ZIRCON FROM THE MANTLE - A NEW WAY TO DATE OLD DIAMONDS

We report the first successful dating by the SHRIMP U-Pb method of a t iny, single zircon inclusion in a natural diamond from a kimberlite at Mbuji Mayi, Zaire. The Mbuji Mayi diamonds are a composite suite of c ontrasting composition, probable age, and origins, which includes diam onds of the typical ultramafic and eclogitic parageneses, as well as m any diamonds featuring a fibrous coating that formed in equilibrium wi th a volatile-rich fluid phase. The zircon-diamond pair is believed to be cogenetic, and the 628 +/- 12 Ma age obtained from the inclusion i s the youngest yet recorded for a diamond originating in the earth's u pper mantle, though still much older than the Cretaceous eruption age of the kimberlite host from which it was recovered. Significantly, the age does not appear to correspond to any known geological event affec ting the Congo craton or adjacent areas as recorded in the currently e xposed rocks. Unlike zircon megacrysts from Mbuji Mayi and elsewhere, the inclusion is U-rich, up to 650 ppm, implying a distinct origin.