Peridotite
Igneous rock | |
Composition | |
---|---|
olivine, pyroxene |
Peridotite (
Peridotite is the dominant rock of the
The word peridotite comes from the gemstone peridot, which consists of pale green olivine.[1] Classic peridotite is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this is because olivine is easily weathered to iddingsite. While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors including blue, brown, and red.
Classification
Coarse-grained igneous rocks in which
Peridotites are further classified as follows:[4][5]
- Dunite: more than 90% olivine
- Dunite is found as prominent veins in the peridotite layer of cumulate in layered intrusions, 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.[7] Dunite almost always contains accessory chromite.[8]
- Kimberlite: formed in volcanic pipes and at least 35% olivine[9]
- Kimberlite is a highly brecciated variant of peridotite formed in volcanic pipes and is known for being the host rock to diamonds. Unlike other forms of peridotite, kimberlite is quite rare.[10]
- 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 basaltic magma has been extracted. It also forms as a cumulate in Type I layered intrusions, forming a layer just above the dunite layer.[11] Harzburgite likely makes up most of the mantle lithosphere underneath continental cratons.[12]
- Wehrlite: less than 5% orthopyroxene
- Wehrlite makes up part of the transition zone between the peridotite layer and overlying
- Harzburgite makes up the bulk of the peridotite layer of
- Lherzolite: intermediate content of clinopyroxene and orthopyroxene
- Lherzolite is thought to make up much of the upper mantle.
- Hornblende peridotite: From 40% to 90% olivine and less than 5% pyroxene
- Hornblende peridotite is found as rare xenoliths in
- Pyroxene hornblende peridotite: Intermediate between pyroxene peridotite and hornblende peridotite
- Pyroxene hornblende peridotite is found as rare xenoliths, such as those of Wilcza Góra in southwest Poland. Here it likely formed by alteration of mantle rock by carbonated hydrous silicic fluids associated with volcanism.[18]
Composition
Mantle peridotite is highly enriched in magnesium, with a typical magnesium number of 89.
Olivine is the essential mineral found in all peridotites. It is a magnesium orthosilicate containing some iron with the variable formula (Mg,Fe)2SiO4. The magnesium-rich olivine of peridotites is typically olive-green in color.[20]
Pyroxenes are chain silicates having the variable formula (Ca,Na,FeII,Mg)(Cr,Al,FeIII,Mg,Mn,Ti,V)Si2O6 comprising a large group of different minerals. These are divided into orthopyroxenes (with an
Hornblende is an amphibole, a group of minerals resembling pyroxenes but with a double chain structure incorporating water. Hornblende itself has a highly variable composition, ranging from tschermakite (Ca2(Mg,Fe)3Al2Si6Al2O22(OH)2) to pargasite (NaCa2(Mg,Fe)4AlSi6Al2O22(OH)2) with many other variations in composition.[22] It is present in peridotite mostly as a consequence of alteration by hydrous fluids.[17][18]
Although peridotites are classified by their content of olivine, pyroxenes, and hornblende, a number of other mineral families are characteristically present in peridotites and may make up a significant fraction of their composition. For example, chromite is sometimes present in amounts of up to 50%. (A chromite composition above 50% reclassifies the rock as a peridotitic chromitite.) Other common accessory minerals include spinel, garnet, biotite, or magnetite. A peridotite containing significant amounts of one of these minerals may have its classification refined accordingly; for example, if a lhertzolite contains up to 5% spinel, it is a spinel-bearing lhertzolite, while for amounts up to 50%, it would be classified as a spinel lhertzolite.[23] The accessory minerals can be useful for estimating the depth of formation of the peridotite. For example, the aluminium in lhertzolite is present as plagioclase at depths shallower than about 20 kilometers (12 mi), while it is present as spinel between 20 km and 60 kilometers (37 mi) and as garnet below 60 km.[24]
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 the higher-pressure mineral wadsleyite.[26]
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.
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.[33] Although kimberlite is a variant of peridotite, kimberlite is also considered as brecciated volcanic material as well,[10] which is why it is referred to as a source of peridotite xenoliths. Peridotite xenoliths contain osmium and other elements whose stable isotope ratios provide clues on the formation and evolution of the Earth's mantle.[34][35] Such xenoliths originate from depths of up to nearly 200 kilometers (120 mi)[36] or more.[37]
The volcanic equivalent of peridotites are komatiites, which were mostly erupted early in the Earth's history and are rare in rocks younger than Archean in age.[38]
Small pieces of peridotite have been found in lunar breccias.[39]
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
Color, morphology, and texture
Most peridotite is green in color due to its high olivine content. However, peridotites can range in color from greenish-gray[44][45] to nearly black[46] to pale yellowish-green.[47] Peridotite weathers to form a distinctive brown crust in subaerial exposures[48] and to a deep orange color in submarine exposures.[49]
Peridotites can take on a massive form or may be in layers on a variety of size scales.
Another texture is a well-annealed texture of equal sized anhedral crystals with straight grain boundaries intersecting at 120°. This may result when slow cooling allowed recrystallization to minimize surface energy. Cataclastic texture, showing irregular fractures and
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
Mantle peridotites are sampled as ophiolites in collisional mountain ranges, as xenoliths in basalt or kimberlite, or as abyssal peridotites (sampled from ocean floor).[27] These rocks represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite).[54] 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.[55]
Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals.[56] They form from mantle-derived magmas, such as those of basalt composition.[57] Peridotites associated with Alaskan-type ultramafic complexes are cumulates that probably formed in the root zones of volcanoes.[58] Cumulate peridotites are also formed in komatiite lava flows.[59]
Associated rocks
Komatiites are high temperature partial melts of peridotite characterized by a high degree of partial melting deep below the surface.[60]
Eclogite, a rock similar to basalt in composition, is composed primarily of omphacite (sodic clinopyroxene) and pyrope-rich garnet. Eclogite is associated with peridotite in some xenolith occurrences;[61] it also occurs with peridotite in rocks metamorphosed at high pressures during processes related to subduction.[62]
Economic geology
Peridotite may potentially be used in a low-cost, safe and permanent method of capturing and storing atmospheric CO2 as part of
Peridotite is named for the gemstone peridot, a glassy green gem originally mined on St. John's Island in the Red Sea[65] and now mined on the San Carlos Apache Indian Reservation in Arizona.[66]
Peridotite that has been hydrated at low temperatures is the protolith for serpentinite, which may include chrysotile asbestos (a form of serpentine)[43] and talc.[67]
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 Bushveld Igneous Complex in South Africa and the Great Dyke of Zimbabwe.[68] The chromite bands found in peridotites are the world's major source of chromium.[69]
References
- ^ Collins Australian Dictionary, 7th edition
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- ^ a b c "Rock Classification Scheme - Vol 1 - Igneous" (PDF). British Geological Survey: Rock Classification Scheme. 1: 1–52. 1999.
- ^ a b Philpotts & Ague 2009, p. 142.
- ^ Philpotts & Ague 2009, pp. 43–44, 372–373.
- ^ a b Philpotts & Ague 2009, p. 385.
- ^ Jackson 1997, "dunite".
- ^ "Peridotite: Igneous Rock - Pictures, Definition & More". geology.com. Retrieved 2022-07-13.
- ^ a b "kimberlite | rock | Britannica". www.britannica.com. Retrieved 2022-07-13.
- ^ Philpotts & Ague 2009, pp. 43–44, 385.
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- ^ Philpotts & Ague 2009, p. 590.
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- ^ a b c Nesse 2000, pp. 261–74.
- ^ Nesse 2000, pp. 277–289.
- ^ Philpotts & Ague 2009, p. 138.
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- doi:10.17863/CAM.44064. Archived from the original on 17 December 2019. Retrieved 13 December 2019.)
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- ^ "Serpentinite". Presidio of San Francisco. National Park Service. Retrieved 3 September 2021.
- ^ a b Nesse 2000, pp. 241–242.
- ^ "Spinel peridotite". National Museum of Natural History. Smithsonian Institution. Archived from the original on 26 February 2022. Retrieved 26 February 2022.
- ^ "Peridotite (Dunite)". Geology: Rocks and minerals. University of Auckland. Retrieved 26 February 2022.
- ^ Sepp, Siim. "Peridotite - Igneous Rocks". www.sandatlas.org. Retrieved 26 February 2022.
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- ^ "Rocks Could Be Harnessed To Sponge Vast Amounts Of Carbon Dioxide From Air". Science Daily. November 6, 2008. Retrieved 24 February 2022.
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- ^ St. John's Island peridot information and history at Mindat.org
- ^ Finlay, Victoria. Jewels: A Secret History (Kindle ed.). Random House Publishing Group. pp. 2543–2546.
Although some good olive-colored crystals are found in a few other places, like Burma, China, Zambia, and Pakistan, ninety percent of all known peridots are found in just one place. It is a Native American reservation, and it is located in a little-visited corner of the United States. San Carlos
- ^ Nesse 2000, pp. 242–243.
- ^ Nesse 2000, pp. 387–388.
- ^ Nesse 2000, pp. 361–362.
Further reading
- Anderson, A. T. Jr. (2019). "Peridotite". AccessScience. McGraw-Hill. .
- 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
External links
Media related to Peridotite at Wikimedia Commons