THE CARBON ISOTOPIC COMPOSITION AND NITROGEN-CONTENT OF LITHOSPHERIC AND ASTHENOSPHERIC DIAMONDS FROM THE JAGERSFONTEIN AND KOFFIEFONTEIN KIMBERLITE, SOUTH-AFRICA

Asthenospheric diamonds, from the Jagersfontein kimberlite, South Africa, have a delta-C-13 range from -18.9 to -24.4 parts per thousand vs. PDB. Deep diamonds from the nearby Koffiefontein mine have delta-C-13 values between -2.9 and -6.1 parts per thousand. At these two locations asthenospheric diamonds are more frequently nitrogen Type II than Type I. If they are Type I, the nitrogen content is low and its state of aggregation is high. Diamonds from the Jagersfontein kimberlite have a bimodal delta-C-13 distribution. On the basis of the carbon isotopic composition of the hosts and their mineral inclusion composition, one can suggest that as many as six distinct diamond source regions were sampled by the kimberlite. In the mantle beneath Jagersfontein, the relative abundance of low delta-C-13 diamonds may be higher in the asthenosphere than in the lithosphere. In the Koffiefontein sample suite, asthenospheric and lithospheric diamonds are indistinguishable in delta-C-13. The differences in the nature of the relationships between isotope and inclusion chemistry demonstrated by deep diamonds from two diatremes, which are very closely associated in space and time, indicate a high degree of complexity in mantle carbon geochemistry even within the asthenosphere.