Peridotite xenolith from San Carlos, southwestern United States. The rock is typical olivine-rich peridotite, cut by a centimeter-thick layer of greenish-black pyroxenite.

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. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole. Image File history File linksMetadata No higher resolution available. ... Image File history File linksMetadata No higher resolution available. ... The rocky side of a mountain creek near Orosí, Costa Rica. ... The mineral olivine (also called chrysolite and, when gem-quality, peridot) is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... Ultramafic (or ultrabasic) rocks are igneous rocks with very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). ... The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ... General Name, Symbol, Number magnesium, Mg, 12 Chemical series alkaline earth metals Group, Period, Block 2, 3, s Appearance silvery white Atomic mass 24. ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... Earth cutaway from core to exosphere. ... Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... Chromite, iron magnesium chromium oxide: (Fe,Mg)Cr2O4, is an oxide mineral belonging to the spinel group. ... Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). ... For the logical fallacy, see Amphibology. ...

Peridotite is the dominant rock of the upper part of the Earth's mantle. The compositions of peridotite nodules found in certain basalts and diamond pipes (kimberlites) are of special interest, because they provide samples of the Earth's mantle roots of continents brought up from depths from about 30 km or so to depths at least as great as about 200 km. Some of the nodules preserve isotope ratios of osmium and other elements that record processes over three billion years ago, and so they are of special interest to paleogeologists because they provide clues to the composition of the Earth's early mantle and the complexities of the processess that were involved. Earth cutaway from core to exosphere. ... A nodule describes an aggregation of similar cells or particles in a number of scientific fields: In medicine it refers to a small aggregation of cells. ... Earth cutaway from core to exosphere. ... Isotopes are any of the several different forms of an element each having different atomic mass. ... General Name, Symbol, Number osmium, Os, 76 Chemical series transition metals Group, Period, Block 8, 6, d Appearance silvery, blue cast Atomic mass 190. ... Paleogeology is the historical geological state of a particular area, or the study of the geologic history and origins of the earth, including such things as reconstructions of previous configurations of the earths continents, the history and evolution of the earths surface geological configuration, the formation and development...

Types of Peridotite

• Dunite - More than 90% olivine, typically with Mg/Fe ratio of about 9:1.
• Wehrlite - predominantly composed of olivine plus clinopyroxene.
• Harzburgite - predominantly composed of olivine plus orthopyroxene, and relatively low proportions of basaltic ingredients (because garnet and clinopyroxene are minor).
• Lherzolite - predominantly composed of olivine, orthopyroxene (commonly enstatite),and clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients (garnet and clinopyroxene). Partial fusion of lherzolite and extraction of the melt fraction can leave a solid residue of harzburgite.
  

  

  Classification diagram for peridotite and pyroxenite, based on proportions of olivine and pyroxene. The pale green area encompasses the most common compositions of peridotite in the upper part of the Earth's mantle (partly adapted from Bodinier and Godard (2004)).

Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse grained or phaneritic texture. ... Peridotite xenolith from San Carlos, southwestern United States. ... Lherzolite is a type of ultramafic igneous rock. ... Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. ... Image File history File linksMetadata No higher resolution available. ... Image File history File linksMetadata No higher resolution available. ...

Composition

Peridotites are rich in magnesium, reflecting the high proportions of magnesium-rich olivine. The compositions of peridotites from layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole. 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. The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ... Chromite, iron magnesium chromium oxide: (Fe,Mg)Cr2O4, is an oxide mineral belonging to the spinel group. ... Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. ... For the logical fallacy, see Amphibology. ... Phlogopite is a yellow, greenish or reddish brown member of the mica family of phyllosilicates. ...

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. Pyroxenite is a rock consisting essentially of minerals of the pyroxene group, such as augite and diallage, hypersthene, bronzite or enstatite. ... The spinels are any of a class of minerals which crystallize in the isometric system with an octahedral habit. ...

Distribution and location

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. The peridotite below the ocean crust, "abyssal peridotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usually subducted back into the mantle in subduction zones. However, pieces can be emplaced into or overthrust on continental crust by a process called obduction, rather than carried down into the mantle; the emplacement may occur during orogenies, as during collisions of one continent with another or with an island arc. The pieces of oceanic plates emplaced within continental crust are referred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated rocks such as gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert. Other masses of peridotite have been emplaced into mountain belts as solid masses but do not appear to be related to ophiolites, and they have been called "orogenic peridotite massifs" and "alpine peridotites." Peridotites also occur as fragments (xenoliths) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite. Certain volcanic rocks, sometimes called komatiites, are so rich in olivine and pyroxene that they also can be termed peridotite. Small pieces of peridotite have even been found in lunar breccias. The Juan de Fuca plate sinks below the North America plate at the Cascadia subduction zone. ... The continental crust is the layer of granitic, sedimentary and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. ... Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks. ... Orogeny is the process of mountain building, and as such is both a tectonic structural event, a geographical event and a chronological event, in that orogenic events happen within a time frame, affect certain regions of rocks and crust, and cause distinctive structural phenomena and related tectonic activity. ... An island arc is a type of archipelago formed by plate tectonics as one oceanic tectonic plate subducts under another and produces magma. ... Ophiolites are sections of the oceanic crust and the subjacent upper mantle that have been uplifted or emplaced to be exposed within continental crustal rocks. ... Gabbro specimen. ... Basalt Columnar basalt at Sheepeater Cliff in Yellowstone Basalt (IPA: ) is a common gray to black volcanic rock. ... A xenolith A xenolith (Greek: foreign rock) is a rock fragment which becomes enveloped in a larger rock during the latters development and hardening. ... Basalt Columnar basalt at Sheepeater Cliff in Yellowstone Basalt (IPA: ) is a common gray to black volcanic rock. ... Hewn kimberlite core sample from the James Bay Lowlands region of Northern Ontario, Canada. ...

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. A sample of serpentinite rock, partially made up of chrysotile Serpentinite is a rock comprised of one or more serpentine minerals. ... Serpentine Serpentine is a group of common rock-forming hydrous magnesium iron phyllosilicate ((Mg, Fe)3Si2O5(OH)4) minerals; it is also often rich in other metal ores, including chromium, manganese, cobalt and nickel. ... Chrysotile Asbestos Chrysotile is an asbestiform sub-group within the serpentine group of minerals. ...

Morphology and texture

Some peridotites are layered or are themselves layers; others are massive. Many layered peridotites occur near the base of bodies of stratified gabbroic complexes. Other layered peridotites occur isolated, but possibly once composed part of major gabbroic complexes. Both layered and massive peridotites can have any of three principal textures: (1) rather well formed crystals of olivine separated by other minerals. This probably reflects the original deposition of olivine sediment from magma. (2) Equigranular crystals with straight grain boundaries intersecting at about 120°. This may result from slow cooling whereby recrystallization leads to a minimization of surface energy. (3) Long crystals with ragged curvilinear boundaries. This probably results from internal deformation.

Many peridotite occurrences have characteristic textures. For example, peridotites with well-formed olivine crystals occur mainly as layers in gabbroic complexes. "Alpine" peridotites generally have irregular crystals that occur as more or less serpentinized lenses bounded by faults in belts of folded mountains such as the Alpines, the Pacific coast ranges, and in the Appalachian piedmont. Peridotite nodules with irregular equigranular textures are often found in alkaline basalts and in kimberlite pipes. Some peridotites rich in amphibole have a concentric layered structure and form parts of plutons called Alaskan-type zoned ultramafic complexes.

Origin

Peridotites have two primary modes of origin, as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas; these magmas are ultimately derived from the upper mantle by partial melting of mantle peridotites.

Mantle peridotites are sampled as alpine-type massifs in collisional mountain ranges or as xenoliths in basalt or kimberlite. In all cases these rocks are pyrometamorphic (that is, metamorphosed in the presence of molten rock) and represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite). Alpine peridotites may be either of the ophiolite association and representing the uppermost mantle below ocean basins, or masses of subcontinental mantle emplaced along thrust faults in mountain belts.

Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals. Some peridotite forms by precipitation and collection of cumulate olivine and pyroxene from mantle-derived magmas, such as those of basalt composition. Peridotites associated with Alaskan-type ultramafic complexes are cumulates that probably formed in the root zones of volcanoes. Cumulate peridotites are also formed in komatiite lava flows. In geology a cumulate is an igneous rock formed by the accumulation of crystals from a magma. ... Komatiites are ultramafic mantle-derived volcanic rocks. ...

Mantle lherzolites may be the principal source rock for basaltic magmas, whereas mantle harzburgites probably form both from the crystalline residue left after basaltic magma migrates out of lherzolite and from a crystalline accumulation of early solidification products of some basaltic magmas within the mantle.

Associated rocks

Komatiites are the rare volcanic equivalent of peridotite. Komatiites are ultramafic mantle-derived volcanic rocks. ... This article is about volcanoes in geology. ...

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. Eclogite is a coarse-grained, mafic-to-ultramafic grouping of metamorphic rocks of special interest on account of the variety of minerals they contain and their microscopic structures and geological relationships. ...

Economic geology

Peridotite is named for the gemstone peridot, a glassy green gem mined in Asia and Arizona (Peridot Cove). Some peridotite is mined for ornamental stone.

Peridotite that has been hydrated at low temperatures forms serpentine, which may include chrysotile asbestos (a form of serpentine) and talc.

Layered intrusions with cumulate peridotite are typically associated with sulfide or chromite ores. Sulfides associated with peridotites form nickel ores and platinoid metals; most of the platinum used in the world today is mined from the Great Dyke of Zimbabwe. The chromite bands commonly associated with peridotites are the world's major ores of chromium. --195. ...

References

• Alfred T. Anderson, Jr., 2002. "Peridotite", AccessScience@McGraw-Hill, DOI 10.1036/1097-8542.498300.
• Harvey Blatt and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, ISBN 0-7167-2438-3
• J.-L. Bodinier and M. Godard, 2004, Orogenic, Ophiolitic, and Abyssal Peridotites, in The Mantle and Core (ed. R. W. Carlson), Treatise on Geochemistry v. 2, Elsevier-Pergamon, Oxford ISBN 0-08-043751-6