Spinifex metakomatiite (serpentinite) (Komati Formation, Paleoarchean, 3.481-3.482 Ga; Komati River Valley, South Africa) 24

            Spinifex metakomatiite (serpentinite) with weathering rind from the Precambrian of South Africa. (cut surface; USNM 116690-1, National Museum of Natural History, Smithsonian Institution, Washington D.C., USA; public domain photo provided by the USNM)

Komatiites are very rare, magnesium-rich, ultramafic, extrusive igneous rocks.  They are named after the Komati River Valley in South Africa.  Komatiite is an exceedingly rare type of lava.  No volcano on Earth erupts this material today.  Komatiites are essentially restricted to the Archean (4.55 to 2.5 billion years ago).  Experimental evidence has shown that komatiite lavas, when originally erupted, were considerably hotter (~1600º C) than any modern lava type on Earth.  This indicates that Earth’s mantle was much hotter than at present.  Other geologic evidence indicates that early Earth’s heat flux was much higher than now.
 
Komatiite lava had a very low viscosity - it could flow like an ultradense gas.  This property permitted the solidification of some individual lava flows that are only 1 centimeter thick.
 
The classic texture of komatiites is spinifex texture, named after clumps of long, spiky grasses.  Komatiites with spinifex texture have relatively thin plates of olivine and pyroxene - in cross-section, they look spike-shaped.
 
All Archean komatiites are metamorphosed - the original igneous mineralogy (olivine, pyroxene, minor chromite, etc.) is gone to mostly gone.  Such rocks are best termed metakomatiites, but the prefix “meta-” is usually not specified.
 
Komatiites have economic significance, as many are closely associated with copper-nickel minerals (chalcopyrite & pentlandite), plus minor platinum-group elements, arsenides, bismuthides, and maybe a little gold and silver.  Komatiites are a world-class source of nickel in Canada and Western Australia.

The sample seen here is from the Komati Formation at the type locality - South Africa's Komati River Valley.  The original igneous rock consisted of blades of olivine mixed with pyroxene.  Greenschist-facies regional metamorphism has altered the original olivine spinifex komatiite into serpentinite, but the spinifex texture still preserved (click once or twice on the photo to zoom in).

Komati Formation komatiites are the oldest well-preserved examples in the world.  Published research indicates that these ultramafic lavas erupted on an ancient oceanic plateau (a submarine lava plain environment).

Stratigraphy: Komati Formation, Onverwacht Volcanics Group, Swaziland sequence, Barberton Greenstone Belt, Paleoarchean, 3.481-3.482 Ga

Locality: Spinifex Stream Area, Komati River Valley, northeastern South Africa

Taken on: March 7, 2003

Tags:

spinifex

metakomatiite

serpentinite

Komati

Formation

Precambrian

Archean

Paleoarchean

River

Valley

South

Africa

Stream

Area

Barberton

Greenstone

Belt

Onverwacht

Volcanics

Group

Swaziland

sequence « less

Sets appears in: • South Africa Rocks & Fossils • Archean Rocks, Minerals, Fossils more » • Komatiites « less

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