Komatiite Lava in South Africa - CSIRO ScienceImage 11033

Komatiite is a type of

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

mantle-derived

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

defined as having crystallised from a

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

of at least 18 wt%

magnesium oxide Magnesium oxide (MgO), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...

(MgO). It is classified as a 'picritic rock'. Komatiites have low

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

and

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

, and high to extremely high

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

content. Komatiite was named for its type locality along the

Komati River The Komati River, also known as the Inkomati River or Incomati River (in Mozambique, from Portuguese Rio Incomati), is a river in South Africa, Eswatini and Mozambique. Originating in north-western Eswatini, it is joined by the Crocodile Riv ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

, and frequently displays spinifex texture composed of large dendritic plates of

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

and

pyroxene The pyroxenes (commonly abbreviated Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron ( ...

. Komatiites are rare rocks; almost all komatiites were formed during the Archaean Eon (4.03–2.5 billion years ago), with few younger (

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

Phanerozoic The Phanerozoic is the current and the latest of the four eon (geology), geologic eons in the Earth's geologic time scale, covering the time period from 538.8 million years ago to the present. It is the eon during which abundant animal and ...

) examples known. This restriction in age is thought to be due to cooling of the mantle, which may have been hotter during the Archaean. The early Earth had much higher heat production, due to the residual heat from planetary accretion, as well as the greater abundance of

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

isotopes, particularly shorter lived ones like uranium 235 which produce more

decay heat Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha particle, alpha, Beta particle, beta or gamma radiation is converted into the thermal movement ...

. Lower temperature mantle melts such as

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

and picrite have essentially replaced komatiites as an eruptive lava on the Earth's surface. Geographically, komatiites are predominantly restricted in distribution to the Archaean

shield A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry like spears or long ranged projectiles suc ...

areas, and occur with other

and high-magnesian

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

volcanic rocks in Archaean

greenstone belt Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies. The name comes from the green h ...

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

oceanic plateau An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides. There are 184 oceanic plateaus in the world, covering an area of or about 5.11% of the o ...

off the Pacific coast of

Colombia Colombia, officially the Republic of Colombia, is a country primarily located in South America with Insular region of Colombia, insular regions in North America. The Colombian mainland is bordered by the Caribbean Sea to the north, Venezuel ...

, and a rare example of Proterozoic komatiite is found in the Winnipegosis komatiite belt in

Manitoba Manitoba is a Provinces and territories of Canada, province of Canada at the Centre of Canada, longitudinal centre of the country. It is Canada's Population of Canada by province and territory, fifth-most populous province, with a population ...

, Canada.

Petrology

Magmas of komatiitic compositions have a very high

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

, with calculated eruption temperatures up to, and possibly in excess of 1600 °C.

Basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

s 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 gradient Geothermal gradient is the rate of change in temperature with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plat ...

s in the Archaean Earth. Komatiitic lava was extremely fluid when it erupted (possessing the

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

close to that of water but with the density of rock). Compared to the basaltic lava of the Hawaiian plume basalts at ~1200 °C, which flows the way

treacle Treacle () is any uncrystallised syrup made during the refining of sugar.Oxford Dictionary The most common forms of treacle are golden syrup, a pale variety, and black treacle, a darker variety similar to molasses. Black treacle has a distinctiv ...

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 A lava tube, more rarely called a pyroduct, is a 'roofed conduit through which molten lava travels away from its vent'. If lava in the tube drains out, it will leave an empty cave. Lava tubes are common in low-viscosity volcanic systems. La ...

, 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 Partial melting is the phenomenon that occurs when a rock is subjected to temperatures high enough to cause certain minerals to melt, but not all of them. Partial melting is an important part of the formation of all igneous rocks and some metamorp ...

. 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 element In petrology and geochemistry, an incompatible element is one that is unsuitable in size and/or charge to the cation sites of the minerals in which it is included. It is defined by a partition coefficient between rock-forming minerals and melt ...

s. 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 petrological 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 Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

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 chrome spinels 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 Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

. Boninites with 10–18% MgO tend to have higher large-ion lithophile elements (LILE: Ba, Rb, Sr) than komatiites.

Mineralogy

The pristine volcanic mineralogy of komatiites is composed of forsteritic olivine (Fo90 and upwards), calcic and often chromian

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

chromite Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of Iron, FeChromium, Cr2Oxygen, O4. It is an oxide mineral belonging to the spinel group. The ...

. A considerable population of komatiite examples show a cumulate texture and

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

. The usual cumulate

mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific s ...

is highly magnesium rich

forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...

olivine, though chromian pyroxene cumulates are also possible (though rarer). Volcanic rocks rich in magnesium may be produced by accumulation of olivine

phenocrysts 300px, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid shapes when cut through). 1 euro coin (diameter 2.3 cm) for scale. A phenocryst is an early forming, relatively large and usually conspic ...

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 Texture may refer to: Science and technology * Image texture, the spatial arrangement of color or intensities in an image * Surface texture, the smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface c ...

, a texture attributable to rapid

crystallization Crystallization is a process that leads to solids with highly organized Atom, atoms or Molecule, molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regu ...

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 Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

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 A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

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 A stratigraphic section is a sequence of layers of rocks in the order they were deposited. It is based on the principle of original horizontality, which states that layers of sediment are originally deposited horizontally under the action of ...

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 Augite, also known as Augurite, is a common rock-forming pyroxene mineral with formula . The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees. Characteristics Augite is a solid soluti ...

, pigeonite and bronzite,

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printi ...

and rarely pargasitic

amphibole Amphibole ( ) is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

. Secondary (metamorphic) minerals include serpentine,

chlorite The chlorite ion, or chlorine dioxide anion, is the halite (oxyanion), halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as s ...

, amphibole, sodic plagioclase,

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, iron oxides and rarely

phlogopite Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica. Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3 ...

baddeleyite Baddeleyite is a rare zirconium oxide mineral (ZrO2 or zirconia), occurring in a variety of monoclinic prismatic crystal forms. It is transparent to translucent, has high Index of refraction, indices of refraction, and ranges from colorless to yel ...

, and

pyrope The mineral pyrope is a member of the garnet group. Pyrope is the only member of the garnet family to always display red colouration in natural samples, and it is from this characteristic that it gets its name: from the Greek words for ''fire'' ...

or hydrogrossular

Metamorphism

All known komatiites have been

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

, therefore should technically be termed 'metakomatiite' though the prefix meta is inevitably assumed. Many komatiites are highly altered and serpentinized or carbonated from metamorphism and

metasomatism Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is traditionally defined as metamorphism which involves a change in the chemical com ...

. This results in significant changes to the mineralogy and the texture.

Hydration vs. carbonation

The metamorphic mineralogy of ultramafic rocks, particularly komatiites, is only partially controlled by composition. The character of the

connate fluids In geology and sedimentology, connate fluids are liquids that were trapped in the pores of sedimentary rocks as they were deposited. These liquids are largely composed of water, but also contain many mineral components as ions in solution. As r ...

which are present during low temperature metamorphism whether prograde or retrograde control the metamorphic assemblage of a metakomatiite (''hereafter the prefix meta- is assumed''). The factor controlling the mineral assemblage is the

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

within the metamorphic fluid, called the XCO₂. If XCO₂ is above 0.5, the metamorphic reactions favor formation of

talc Talc, or talcum, is a clay mineral composed of hydrated magnesium silicate, with the chemical formula . Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent and lubricant ...

magnesite Magnesite is a mineral with the chemical formula ( magnesium carbonate). Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts. Occurrence Magnesite occurs as veins in and an alteration product of ultramafic r ...

(magnesium carbonate), and

tremolite Tremolite is a member of the amphibole group of silicate minerals with composition Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz, and occurs in two distinct forms, crystals and fib ...

amphibole. These are classed as talc-carbonation reactions. Below XCO₂ of 0.5, metamorphic reactions in the presence of water favor production of

serpentinite Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

. 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- antigorite and

brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Magnesium, Mg(hydroxyl, OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal Vein (geology), vein mineral in metamorphosed li ...

. 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 Anthophyllite is an orthorhombic amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anth ...

, enstatite, olivine and diopside dominate as the rock mass dehydrates.

Mineralogic variations in komatiite flow facies

Komatiite tends to fractionate 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 volcanological

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

and stratigraphic position. Typical metamorphic mineralogy is

, or

-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-

+/-

if hydrated, or talc-

if carbonated. The upper flow facies tend to be dominated by talc, chlorite, tremolite, and other magnesian amphiboles (

cummingtonite Cummingtonite ( ) is a metamorphic amphibole with the chemical composition , magnesium iron silicate hydroxide. Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common ...

, gedrite, etc.). For example, the typical flow facies (see below) may have the following mineralogy;

Geochemistry

Komatiite can be classified according to the following

IUGS The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to global cooperation in the field of geology. As of 2023, it represents more than 1 million geoscientists around the world. About Fo ...

geochemical criteria: * SiO₂ less than 52 wt% * MgO more than 18 wt% * K₂O + Na₂O less than 1 wt% * TiO₂ less than 1 wt% 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 crystal accumulation (as in

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

). 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

lamprophyre Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica- undersaturated mafic or ultramafic rocks with high magnesium o ...

kimberlite Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley, Northern Cape, Kimberley in South Africa, where the discovery of an 83.5-Car ...

s or other rare ultramafic, potassic or

ultrapotassic Ultrapotassic igneous rocks are a class of rare, volumetrically minor, generally ultramafic or mafic silica-depleted igneous rocks. While there are debates on the exact classifications of ultrapotassic rocks, they are defined by using the chemica ...

rocks.

Morphology and occurrence

Komatiites often show

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discontinu ...

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,

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

s and tholeiitic basalts. Komatiites were produced from a relatively wet mantle. Evidence of this is from their association with

felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

s, occurrences of komatiitic

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

niobium Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...

anomalies and by S- and H₂O-borne rich mineralizations.

Textural features

CSIRO ScienceImage 1484 Komatiite Pyroxene Needles

A common and distinctive texture is known as ''spinifex texture'' and consists of long acicular phenocrysts of olivine (or

pseudomorph In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced b ...

s 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 or sill. ''Harrisite texture'', first described from

intrusive rock Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''Igneous intrusion, intrusions'', such as batholiths, dike (geology), dikes, Sill (geology), sills, laccoliths, and volcanic necks.I ...

s (not komatiites) at Harris Bay on the island of

Rùm Rùm (), a Scottish Gaelic language, Scottish Gaelic name often Anglicisation, anglicised to Rum ( ), is one of the Small Isles of the Inner Hebrides, off the west coast of Scotland, in the district of Lochaber. For much of the 20th century the ...

Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...

, is formed by nucleation of crystals on the floor of a

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it u ...

. 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 cumulates has occurred. KomatiiteA2

Volcanology

Komatiite

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

morphology is interpreted to have the general form and structure of a

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...

, typical of most large

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 olivine adcumulate 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; * A1 – pillowed and variolitic chilled flow top, often grading and transitional with sediment * A2 – Zone of quickly chilled, feathery acicular olivine-clinopyroxene-glass representing a

chilled margin A chilled margin is a shallow intrusive or volcanic rock texture characterised by a glassy or fine-grained zone along the margin where the magma or lava has contacted air, water, or particularly much cooler rock. This is caused by rapid sol ...

on the top of the flow unit * A3 – Olivine spinifex sequence composed of sheaf and book-like olivine spinifex, representing a downward-growing crystal accumulation on the flow top * B1 – Olivine mesocumulate to orthocumulate, representing a harrisite grown in flowing liquid melt * B2 – Olivine adcumulate composed of >93% interlocking equant olivine crystals * B3 – Lower chill margin composed of olivine adcumulate to mesocumulate, with finer grain size. 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.

Intrusive komatiites

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 the Yilgarn craton have revealed that the majority of komatiite olivine adcumulate occurrences are likely to be subvolcanic to intrusive in nature. This is recognised at the Mt Keith

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

deposit where wall-rock intrusive textures and

xenolith A xenolith ("foreign rock") is a rock (geology), rock fragment (Country rock (geology), country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusi ...

s of

country rocks have been recognised within the low-strain contacts.Rosengren, N. M., Beresford, S. W., Grguric, B. A., & Cas, R.A.F. 2005. An intrusive origin for the komatiitic dunite-hosted Mount Keith disseminated nickel sulfide deposit, Western Australia. Economic Geology, 100(1), 149–156. https://doi.org/10.2113/100.1.0149 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.

Economic importance

The economic importance of komatiite was first widely recognised in the early 1960s with the discovery of massive nickel sulfide mineralisation at Kambalda, Western Australia. Komatiite-hosted nickel-copper sulfide mineralisation today accounts for about 14% of the world's

production, mostly from Australia, Canada and South Africa. Komatiites are associated with nickel and

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

deposits in Australia, Canada, South Africa and most recently in the

Guiana shield The Guiana Shield (; ; ; ) is one of the three cratons of the South American Plate. It is a 1.7 billion-year-old Precambrian geological formation in northeast South America that forms a portion of the northern coast. The higher elevations on ...

of South America.

References

Bibliography

* Hess, P. C. (1989), ''Origins of Igneous Rocks'', President and Fellows of Harvard College (pp. 276–285), . * Hill R.E.T, Barnes S.J., Gole M.J. and Dowling S.E. (1990), ''Physical volcanology of komatiites; A field guide to the komatiites of the Norseman-Wiluna Greenstone Belt, Eastern Goldfields Province, Yilgarn Block, Western Australia.'', Geological Society of Australia. * Blatt, Harvey and Robert Tracy (1996), ''Petrology'', 2nd ed., Freeman (pp. 196–7), . * 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. * Arndt, N.T., and Nisbet, E.G. (1982), ''Komatiites''. Unwin Hyman, . Hardcover. * Arndt, N.T., and Lesher, C.M. (2005), Komatiites, in Selley, RC, Cocks, L.R.M., Plimer, I.R. (Editors), ''Encyclopedia of Geology'' 3, Elsevier, New York, pp. 260–267 * Faure, F., Arndt, N.T. Libourel, G. (2006), Formation of spinifex texture in komatiite: An experimental study. J. Petrol 47, 1591–1610. * Arndt, N.T., Lesher, C.M. and Barnes, S.J. (2008), ''Komatiite'', Cambridge University Press, Cambridge, 488 pp., .

External links

accessed 7-25-2005
Komatiites and astrobiology

(with text in French) retrieved 2009-05-17 {{Volcanoes Ultramafic rocks Volcanology Igneous petrology Metamorphic rocks Volcanic rocks Subvolcanic rocks