Nitrogen and 13C content of Finsch and Premier diamonds and their implications

Abstract

The nitrogen content and state of aggregation have been determined spectroscopically for diamonds from the Finsch (93) and the Premier (116) kimberlite, South Africa. The ranges of concentration for diamonds with eclogitic (E-Type) and peridotitic (P-Type) mineral inclusions are: Finsch: E-Type, 29 to 1639 ppm, P-Type, 7 to 1206 ppm; Premier: E-Type, 15 to 1359 ppm, P-Type, 7 to 1086 ppm. In both kimberlites nitrogen-free diamonds (Type II) contain less frequently eclogitic inclusions. Among E-Type diamonds there are none of Type II in the Finsch and 1% in the Premier sample suite; for P-Type diamonds the respective percentages are 29 and 25%. There is no significant difference in the nitrogen aggregation state between E-Type and P-Type diamonds within each kimberlite; however, there is a significant difference between the two locations. The nitrogen in the B form amounts to 26 ± 29% (n = 65) in Finsch and to 48 ± 26% (n = 101) in Premier diamonds. No correlation between δ 13C and nitrogen content or aggregation state is observed. However, within the eclogitic and peridotitic paragenesis, groups of diamonds of very similar nitrogen concentration, δ 13C, and mineral inclusion composition can be recognized. One of these may be common to both kimberlites. Some of the diamonds in the two kimberlites may have had a common origin but subsequent thermal histories, which differed for the sample suites from the two diatremes. The nitrogen content of the diamonds is a function of local nitrogen concentration, temperature and oxygen fugacity. The dependence on the last of these can cause complex relationships between δ 15N, δ 13C and nitrogen content of diamonds.