Editing Peridotite (section)

==Classification==
[[File:Peridotite Olivine-Orthopyroxene-Clinopyroxene - Common Peridotites highlighted.png|thumb|upright=1.4|right|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)).]]

Igneous rocks rich in [[magnesium]] and [[iron]] with a [[color index (geology)|color index]] greater than 90 are defined as [[ultramafic rocks|ultramafic]].<ref name=Downes>{{cite book |editor1-last=Downes |editor1-first=Hilary |title=Encyclopedia of Geology |date=2021 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=978-0-08-102909-1 |edition=Second }}</ref>  Ultramafic rocks may be further classified by their relative proportions of [[olivine]], [[orthopyroxene]], [[clinopyroxene]], and [[hornblende]], which are the most abundant families of mafic minerals in most ultramafic rocks. Peridotite is then defined as coarse-grained ultramafic rock in which olivine makes up 40% or more of the total volume of these four mineral families in the rock.<ref name=PhilpottsAgue2009ch6>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |pages=137–142}}</ref><ref name="bgs">{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref>

Peridotites are further classified as follows:<ref name="bgs"/>{{sfn|Philpotts|Ague|2009|p=142}}

* [[Dunite]]: more than 90% olivine
: Dunite is found as prominent veins in the peridotite layer of [[ophiolites]], which are interpreted as slices of oceanic lithosphere (crust and upper mantle) thrust onto continents.{{sfn|Philpotts|Ague|2009|pp=43–44, 372–373}} Dunite also occurs as a [[cumulate]] in [[layered intrusion]]s, where olivine crystallized out of a slowly cooling body of [[magma]] and accumulated on the floor of the magma body to form the lowest layer of the intrusion.{{sfn|Philpotts|Ague|2009|p=385}} Dunite almost always contains accessory [[chromite]].{{sfn|Jackson|1997|loc="dunite"}}

*[[Kimberlite]]: formed in [[volcanic pipe]]s and at least 35% olivine<ref>{{Cite web |title=Peridotite: Igneous Rock - Pictures, Definition & More |url=https://geology.com/rocks/peridotite.shtml |access-date=2022-07-13 |website=geology.com}}</ref>
: Kimberlite is a highly [[breccia]]ted variant of peridotite formed in [[volcanic pipe]]s and is known for being the host rock to diamonds. Unlike other forms of peridotite, kimberlite is quite rare.<ref name=":0">{{Cite web |title=kimberlite {{!}} rock {{!}} Britannica |url=https://www.britannica.com/science/kimberlite |access-date=2022-07-13 |website=www.britannica.com |language=en}}</ref>

* Pyroxene peridotite: From 40% to 90% olivine and less than 5% hornblende
** [[Harzburgite]]: less than 5% clinopyroxene
:: Harzburgite makes up the bulk of the peridotite layer of [[ophiolites]]. It is interpreted as depleted mantle rock, from which [[basaltic]] magma has been extracted. It also forms as a cumulate in Type I layered intrusions, forming a layer just above the dunite layer.{{sfn|Philpotts|Ague|2009|pp=43–44, 385}} Harzburgite likely makes up most of the mantle [[lithosphere]] underneath continental [[craton]]s.<ref name="Herzberg2004">{{cite journal |last1=Herzberg |first1=Claude |title=Geodynamic Information in Peridotite Petrology |journal=Journal of Petrology |date=December 2004 |volume=45 |issue=12 |pages=2507–2530 |doi=10.1093/petrology/egh039|doi-access=free }}</ref>
:* [[Wehrlite]]: less than 5% orthopyroxene
:: Wehrlite makes up part of the transition zone between the peridotite layer and overlying [[gabbro]] layer of [[ophiolites]]. {{sfn|Philpotts|Ague|2009|p=178}} In  Type II layered intrusions, it takes the place of harzburgite as the layer just above the dunite layer.{{sfn|Philpotts|Ague|2009|p=385}}

:* [[Lherzolite]]: intermediate content of clinopyroxene and orthopyroxene
:: Lherzolite is thought to make up much of the upper mantle.{{sfn|Philpotts|Ague|2009|p=590}} It has almost exactly the composition of a mixture of three parts harzburgite and one part [[tholeiitic basalt]] (''[[pyrolite]]'') and is the likely source rock for basaltic magma. It is found as rare [[xenolith]]s in basalt, such as those of [[Kilbourne Hole]] in southern New Mexico, US,{{sfn|Philpotts|Ague|2009|p=604}} and at [[Oahu]], Hawaii, US.<ref>{{cite journal |last1=Yang |first1=H.-J. |last2=Sen |first2=G. |last3=Shimizu |first3=N. |title=Mid-Ocean Ridge Melting: Constraints from Lithospheric Xenoliths at Oahu, Hawaii |journal=Journal of Petrology |date=1 February 1998 |volume=39 |issue=2 |pages=277–295 |doi=10.1093/petroj/39.2.277|doi-access=free }}</ref>

* Hornblende peridotite: From 40% to 90% olivine and less than 5% pyroxene
: Hornblende peridotite is found as rare xenoliths in [[andesite]]s above [[subduction zones]]. They are direct evidence of alteration of mantle rock by fluids released by the subducting [[Slab (geology)|slab]].<ref name=BlatterCarmichael1998>{{cite journal |last1=Blatter |first1=Dawnika L. |last2=Carmichael |first2=Ian S. E. |title=Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle |journal=Geology |date=1 November 1998 |volume=26 |issue=11 |pages=1035–1038 |doi=10.1130/0091-7613(1998)026<1035:HPXFCM>2.3.CO;2|bibcode=1998Geo....26.1035B }}</ref>

* Pyroxene hornblende peridotite: Intermediate between pyroxene peridotite and hornblende peridotite
: Pyroxene hornblende peridotite is found as rare xenoliths, such as those of [[Wilcza Góra, Silesian Voivodeship|Wilcza Góra]] in southwest Poland. Here it likely formed by alteration of mantle rock by carbonated hydrous silicic fluids associated with volcanism.<ref name=MatusiakMatekEtal2017>{{cite journal |last1=Matusiak-Małek |first1=Magdalena |last2=Puziewicz |first2=Jacek |last3=Ntaflos |first3=Theodoros |last4=Grégoire |first4=Michel |last5=Kukuła |first5=Anna |last6=Wojtulek |first6=Piotr Marian |title=Origin and evolution of rare amphibole-bearing mantle peridotites from Wilcza Góra (SW Poland), Central Europe |journal=Lithos |date=August 2017 |volume=286-287 |pages=302–323 |doi=10.1016/j.lithos.2017.06.017|bibcode=2017Litho.286..302M }}</ref>