Komatiites: An early precambrian phenomenon

Using the presently accepted definition, komatiites are ultramafic volcanic rocks characterised by bladed and/or skeletal olivine texture (spinifex texture). Where textural evidence is absent,the geochemical definition is equivocal and as a result there is considerable ambiguity, particularly among the basaltic types. If the added control of Al2O3/TiO2 ? 25 is added to the definition, this eliminates rocks of the boninite group and also the low-Ti basalts found in some ophiolite complexes. The bulk of komatiites occur in the Archaean and the most obvious explanation is that the eruption of high (up to 30%) MgO melts was controlled by an enhanced geothermal gradient. There is some evidence to suggest that the high CaO/Al2O3 ratio characteristic of some komatiites (e.g. Barberton) is found only in those greenstone belts older than 3.0 b.y