Komatiites

Komatiites are ultramafic volcanic rocks named for the Komati River area of South Africa. They are characterized by very high magnesium oxide contents, typically greater than about 18–20 wt%, and by primitive, high-temperature mantle-derived compositions. Most well-preserved examples are from the Archean crust, though later occurrences are known, making them important indicators of early Earth mantle dynamics.

A hallmark feature of komatiites is their distinctive texture, especially spinifex texture, in which elongated, needle-like olivine (and sometimes pyroxene) crystals form during rapid cooling of hot, relatively low-silica melts. This texture, together with their high MgO, distinguishes them from more common basaltic rocks. Their chemistry generally shows low silica and alkali contents, with compatible elements and nickel-sulfur enrichment reflecting mantle-derived, ultramafic melts.

Formation occurs when unusually hot mantle melts, produced at high degrees of partial melting, erupt or intrude into crust before significant crystallization. The result is lava flows and sills that can preserve spinifex textures. Because the Earth’s mantle cooled over geological time, komatiites are far more common in the Archean crust and are rare in younger rocks.

Geological significance includes their use as proxies for high-temperature mantle processes and their association with nickel-copper-PGE sulfide deposits. Komatiites are widely distributed in ancient greenstone belts across continents, including South Africa, Australia, Canada, Greenland, and Finland, offering clues about early crust formation and tectonics.