KOMATIITE
Komatiites are very rare igneous rocks. They are
one variety of extrusive ultramafic igneous rock (although a komatiite in
Canada has been interpreted to be an intrusive sill). They are named
after the Komati River in South Africa, the type locality. 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), when Earth's heat flux was much higher.
Based on experiments, it's been determined that
komatiite lava, when originally erupted, was considerably hotter than any
modern lava type on Earth. Komatiite lava also had a very low viscosity,
flowed like an ultradense gas, and could solidify into individual lava flows
only 1 cm thick.
The classic texture of komatiites is spinifex
texture, named after clumps of long, spiky (& painful!) grasses common
in Australia. Komatiites with spinifex texture are characterized by
having long blades of olivine mixed with smaller-scale blades of pyroxene.
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.
Two komatiite samples from South Africa and Ontario,
both having excellent spinifex texture, are shown below.
Komatiite (6.3 cm across) showing spinifex texture - long blades of olivine
mixed with pyroxene. This sample is from northeastern South Africa's Barberton
Greenstone Belt. It comes from the Komati Formation (Onverwacht
Volcanics Group). Published dates on this unit range from 3.482 to 3.657
billion years (Paleoarchean). Research on the komatiites from here
indicates that these ultramafic lavas erupted on an ancient oceanic plateau (a
submarine lava plain eruption).
Komatiite (same sample as above; field of view: ~1.8 cm across) showing spinifex
texture (long blades of olivine mixed with pyroxene).
Komatiite (field of view: 3.0 cm across) with spinifex texture. This rock
is from the Pyke Hill Komatiite in Munro Township, east of Timmins, southern
Cochrane District, eastern Ontario, southeastern Canada. Pyke Hill is a
world-class locality for komatiites. The rocks there are part of the
Abitibi Greenstone Belt of Late Archean age. Published dates on
komatiites from Pyke Hill range from 2.675 to 2.746 billion years. The
best available age for this rock appears to be between 2.703 and 2.715 billion
years.
Komatiite (same sample as above; field of view: 3.0 cm across)