Mantle-related carbonados? Geochemical insights from diamonds from the Dachine komatiite (French Guiana)
Résumé
Carbonado is a unique type of polycrystalline diamond characterised, among others, by 13C-depleted isotope compositions (δ13C not, vert, similar -25‰ vs. PDB), little advanced nitrogen aggregation (Ib-IaA) and sintered (ceramic-like) diamond grains. Its origin remains an enigma, with models proposing a formation either in the Earth's crust or even within an exploding super-nova. The possibility that carbonado formed in the Earth's mantle is often rejected because diamond with carbonado-like geochemical features has never been found in rocks, such as kimberlites, that carry diamonds from the mantle. In this study, it is shown that the C- and N- stable isotope compositions, nitrogen contents and nitrogen speciation of diamonds from the Dachine komatiite (French Guyana) exhibit unambiguous similarities with carbonados. These include C-isotopes (from -32.6 to + 0.15‰, mode at not, vert, similar-27‰), N-aggregation (only Ib-IaA diamonds, from 2 to 76% of N-pairs) and N-isotopes (from -4.1 to + 6.9‰, average not, vert, similar + 2.1 ± 2.9‰), which all strikingly match the carbonado data. This evidence illustrates that the main geochemical arguments usually called to reject a mantle origin of carbonado are no longer valid. A model linking carbonado crystallisation from komatiite volatiles is developed. In this model, the sintering is produced by the high temperature of the komatiite magma thus accounting for their absence in colder kimberlites. The low δ13C compositions of carbonados would be inherited from the transition zone (> 300 km depths), which is known to yield diamonds with distinct C-isotope distributions compared to most lithospheric diamonds (150–300 km depths). This model can account for most available observations of carbonados, including their large size, sintering, photoluminescense/cathodoluminescence features and geochemical characteristics.