Carbon isotopic composition, nitrogen content and inclusion composition of diamonds from the Roberts Victor kimberlite, South Africa: Evidence for ¹³C depletion in the mantle

The C isotopic composition and N content of a suite of diamonds of known inclusion mineral composition from the Roberts Victor kimberlite have been measured. The mean ¹³C-content of diamonds containing peridotitic minerals ( X ̄ = -5.4%. vs. PDB, s = ±0.9%., n = 65) does not differ significantly from those containing sulfides ( X ̄ = -4.9%., s = ± 0.9%., n = 20). Diamonds containing eclogitic minerals can be subdivided into two groups based on their carbon isotopic composition: Group-A ( X ̄ = -15.5%., s = ±0.4%., n = 10) and Group-B ( X ̄ = -5.6%., s = ±0.6%., n = 4). The clinopyroxenes occluded by the Group-A diamonds are depleted in SiO₂, MgO, and CaO and significantly enriched in Al₂O₃, FeO, and MnO compared to clinopyroxenes occluded by Group-B diamonds. Carbon in two graphite-diamond eclogites has a mean isotopic composition of -5.31%.; in both samples graphite shows a slight enrichment in ¹³C compared to the coexisting diamond. The range of N concentrations of diamonds containing peridotitic or eclogitic minerals is essentially the same (0 to 750 ppm), while it is considerably larger for sulfide-containing diamonds (0 to 1700 ppm). There is no difference in the C isotopic composition between Type I and Type II diamonds for sulfide and peridotitic minerals occluding diamonds. All Type II diamonds containing eclogitic minerals belong to Group-A. No correlation between N content and C isotopic composition could be established, although a large range in both variables is observed for the sample suite. The composition of eclogitic minerals included in diamonds of low ¹³C-content differs from that of eclogite xenoliths characterized by ¹⁸O-depletions, which have been related to subduction processes. Hence the data available do not suggest a common cause for the depletion of the heavy isotopes of the two elements. The chemical and isotopic characteristics of the suite of diamond samples reflect different mantle environments. Diamonds depleted in ¹³C (δ¹³C = -15 to -16%.) come from a region at greater depth than those of ¹³C contents of -5 to -6%.. The source region of the former is characterized by higher Fe, Mn, Al, and lower Mg, Ca, Si, and N contents than that of the latter.