komatiitic

Komatiitic is a geological descriptor used for rocks, magmas, and related processes that have a komatiite composition. Komatiites are ultramafic volcanic rocks characterized by very high magnesium content, low silica, and typically ancient crustal associations. The name derives from the Komati River in South Africa, where such rocks were first described. They are dominated by olivine and pyroxene, often with accessory chromite or spinel, and many display spinifex textures—elongated, needle-like crystals formed during rapid cooling of highly viscous magmas.

Geochemically, komatiitic rocks usually have MgO contents above roughly 18–20% and SiO2 well below 50%, highlighting their ultramafic nature. They are most abundant in Archean greenstone belts, dating from about 4.0 to 2.5 billion years ago, though younger examples exist. Their prevalence is interpreted as evidence of higher mantle temperatures in the early Earth, allowing unusually magnesium-rich melts to form and crystallize.

Formation and emplacement involve high-temperature mantle-derived magmas that often erupt as lava flows or intrude as ultramafic sills. The textures and mineralogy reflect rapid cooling and extreme early-domain melting conditions. Komatiites are also noted for their potential economic significance, as some host nickel-copper-PGE sulfide deposits associated with ultramafic magmas and sulfide saturation.

In summary, komatiitic describes rocks, magmas, or settings characterized by komatiite-like ultramafic composition, with high Mg, low Si, distinctive textures, and a key place in studies of early Earth geodynamics.