Geochemistry
Komatiite can be classified according to the following geochemical criteria;
- SiO2; typically 40 - 45%
- MgO greater than 18%
- Low K2O (<0.5%)
- Low CaO and Na2O (<2% combined)
- Low Ba, Cs, Rb (incompatible element) enrichment; ΣLILE <1,000ppm
- High Ni (>400ppm), Cr (>800ppm), Co (>150ppm)
The above geochemical classification must be the essentially unaltered magma chemistry and not the result of crystal accumulation (as in peridotite). Through a typical komatiite flow sequence the chemistry of the rock will change according to the internal fractionation which occurs during eruption. This tends to lower MgO, Cr, Ni towards the top, and increases Al, K2O, Na and CaO and SiO2 toward the top of the flow.
Rocks with high MgO, high K2O and Ba, Cs, Rb etc. may be lamprophyres, kimberlites or other rare ultramafic, potassic or ultrapotassic rocks.
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