Zones of crustal weakness, representing foci of the mantle, have persisted for at least 2500 m.y., as shown by orogens and abundance of mafic igneous rocks. Karroo lavas are especially indicative of such zones. Diamonds in kimberlite and nickel associated with mafic and ultramafic rocks suggest a mantle source. The distribution of known diamond and nickel deposits is closely related to the zones of crustal weakness, and particularly to areas where two or more zones intersect. Such areas are exploration targets.

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