A komatiite is a high-magnesium, ultramafic lava exhibiting spinifex (a bladed quench pattern, like ice on a window pane) 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 on the Komati River in Barberton, South Africa. Komatiites are very rare in Phanerozoic oro-genic belts and have been recovered from few places, such as fracture zones, on the modern sea floor. 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 2,912-3,272°F (1,600-1,800°C). Since these temperatures are much higher than those in the melting region of the mantle today, and since komatiites are more abundant in Archean greenstone belts than younger orogenic belts, some workers used komatiites as evidence that the Archean mantle was much hotter than the mantle is today. More recent petrological work has shown that the earlier estimates were based on dry melting experiments, and komatiites have water in their structure. Adding water to the melting calculations, new estimates of komatiite source region melting temperatures fall in the range of 2,192-2,552°F (1,200-1,400°C), much more similar to present-day mantle temperatures.

See also Archean; atmosphere; continental crust; craton; greenstone belts; life's origins and early evolution; Proterozoic.

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