Petrology
Magmas of komatiite compositions have a very high melting point with calculated eruption temperatures in excess of 1600 °C. Basaltic lavas 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 Archean Earth.
Komatiitic lava would have behaved as a supercritical fluid when it erupted (possessing the viscosity of gas but with the density of rock). Compared to the basaltic lava of the Hawaiian plume basalts at ~1200 °C which behaves as treacle or honey, the komatiitic lava would have flowed swiftly 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.
Komatiite chemistry is thought to be 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. Kimberlite, another magnesium-rich igneous rock, is relatively rich in potassium and in other incompatible elements, and is thought to form as a result of less than a percent or so of partial melting fluxed by water and carbon dioxide.
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 modeled by melting experiments as being produced by high degrees of partial melting of hydrous mantle at low pressure where Al-bearing pyroxenes in the source are not melted, whereas Al-undepleted komatiites are produced by high degree partial melts at greater depth, allowing melting of Al-rich pyroxene.
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 large-ion lithophile elements (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.
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