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Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt%
/ref> It is classified as a 'picritic rock'. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa,Viljoen, M. J., & Viljoen, R. P. 1969a. Evidence for the existence of a mobile extrusive peridotitic magma from the Komati Formation of the Onvernacht Group. Geological Survey of South Africa, Special Publication, 21, 87 – 112. and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.Arndt, N., Lesher, C. M., & Barnes, S. J. 2008. Komatiite. Cambridge: Cambridge University Press.
Komatiites are rare rocks; almost all komatiites were formed during the Archean Eon (4.03–2.5 billion years ago), with few younger (Proterozoic or
/ref> The early Earth had much higher heat production, due to the residual heat from planetary accretion, as well as the greater abundance of radioactive isotopes, particularly shorter lived ones like uranium 235 which produce more decay heat. Lower temperature mantle melts such as basalt and Picrite basalt have essentially replaced komatiites as an eruptive lava on the Earth's surface.
Geographically, komatiites are predominantly restricted in distribution to the Archaean shield areas, and occur with other Ultramafic rock and high-magnesian mafic volcanic rocks in Archaean . The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau off the Pacific coast of Colombia, and a rare example of Proterozoic komatiite is found in the Winnipegosis komatiite belt in Manitoba, Canada.
/ref>Sossi, P. A., Eggins, S. M., Nesbitt, R. W., Nebel, O., Janet Hergt, Campbell, I. H., O'
/ref> normally have eruption temperatures of about 1100 to 1250 °C. The higher melting temperatures required to produce komatiite have been attributed to the presumed higher geothermal gradients in the Archaean Earth.
Komatiitic lava was extremely fluid when it erupted (possessing the viscosity close to that of water but with the density of rock). Compared to the basaltic lava of the Hawaiian Mantle plume basalts at ~1200 °C, which flows the way treacle or honey does, the komatiitic lava would have flowed swiftly across the surface, leaving extremely thin lava flows (down to 10 mm thick). The major komatiitic sequences preserved in Archaean rocks are thus considered to be lava tubes, ponds of lava etc., where the komatiitic lava accumulated.
Komatiite chemistry is different from that of basaltic and other common mantle-produced magmas, because of differences in degrees 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.
There are two geochemical classes of komatiite; aluminium undepleted komatiite (AUDK) (also known as Group I komatiites) and aluminium depleted komatiite (ADK) (also known as Group II komatiites), defined by their Al2O3/TiO2 ratios. These two classes of komatiite are often assumed to represent a real Petrology source difference between the two types related to depth of melt generation. Al-depleted komatiites have been modeled by melting experiments as being produced by high degrees of partial melting at high pressure where garnet in the source is not melted, whereas Al-undepleted komatiites are produced by high degrees of partial melts at lesser depth. However, recent studies of fluid inclusions in Chromium Spinel group from the cumulate zones of komatiite flows have shown that a single komatiite flow can be derived from the mixing of parental magmas with a range of Al2O3/TiO2 ratios, calling into question this interpretation of the formations of the different komatiite groups. Komatiites probably form in extremely hot mantle plumes or in Archaean subduction zones.
Boninite magmatism is similar to komatiite magmatism but is produced by fluid-fluxed melting above a subduction zone. Boninites with 10–18% MgO tend to have higher large-ion lithophile elements (LILE: Barium, Rubidium, Strontium) than komatiites.
A considerable population of komatiite examples show a cumulate rocks and Geomorphology. The usual cumulate mineralogy is highly magnesium rich forsterite olivine, though chromian pyroxene cumulates are also possible (though rarer).
Volcanic rocks rich in magnesium may be produced by accumulation of olivine phenocrysts in basalt melts of normal chemistry: an example is picrite. Part of the evidence that komatiites are not magnesium-rich simply because of cumulate olivine is textural: some contain spinifex texture, a texture attributable to rapid crystallization of the olivine in a thermal gradient in the upper part of a lava flow. "Spinifex" texture is named after the common name for the Australian grass Triodia, which grows in clumps with similar shapes.
Another line of evidence is that the MgO content of olivines formed in komatiites is toward the nearly pure MgO forsterite composition, which can only be achieved in bulk by crystallisation of olivine from a highly magnesian melt.
The rarely preserved flow top breccia and pillow margin zones in some komatiite flows are essentially volcanic glass, quenched in contact with overlying water or air. Because they are rapidly cooled, they represent the liquid composition of the komatiites, and thus record an anhydrous MgO content of up to 32% MgO. Some of the highest magnesian komatiites with clear textural preservation are those of the Barberton belt in South Africa, where liquids with up to 34% MgO can be inferred using bulk rock and olivine compositions.
The mineralogy of a komatiite varies systematically through the typical stratigraphic section of a komatiite flow and reflects magmatic processes which komatiites are susceptible to during their eruption and cooling. The typical mineralogical variation is from a flow base composed of olivine cumulate, to a spinifex textured zone composed of bladed olivine and ideally a pyroxene spinifex zone and olivine-rich chill zone on the upper eruptive rind of the flow unit.
Primary (magmatic) mineral species also encountered in komatiites include olivine, the pyroxenes augite, pigeonite and bronzite, plagioclase, chromite, ilmenite and rarely pargasitic amphibole. Secondary (metamorphic) minerals include serpentine group, Chlorite group, amphibole, sodic plagioclase, quartz, iron oxides and rarely phlogopite, baddeleyite, and pyrope or hydrogrossular garnet.
The factor controlling the mineral assemblage is the partial pressure of carbon dioxide within the metamorphic fluid, called the XCO2. If XCO2 is above 0.5, the metamorphic reactions favor formation of talc, magnesite (magnesium carbonate), and tremolite amphibole. These are classed as talc carbonate reactions. Below XCO2 of 0.5, metamorphic reactions in the presence of water favor production of serpentinite.
There are thus two main classes of metamorphic komatiite; carbonated and hydrated. Carbonated komatiites and peridotites form a series of rocks dominated by the minerals chlorite, talc, magnesite or dolomite and tremolite. Hydrated metamorphic rock assemblages are dominated by the minerals chlorite, serpentine group-antigorite and brucite. Traces of talc, tremolite and dolomite may be present, as it is very rare that no carbon dioxide is present in metamorphic fluids. At higher metamorphic grades, anthophyllite, enstatite, olivine and diopside dominate as the rock mass dehydrates.
Typical metamorphic mineralogy is tremolite-Chlorite group, or talc-chlorite mineralogy in the upper spinifex zones. The more magnesian-rich olivine-rich flow base facies tend to be free from tremolite and chlorite mineralogy and are dominated by either serpentine group-brucite +/- anthophyllite if hydrated, or talc-magnesite if carbonated. The upper flow facies tend to be dominated by talc, chlorite, tremolite, and other magnesian amphiboles (anthophyllite, cummingtonite, gedrite, etc.).
For example, the typical flow facies (see below) may have the following mineralogy;
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When meeting the above, but the TiO2 is more than 1 wt%, it is classified as meimechite.
A similar high-Mg volcanic rock is boninite, having 52–63 wt% SiO2, more than 8 wt% MgO and less than 0.5 wt% TiO2.
The above geochemical classification must be the essentially unaltered magma chemistry and not the result of cumulate rocks (as in peridotite). Through a typical komatiite flow sequence the chemistry of the rock will change according to the internal fractionation which occurs during eruption. This tends to lower MgO, Cr, Ni, and increase Al, K2O, Na, CaO and SiO2 toward the top of the flow.
Rocks rich in MgO, K2O, Ba, Cs, and Rb may be , or other rare ultramafic, potassic or ultrapotassic rocks.
Harrisite texture, first described from (not komatiites) at Harris Bay on the island of Rùm in Scotland, is formed by nucleation of crystals on the floor of a magma chamber. Harrisites are known to form megacrystal aggregates of pyroxene and olivine up to 1 metre in length. Harrisite texture is found in some very thick lava flows of komatiite, for example in the Norseman-Wiluna Greenstone Belt of Western Australia, in which crystallization of has occurred.
However, the initial flux of the most magnesian magmas is interpreted to form a channelised flow facie, which is envisioned as a fissure vent releasing highly fluid komatiitic lava onto the surface. This then flows outwards from the vent fissure, concentrating into topographical lows, and forming channel environments composed of high MgO olivine Cumulate rock flanked by a 'sheeted flow facies' aprons of lower MgO olivine and pyroxene thin-flow spinifex sheets.
The typical komatiite lava flow has six stratigraphically related elements;
The channel and sheeted flows are then covered by high-magnesian basalts and tholeiitic basalts as the volcanic event evolves to less magnesian compositions. The subsequent magmatism, being higher silica melts, tends to form a more typical shield volcano architecture.
This is recognised at the Mt Keith nickel deposit where wall-rock intrusive textures and of
/ref> The previous interpretations of these large komatiite bodies was that they were "super channels" or reactivated channels, which grew to over 500 m in stratigraphic thickness during prolonged volcanism.
These intrusions are considered to be channelised sills, formed by injection of komatiitic magma into the stratigraphy, and inflation of the magma chamber. Economic nickel-mineralised olivine adcumulate bodies may represent a form of sill-like conduit, where magma pools in a staging chamber before erupting onto the surface.
Komatiites are associated with nickel and gold deposits in Australia, Canada, South Africa and most recently in the Guiana Shield of South America.
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