Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO₂, low K₂O, low Al₂O₃, and high to extremely high MgO. Ultramafic rocks are igneous rocks with very low silica content (less than 45%) and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). ... This article is about volcanoes in geology. ...

Komatiites were named for their type locality along the Komati River in South Africa. The Komati River is a river in South Africa. ...

True komatiites are very rare and essentially restricted to rocks of Archaean age and most are greater than three billion years old, restricted in distribution to the Archaean shield areas. Komatiites occur with other ultramafic and high-magnesium mafic volcanic rocks in Archaean greenstone belts. The Archean is a geologic eon; it is a somewhat antiquated term for the time span between 2500 million years before the present and 3800 million years before the present. ... Shields are shown in orange. ...

Komatiites are restricted to the Archaean, with few Proterozoic and few Mesozoic or Phanerozoic komatiites known (although high-magnesian lamprophyres are known from the Mesozoic). This restriction in age is thought to be due to secular cooling of the mantle, which may have been up to 500°C hotter during the early to middle Archaean (4.5 to 2.6 Ga). Lamprophyres (Greek Lampros, bright, and the terminal part of the word porphyry, meaning rocks containing bright porphyritic crystals) are a group of rocks containing phenocrysts, usually of biotite and hornblende (with bright cleavage surfaces), often also of olivine and augite, but not of feldspar. ...

The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau.

Petrology

Magmas of komatiite composition have a very high melting point with calculated eruption temperatures in excess of 1,600 °C. Basaltic lavas normally have eruption temperatures of about 1,100 °C. It is thought that the early Earth had a higher geothermal gradient and thus the higher melt temperatures could have been reached. At present Io is believed to be producing komatiite lavas with temperatures of up to 1,700 °C. The melting point of a solid is the temperature at which it changes state from solid to liquid. ... Basalt Basalt is a common gray to black volcanic rock. ... Look up lava, Aa, and pahoehoe in Wiktionary, the free dictionary. ... The geothermal gradient is the heating up of the Earths top soil and rock layer the deeper you go down, for instance by descending into a cave or drilling a well. ... Atmospheric characteristics Atmospheric pressure trace Sulfur dioxide 90% Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter. ...

Komatiitic lava would have behaved as a superfluid when erupted; it would have behaved as fluidly as water. Compared to the basaltic lava of the Hawaiian plume basalts at ~1200 °C which behaves as treacle or honey, the komatiitic lava would have been incredibly swift in travelling across the surface, leaving extremely thin lava flows (down to 10mm thick). The major komatiite sequences preserved in Archaean rocks are thus considered to be lava tubes, ponds of lava or other conduits, where the komatiitic lava accumulated.

The distinction here is on degree of partial melting. Komatiites are considered to have been formed by high degrees of partial melting, usually greater than 50%, and hence have high MgO with low K₂O and other incompatible elements. Generally, high-K₂O ultramafic rocks are formed by more volatile-driven partial melting and may be the result of less than 10% partial melting.

There are two geochemical classes of komatiite; aluminium undepleted komatiite (AUDK) and aluminium depleted komatiite (ADK). These two classes of komatiite represent a real petrological source difference between the two types related to depth of melt generation. Al-depleted komatiites have been modelled by melting experiements as being produced by high degrees of partial melting of hydrous mantle at low pressure (allowing melting of Al-rich pyroxene), whereas Al-undepleted komatiites are produced by high degree partial melts at greater depth, where Al-bearing pyroxenes in the source are not melted.

Boninite magmatism is similar to komatiite magmatism but is driven more by melting induced by volatile flows above a subduction zone than by decompression melting. Boninites with 10-18% MgO tend to have higher LILE (Ba, Rb, Sr) than komatiites.

Komatiitic magmas are considered to be a source for spatially associated tholeiite basalts based on a study linking the two rock types in the Karelian greenstone belt of northwest Russia. Magma is molten rock often located inside a magma chamber beneath the surface of the Earth. ... Karelia - Wikipedia /**/ @import /skins-1. ... New Zealand greenstone is formed by the metamorphism of basalt. ...

Mineralogy

Komatiites are composed of forsteritic olivine (Fo90 and upwards), calcic and often chromian pyroxene, anorthite (An85 an upwards) and chromite. Olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... Anorthite is one of the plagioclase feldspars, an important group of minerals abundant in the Earths crust. ... Chromite, iron magnesium chromium oxide: (Fe,Mg)Cr2O4, is an oxide mineral belonging to the spinel group. ...

They are usually highly altered and serpentinized from metamorphism. A considerable population of komatiite examples show a cumulate texture and morphology. The usual cumulate mineralogy is highly magnesium rich forsterite olivine, though chromian pyroxene cumulates are also possible (though rarer). Serpentinite is a rock comprised of an admixture of serpentine minerals. ... In geology a cumulate is an igneous rock formed by the accumulation of crystals from a magma. ... Forsterite (Mg2SiO4) is the magnesium rich end-member of the olivine solid-solution series. ...

Mineral species also encountered in komatiites include pargasitic amphibole (amphibole with >20%MgO), phlogopite, baddeleyite, ilmenite and pyrope garnet. For the logical fallacy, see Amphibology. ... Phlogopite is a yellow, greenish or reddish brown member of the mica family of phyllosilicates. ... Baddeleyite is a mineral that consists of zirconia (ZrO2). ... Ilmenite (FeTiO3) is a weakly magnetic iron-black or steel-gray mineral found in metamorphic and plutonic igneous rocks. ... The pyrope is a garnet. ... The garnet group of minerals show crystals with a habit of rhombic dodecahedrons and trapezohedrons. ...

Geochemistry

Komatiite can be classified according to the following geochemical criteria;

• SiO₂; typically 40 - 45%
• MgO greater than 18%
• Low K₂O (<0.5%)
• Low CaO and Na₂O (<2% combined)
• Low Ba, Cs, Rb (incompatible element) enrichment; ΣLILE <1,000ppm
• High Ni (>400ppm), Cr (>800ppm), Co (>150ppm)

The above geochemical classification must be the essentially unaltered magma chemistry and not the result of crystal accumulation (as in peridotite). Incompatible element is a term used in petrology and geochemistry. ... NI may refer to: A region of the United Kingdom, see Northern Ireland. ... For information on cleanup resources in Wikipedia, see Wikipedia Cleanup Resources CR can mean: Caloric restriction Carriage return Castle Rock, the name of several towns Challenge Rating, a term representing difficulty in Dungeons & Dragons Change Request, in industry programming terminology Channel Representative Cocoa Roach (Carracho X) College Republicans Comfort Room... CO can stand for: Carbon monoxide (molecular formula) Central Office (telecommunications and telephony) Chesapeake and Ohio Railroad (AAR reporting mark CO) Colombia (ISO 3166-1 digram and NATO country code) Colorado ( U.S. state postal symbol) Commanding Officer (military) Conscientious Objector (military) Continental Airlines (IATA airline code) Collaborative Browsing Co... In geology a cumulate is an igneous rock formed by the accumulation of crystals from a magma. ... Peridotite Peridotite is a dense, coarse-grained rock, consisting mostly of the minerals olivine and pyroxene. ...

Rocks with high MgO, high K₂O and Ba, Cs, Rb etc. may be lamprophyres, kimberlites or other rare ultramafic, potassic or ultrapotassic rocks. Lamprophyres (Greek Lampros, bright, and the terminal part of the word porphyry, meaning rocks containing bright porphyritic crystals) are a group of rocks containing phenocrysts, usually of biotite and hornblende (with bright cleavage surfaces), often also of olivine and augite, but not of feldspar. ... Hewn kimberlite core sample from the James Bay Lowlands region of Northern Ontario, Canada. ...

Morphology and occurrence

Komatiites often show pillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows, under which considerable lava tubes and pools accumulate. Proximal volcanic facies are thinner and interleaved with sulphidic sediments, black shales, cherts and tholeiitic basalts. Chert Chert is a fine-grained silica-rich cryptocrystalline sedimentary rock that may contain small fossils. ... Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ...

A common and distinctive texture is known as spinifex texture and consists of long acicular phenocrysts of olivine (or pseudomorphs of alteration minerals after olivine) which give the rock a bladed appearance especially on a weathered surface. The spinifex texture is the result of rapid crystallization of a supercooled liquid.

Crystal growth is retarded due to the superfluid nature of the komatiite, and proceeds in a 'flash freeze' to form the spinifex texture.

Harrisite texture, first described from the locality of Harris, Scotland, is formed by nucleation of crystals on the floor of the lava flow chamber. Harrisites are known to form megacrystal aggregates of pyroxene and olivine up to 1 metre in length.

Economic importance

The economic importance of kamotiite was first widely recognised in the early 1960's with the discovery of massive nickel sulphide mineralisation at Kambalda, Western Australia. Komatiite-hosted nickel-cuppoer sulphide mineralisation todayt accounts for about 14% of the world's nickel production, mostly from Australia, Canada and South Africa. Motto: Cygnis Insignis (Distinguished by its swans) Nickname: Wildflower State Other Australian states and territories Capital Perth Government Governor Premier Const. ...

Komatiites are associated with nickel and gold deposits in Australia, Canada, South Africa and most recently in the Guiana shield of South America. General Name, Symbol, Number nickel, Ni, 28 Chemical series transition metals Group, Period, Block 10, 4, d Appearance lustrous, metallic Atomic mass 58. ... General Name, Symbol, Number gold, Au, 79 Chemical series transition metals Group, Period, Block 11, 6, d Appearance metallic yellow Atomic mass 196. ... South America South America is a continent crossed by the equator, with most of its area in the Southern Hemisphere. ...

See also

• Komatiitic Ni-Cu-PGE mineralisation
• Rock microstructure
• List of rock textures
• List of rocks
• Igneous rocks
• Definition of ultramafic rocks
• Cumulate rocks

The various theories of ore genesis explain how the various types of mineral deposits form within the Earths crust. ... Rock microstructure includes the texture of a rock and the small scale rock structures. ... This page is intended to be a list of rock textural and morphological terms. ... Banded gneiss with dike of granite orthogneiss This page is intended as a list of all rock types. ... 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. ... Ultramafic rocks are igneous rocks with very low silica content (less than 45%) and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). ... In geology a cumulate is an igneous rock formed by the accumulation of crystals from a magma. ...

References

• Blatt, Harvey and Robert Tracy (1996), Petrology, 2nd ed., Freeman (pp. 196-7), ISBN 0-7167-2438-3.
• S. A. Svetov, A. I. Svetova, and H. Huhma, 1999, Geochemistry of the Komatiite–Tholeiite Rock Association in the Vedlozero–Segozero Archean Greenstone Belt, Central Karelia, Geochemistry International, Vol. 39, Suppl. 1, 2001, pp. S24–S38. PDF accessed 7-25-2005
• Vernon R.H., 2004, A Practical Guide to Rock Microstructure, (pp. 43-69, 150-152) Cambridge University Press. ISBN 0-521-81443-X
• Unusual lava types accessed 7-25-2005
• Komatiite and diamond accessed 7-25-2005
• Komatiites and astrobiology
• Komatiites and the Plume Debate
• Volcanic fireworks on Io