesium with appreciable iron. Peridotite is derived from the mantle of the earth, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle.
Members of the peridotite family include (see the classification diagram below):
- Dunite - More than 90% olivine, typically with Mg/Fe equal to about 9.
- Harzburgite - predominantly composed of olivine plus orthopyroxene.
- Wehrlite - predominantly composed of olivine plus clinopyroxene.
- Lherzolite - predominantly composed of olivine, orthopyroxene (commonly enstatite),and clinopyroxene (diopside). Partial fusion of lherzolite and extraction of the melt fraction can leave a solid residue of harzburgite.
Minor minerals and mineral groups in peridotite include plagioclase, spinel (commonly the variety chromite), garnet (especially the variety pyrope), amphibole, and phlogopite. In peridotite, plagioclase is stable at relatively low pressures (crustal depths), aluminous spinel at higher pressures (to depths of 60 km or so), and garnet at yet higher pressures.
Some peridotite forms by precipitation and collection of cumulate olivine and pyroxene from mantle-derived magmas, such as those of basalt composition. Cumulate peridotites are also formed in lava conduits of komatiite flows.
Pyroxenites are related ultramafic rocks which are composed largely of orthopyroxene and/or clinopyroxene; minerals that may be present in lesser abundance include olivine, garnet, plagioclase, amphibole, and spinel.
Image:Umr.gif Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine is converted to a higher-pressure mineral.
Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. Oceanic plates are usually subducted back into the mantle in subduction zones. However, pieces can be emplaced into or overthrust on continental crust rather than carried down into the mantle, by a process called obduction; the emplacement may occur during orogenies, as during collisions of one continent with another or with an island arc. The pieces of oceanic plates within continental crust are referred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated gabbro and/or basalt.
Peridotites also occur as fragments (xenoliths) carried up by magmas from the mantle. Rocks that commonly include peridotite xenoliths include basalt and kimberlite.
Eclogite, a rock similar to basalt in composition, is composed primarily of sodic clinopyroxene and garnet. Eclogite is associated with peridotite in some xenolith occurrences; it also occurs with peridotite in rocks metamorphosed at high pressures during processes related to subduction.
The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to serpentinite, a process in which the pyroxenes and olivines are converted to green serpentine. This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock. One mineral in the serpentine group, chrysotile, is a type of asbestos.
Komatiites are the rare volcanic equivalent of peridotite.
Reference
- Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, ISBN 0716724383et:Peridotiit