Metasomatic diamond growth: A multi-isotope study ( 13C, 15N, 33S, 34S) of sulphide inclusions and their host diamonds from Jwaneng (Botswana)

• Published in: Earth and planetary science letters 2009-05, Vol.282 (1), p.79-90
• Main Authors: Thomassot, E., Cartigny, P., Harris, J.W., Lorand, J.P., Rollion-Bard, C., Chaussidon, M.
• Format: Article
• Language: English
• Subjects: isotopes (carbon, nitrogen, sulfur); mass-independent fractionation of sulphur isotopes; metasomatic diamond growth; suphide-bearing diamond
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2009.03.001

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 ( δ 13C, δ 15N and multiple S-isotopes as δ 34S, δ 33S, Δ 33S) 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‰ < Δ 33S < + 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 ( δ 13C, δ 15N, 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