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Komatiite

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Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO.

Komatiites were named for their type locality along the Komati 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.

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).

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

Contents

Petrology

Magmas of komatiite compositions 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.

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 10 mm 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) (also known as Group I komatiites) and aluminium depleted komatiite (ADK) (also known as Group II komatiites). 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.

Mineralogy

Komatiites are composed of forsteritic olivine (Fo90 and upwards), calcic and often chromian pyroxene, anorthite (An85 an upwards) and chromite.

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).

Mineral species also encountered in komatiites include pargasitic amphibole (amphibole with >20%MgO), phlogopite, baddeleyite, ilmenite and pyrope garnet.

Geochemistry

Komatiite can be classified according to the following geochemical criteria; The above geochemical classification must be the essentially unaltered magma chemistry and not the result of crystal accumulation (as in peridotite).

Rocks with high MgO, high K2O and Ba, Cs, Rb etc. may be lamprophyres, kimberlites or other rare ultramafic, potassic or ultrapotassic rocks.

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 sulfidic sediments, black shales, cherts and tholeiitic basalts.

Textural features

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.

Volcanology

Komatiite volcano morphology is interpreted to have the general form and structure of a shield volcano, typical of most large basalt 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 facies, 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;

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 sheild 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 has 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 deposit where wall-rock intrusive textures and xenoliths of felsic country rocks have been recognised within the low-strain contacts. The previous interpretations of these large komatiite bodies was that they were "super channels" or reactivated channels, which grew to over 500m 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 1960's 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 nickel production, mostly from Australia, Canada and South Africa.

Komatiites are associated with nickel and gold deposits in Australia, Canada, South Africa and most recently in the Guiana shield of South America.

See also

References

External Links

 

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