Origin of diamonds in old enriched mantle

Sub-calcic garnets encapsulated by diamonds from relatively young (90 Myr) kimberlites in southern Africa, yield ancient Sm-Nd and Rb-Sr model ages (3,200-3,300 Myr). The chemistry and distribution of these and associated sub-calcic garnets from kimberlite concentrate, indicate diamonds formed following enrichment of residual sub-cratonic mantle such as that remaining after widespread extraction of 3,500-Myr komatiitic lavas.