The formation of diamond through metasomatic events, from volatiles-enriched fluids/melts brought into pre-existing mantle rocks, has been suggested from a series of independent studies. However, the link between these hypothetical volatile-rich fluids and deep-seated mineral inclusions (mostly silicate and sulphides) entrapped in diamonds remains unclear, yet the relationship between these two species may provide the key to our understanding of diamond crystallization. In order to address the relationship between the origin and formation of diamonds and their mineral inclusions, we carried out the first coupled stable isotopic study (δ13C, δ15N and multiple S-isotopes as δ34S, δ33S, Δ33S) of diamonds containing sulphide inclusions, using samples from the Jwaneng kimberlite as our model system. Sulphides extracted from the present collection of 55 diamonds belong to either eclogitic (E-type, 52 diamonds) or peridotitic (P-type, 3 diamonds) paragenetic suites, as attested by their Cr and Ni content (Cr < 0.03 wt.% for E-types and Cr > 0.18 wt.% and Ni > 14 wt.% for P-types). Sulphur isotopic compositions have been measured in-situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds (n = 4) lie on the terrestrial fractionation line whereas Mass Independent sulphur isotope Fractionations (MIF) are preserved inside eclogitic sulphides inclusions (- 0.5‰ < Δ33S < + 0.9‰, n = 33). Such large MIF values, extended here to the negative values recorded in Archean sediments, are thought to be produced through photochemical reactions in the Archean atmosphere, which implies that sulphides inclusions contain an Archean sedimentary sulphur component that was transferred to the Earth's mantle. In contrast to this isotopic dichotomy between E-type and P-type sulphides, the geochemical characteristics of their host diamonds (δ13C, δ15N, N-content, N-speciation) are largely homogeneous throughout the population, and differ from silicate-bearing diamonds previously studied in the same locality. Sulphide-bearing diamonds appear to define a distinct population, which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values. The lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide-bearing diamonds leads us to infer that sulphide-bearing Jwaneng diamonds were not derived from the same chemical reservoir as their inclusions. One possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle-derived carbon-bearing fluid associated with pre-existing sedimentary sulphide minerals. This model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using Re-Os systematics. Here we suggest that the incorporation of a significant amount of mantle-related sulphur during a diamond growth event into the original sulphide could lead to a re-equilibration of the Re-Os isotope composition in any inclusion, and accordingly, account for some scatter on Re-Os isochrons.
- isotopes (carbon, nitrogen, sulfur)
- mass-independent fractionation of sulphur isotopes
- metasomatic diamond growth
- suphide-bearing diamond