Komatiite

Komatiite lava at the type locality in the Komati Valley, Barberton Mountainland, South Africa, showing the distinctive "spinifex texture" formed by dendritic plates of olivine (scale shown by a hammer on the right edge of photo)

Komatiite /koʊˈmɑːtiˌaɪt/ is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava of at least 18 wt% magnesium oxide (MgO).[1] It is classified as a 'picritic rock'. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa,[2] and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.[3]

Komatiites are rare rocks; almost all komatiites were formed during the Archaean Eon (4.03–2.5 billion years ago), with few younger (Proterozoic or Phanerozoic) examples known. This restriction in age is thought to be due to cooling of the mantle, which may have been 100–250 °C (212–482 °F) hotter during the Archaean.[4][5] The early Earth had much higher heat production, due to the residual heat from planetary accretion, as well as the greater abundance of radioactive isotopes, particularly shorter lived ones like uranium 235 which produce more decay heat. Lower temperature mantle melts such as basalt 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 areas, and occur with other ultramafic and high-magnesian mafic volcanic rocks in Archaean greenstone belts. The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau off the Pacific coast of Colombia, and a rare example of Proterozoic komatiite is found in the Winnipegosis komatiite belt in Manitoba, Canada.

  1. ^ Le Bas, M. J. 2000. IUGS reclassification of the high-Mg and picritic volcanic rocks. Journal of Petrology, 41(10), 1467–1470. https://doi.org/10.1093/petrology/41.10.1467
  2. ^ Viljoen, M. J., & Viljoen, R. P. 1969a. Evidence for the existence of a mobile extrusive peridotitic magma from the Komati Formation of the Onvernacht Group. Geological Survey of South Africa, Special Publication, 21, 87 – 112.
  3. ^ Arndt, N., Lesher, C. M., & Barnes, S. J. 2008. Komatiite. Cambridge: Cambridge University Press.
  4. ^ Davies, G. F. 1999. Plates, Plumes and Mantle Convection. Cambridge: Cambridge University Press.
  5. ^ Herzberg, C., Condie, K., & Korenaga, J. 2010. Thermal history of the Earth and its petrological expression. Earth and Planetary Science Letters, 292(1–2), 79–88. https://doi.org/10.1016/j.epsl.2010.01.022