A high-magnesium ultramafic lava exhibiting
spinifex (quench) textures as shown by bladed olivine or
pyroxene crystals. The composition of komatiite may range
from peridotite, with 30 percent MgO and 44 percent SiO2, to
basalt, with 8 percent MgO and 52 percent SiO2. The name is
from the type section where these rocks were first identified
on the Komati River in Barberton, South Africa. Komatiites
are very rare in Phanerozoic orogenic belts and have been
recovered from few places such as fracture zones on the modern
seafloor. They are more abundant but still rare in Archean
greenstone belts. Early work on komatiites suggested that they
reflected high degrees of partial melting of a high-temperature
mantle, with mantle melting temperatures estimated to be as
high as 2,912°F–3,272°F (1,600°C–1,800°C). Since these temperatures
are much higher than those in the melting region of
the mantle today, and komatiites are more abundant in
Archean greenstone belts than in younger orogenic belts, some
workers used komatiites as evidence that the Archean mantle
was much hotter than the mantle is today. However, more
recent petrological work has shown that the earlier estimates
were based on dry melting experiments, and komatiites have
water in their structure. Therefore, by adding a small percent
of water to the melting calculations, new estimates of komatiite
source region melting temperatures fall in the range of
2,192°F–2,552°F (1,200°C–1,400°C), much more similar to
present-day mantle temperatures.
See also ARCHEAN; CRATONS; GREENSTONE BELTS.
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