Twenty-one sulphide inclusion-bearing diamonds from the Finsch mine, South Africa, were analysed for nitrogen abundances and carbon isotope compositions by microbeam methods. On the basis of sulphide Ni contents, one diamond is of peridotitic affinity, the rest belongs to the eclogitic suite.
FTIR analyses show nitrogen abundances and aggregation states from 21 to 1093 at.ppm and 0% to 83% IaB, statistically indistinguishable from previous results for Finsch eclogitic silicate inclusion-bearing diamonds (Appleyard et al., 2004) but significantly higher than observed before for diamonds of the peridotitic suite (Deines et al., 1989). Detailed analyses revealed marked variations in nitrogen characteristics within individual diamonds, demonstrating a complex mantle residence, consistent with multiple episodes of diamond growth over time.
Linked to the growth stratigraphy of the diamond, SIMS micro-analyses show variations in δ13C from −8.90‰ to −2.80‰ with a mean value of −5.54 ± 1.80‰ (1 standard deviation), closely overlapping the typical worldwide value. The C-isotopic variability within individual diamonds ranges up to 3.26‰. SIMS based nitrogen abundances are 3–2221 at.ppm with heterogeneous distribution within individual diamond.
From the δ13C–[N] co-variations within individual diamonds, three major processes of diamond growth for sulphide inclusion-bearing samples at Finsch are proposed. (1) Some diamonds were precipitated during a single event of open system isotopic fractionation, in fluids that varied from oxidised (carbonatitic) to reduced (CH4-rich). In this growth scenario, nitrogen is either compatible or incompatible during diamond growth. (2) Other diamonds show abrupt δ13C–[N] changes indicative of diamond growth involving mixing of several fluid sources. (3) Some diamonds grow from a combination of the two previous processes.
The models are consistent with metasomatic diamond growth involving single and multiple fluid sources. Multiple growth stages for individual diamonds may have taken place over extended time periods, and which have important implications for diamond dating studies.