Carbon isotope ratios, nitrogen content and aggregation state, and inclusion chemistry of diamonds from Jwaneng, Botswana

A detailed study of diamonds from the Jwaneng kimberlite leads to the conclusion that diamonds formed in at least seven physically or chemically distinct environments and over an extended time interval which may have lasted as long as 3 Ga. The work revealed a δ¹³C range for P-Type (peridotitic) diamonds from -3 to -14 ‰; E-Type (eclogitic) diamonds have a δ¹³C range from -4 to -19‰ and more δ¹³C values lower than -10‰. The nitrogen content of P-Type diamonds tends to be lower than that of the E-Type, but the degree of nitrogen aggregation of the two types is indistinguishable. Among those E-Type diamonds for which a nitrogen aggregation age could be computed, low δ¹³C diamonds tend to show older ages. P-Type diamonds are more often octahedral and green coated or brown and deformed than E-Type diamonds. The latter are more often colorless, partially resorbed, and of irregular shape. It is postulated that diamonds containing olivines formed in at least three different environments based on their nitrogen content, δ¹³C value, and the chemical composition of the inclusions. The Jwaneng peridotitic minerals inclusion suite (ol, gt, chr) is enriched in Fe compared to similar suites from other southern African kimberlites. Considering these suits together one observes that the Fe content of olivine inclusions is negatively correlated with the δ¹³C of their hosts. In the suite of E-Type diamonds no compositional subgroups could be identified, but, the Mg content of the E-Type mineral suite tends to be lower than that of E-Type inclusions in diamonds from other southern African kimberlites. It is suggested that as many as four distinct time/temperature histories for E-Type diamonds can be discerned based on coexisting garnet/clinopyroxene equilibration temperatures, the nitrogen content, and the state of nitrogen aggregation in the host diamonds.