The genesis of diamonds can be elucidated by examining jointly their d13C, nitrogen content and aggregation state, as well as the chemical composition of their inclusion minerals. We have studied in this manner diamond suites from seven Souther African kimberlites and report here our findings for Jwaneng, Botswana.
A detailed investigation of 75 diamonds from the Jwaneng kimberlite revealed a d13C range for P-Type (peridotitic) diamonds from -3 to -14 ”. E-Type (eclogitic) diamonds have a d13C range from -4 to -19 ” and more frequently d13C values lower than -10 ”. The nitrogen content of P-Type diamonds tends to be lower than that of the E-Type, however, the degree of nitrogen aggregation of the two types is indistinguishable. P-Type diamonds are more often of octahedral shape and more frequently deformed than E-Type diamonds. These are predominantly colorless, partially resorbed and of irregular shape. The peridotitic mineral inclusions (ol, gt, chr) are enriched in Fe compared to similar suites from other southern African kimberlites. If the results for all of the southern African diamonds are considered together, one finds that the elevated Fe content of olivine inclusions correlates with the range of d13C of their hosts. Based on the olivine inclusion composition and the properties of the host we suggest that Jwaneng P-Type diamonds formed in at least three distinct environments. In the case of E-Type diamonds from this kimberlite no compositional sub-groups could be identified. However, the Mg content of the E-Type mineral suite from Jwaneng tends to be lower than that of E-Type inclusions in diamonds from other southern African kimberlites.
Isotopic, chemical and plastic deformation evidence lead us to conclude that the diamonds of the Jwaneng kimberlite grew in several distinct environments and over a long time span. The data suggest that diamonds precipitated from reduced fluids invading mantle peridotites and eclogites. The silicate inclusion minerals in diamonds were formed during diamond growth as a result of an interaction of the fluid with the local mantle. This process could have started 3.5 Ga ago and continued to the formation and eruption of the kimberlite magma 250 Ma ago. Jwaneng framesites, fine grained diamond aggregates, probably grew late in this process both in the eclogitic and peridotitic mantle. The last phase in the Jwaneng diamond genesis would be the formation of the fibrous diamonds and the fibrous overgrowth on primary growth forms. The fluids responsible for their growth might well also be responsible for the formation of the kimberlite magma.