Komatiite/koʊˈmɑːtiˌaɪt/ is a type ofultramaficmantle-derivedvolcanic rock defined as having crystallised from alava of at least 18 wt%magnesium oxide (MgO).[1] It is classified as a 'picritic rock'. Komatiites have lowsilicon,potassium andaluminium, and high to extremely highmagnesium content. Komatiite was named for itstype locality along theKomati River in South Africa,[2] and frequently displaysspinifex texture composed of large dendritic plates ofolivine andpyroxene.[3]
Komatiites are rare rocks; almost all komatiites were formed during theArchaean Eon (4.03–2.5 billion years ago), with few younger (Proterozoic orPhanerozoic) examples known. This restriction in age is thought to be due to cooling of the mantle, which may have been 100–250 °C (212–482 °F) hotter during the Archaean.[4][5] The early Earth had much higher heat production, due to the residual heat fromplanetary accretion, as well as the greater abundance ofradioactive isotopes, particularly shorter lived ones likeuranium 235 which produce moredecay heat. Lower temperature mantle melts such asbasalt andpicrite have essentially replaced komatiites as an eruptive lava on the Earth's surface.
Geographically, komatiites are predominantly restricted in distribution to the Archaeanshield areas, and occur with otherultramafic and high-magnesianmafic volcanic rocks in Archaeangreenstone belts. The youngest komatiites are from the island ofGorgona on the Caribbeanoceanic plateau off the Pacific coast of Colombia, and a rare example of Proterozoic komatiite is found in theWinnipegosis komatiite belt inManitoba, Canada.
Magmas of komatiitic compositions have a very highmelting point, with calculated eruption temperatures up to, and possibly in excess of 1600 °C.[6][7][8][9]Basalticlavas normally have eruption temperatures of about 1100 to 1250 °C. The higher melting temperatures required to produce komatiite have been attributed to the presumed highergeothermal gradients in the Archaean Earth.
Komatiitic lava was extremely fluid when it erupted (possessing theviscosity close to that of water but with the density of rock). Compared to the basaltic lava of theHawaiianplume basalts at ~1200 °C, which flows the waytreacle 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 belava 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 ofpartial 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 otherincompatible 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 realpetrological 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 wheregarnet 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 inchromespinels 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.[10] Komatiites probably form in extremely hot mantle plumes[11] or in Archaean subduction zones.[12]
Boninite magmatism is similar to komatiite magmatism but is produced by fluid-fluxed melting above asubduction zone. Boninites with 10–18% MgO tend to have higherlarge-ion lithophile elements (LILE:Ba,Rb,Sr) than komatiites.
The pristine volcanic mineralogy of komatiites is composed of forsteritic olivine (Fo90 and upwards), calcic and often chromianpyroxene,anorthite (An85 and upwards) andchromite.
A considerable population of komatiite examples show acumulate texture andmorphology. The usual cumulatemineralogy is highly magnesium richforsterite olivine, though chromian pyroxene cumulates are also possible (though rarer).
Volcanic rocks rich in magnesium may be produced by accumulation of olivinephenocrysts in basalt melts of normal chemistry: an example ispicrite. Part of the evidence that komatiites are not magnesium-rich simply because of cumulate olivine is textural: some contain spinifextexture, a texture attributable to rapidcrystallization 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 grassTriodia,[13] 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 topbreccia 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 ananhydrous MgO content of up to 32% MgO. Some of the highest magnesian komatiites with clear textural preservation are those of theBarberton 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 typicalstratigraphic 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 pyroxenesaugite,pigeonite andbronzite,plagioclase,chromite,ilmenite and rarely pargasiticamphibole. Secondary (metamorphic) minerals includeserpentine,chlorite, amphibole, sodic plagioclase,quartz, iron oxides and rarelyphlogopite,baddeleyite, andpyrope orhydrogrossulargarnet.
 | This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(November 2020) (Learn how and when to remove this message) |
All known komatiites have beenmetamorphosed, therefore should technically be termed 'metakomatiite' though the prefix meta is inevitably assumed. Many komatiites are highly altered andserpentinized orcarbonated from metamorphism andmetasomatism. This results in significant changes to the mineralogy and the texture.
The metamorphic mineralogy of ultramafic rocks, particularly komatiites, is only partially controlled by composition. The character of theconnate fluids which are present during low temperature metamorphism whetherprograde orretrograde control the metamorphic assemblage of a metakomatiite (hereafter the prefix meta- is assumed).
The factor controlling the mineral assemblage is thepartial pressure ofcarbon dioxide within the metamorphic fluid, called the XCO2. If XCO2 is above 0.5, the metamorphic reactions favor formation oftalc,magnesite (magnesium carbonate), andtremolite amphibole. These are classed astalc-carbonation reactions. Below XCO2 of 0.5, metamorphic reactions in the presence of water favor production ofserpentinite.
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 ordolomite and tremolite. Hydrated metamorphic rock assemblages are dominated by the minerals chlorite,serpentine-antigorite andbrucite. 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 anddiopside dominate as the rock mass dehydrates.
Komatiite tends tofractionate from high-magnesium compositions in the flow bases where olivine cumulates dominate, to lower magnesium compositions higher up in the flow. Thus, the current metamorphic mineralogy of a komatiite will reflect the chemistry, which in turn represents an inference as to its volcanologicalfacies and stratigraphic position.
Typical metamorphic mineralogy istremolite-chlorite, ortalc-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 eitherserpentine-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;
| Facies: | Hydrated | Carbonated |
|---|---|---|
| A1 | Chlorite-tremolite | Talc-chlorite-tremolite |
| A2 | Serpentine-tremolite-chlorite | Talc-tremolite-chlorite |
| A3 | Serpentine-chlorite | Talc-magnesite-tremolite-chlorite |
| B1 | Serpentine-chlorite-anthophyllite | Talc-magnesite |
| B2 | Massive serpentine-brucite | Massive talc-magnesite |
| B3 | Serpentine-brucite-chlorite | Talc-magnesite-tremolite-chlorite |
Komatiite can be classified according to the followingIUGS geochemical criteria:[14]
When meeting the above, but the TiO2 is more than 1 wt%, it is classified asmeimechite.
A similar high-Mg volcanic rock isboninite, 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 ofcrystal accumulation (as inperidotite). 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 belamprophyres,kimberlites or other rare ultramafic, potassic orultrapotassic rocks.
Komatiites often showpillow lava structure, autobrecciated upper margins consistent with underwater eruption forming a rigid upper skin to the lava flows. Proximal volcanic facies are thinner and interleaved with sulfidic sediments, black shales,cherts and tholeiiticbasalts. Komatiites were produced from a relatively wetmantle. Evidence of this is from their association withfelsics, occurrences of komatiitictuffs,niobium anomalies and by S- and H2O-borne rich mineralizations.
A common and distinctive texture is known asspinifex texture and consists of longacicular phenocrysts of olivine (orpseudomorphs of alteration minerals after olivine) or pyroxene which give the rock a bladed appearance especially on a weathered surface. Spinifex texture is the result of rapid crystallization of highly magnesian liquid in the thermal gradient at the margin of the flow orsill.
Harrisite texture, first described fromintrusive rocks (not komatiites) atHarris Bay on the island ofRùm inScotland, is formed by nucleation of crystals on the floor of amagma chamber.[15][16] Harrisites are known to form megacrystalaggregates of pyroxene and olivine up to 1 metre in length.[17] 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 ofcumulates has occurred.[18]
| This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(November 2020) (Learn how and when to remove this message) |
Komatiitevolcano morphology is interpreted to have the general form and structure of ashield volcano, typical of most largebasalt edifices, as the magmatic event which forms komatiites erupts less magnesian materials.
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 olivineadcumulate 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;
Individual flow units may not be entirely preserved, as subsequent flow units may thermally erode the A zone spinifex flows. In the distal thin flow facies, B zones are poorly developed to absent, as not enough through-flowing liquid existed to grow the adcumulate.
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 sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(November 2020) (Learn how and when to remove this message) |
Komatiite magma is extremely dense and unlikely to reach the surface, being more likely to pool lower within the crust. Modern (post-2004) interpretations of some of the larger olivine adcumulate bodies in theYilgarn craton have revealed that the majority of komatiite olivine adcumulate occurrences are likely to besubvolcanic tointrusive in nature.
This is recognised at theMt Keithnickel deposit where wall-rock intrusive textures andxenoliths offelsic country rocks have been recognised within the low-strain contacts.[19] 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 bechannelised 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.
| This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(November 2020) (Learn how and when to remove this message) |
The economic importance of komatiite was first widely recognised in the early 1960s with the discovery ofmassive nickel sulfide mineralisation atKambalda, Western Australia. Komatiite-hosted nickel-copper sulfide mineralisation today accounts for about 14% of the world'snickel production, mostly from Australia, Canada and South Africa.
Komatiites are associated with nickel andgold deposits in Australia, Canada, South Africa and most recently in theGuiana shield of South America.
| Types |  | |
|---|---|---|
| Volcanic rocks | ||
| Lists and groups | ||
