Multiple carbon and nitrogen sources associated with the parental mantle fluids of fibrous diamonds from Diavik, Canada, revealed by SIMS microanalysis

Fibrous diamonds are often interpreted as direct precipitates of primary carbonate-bearing fluids in the lithospheric mantle, sourced directly from common reservoirs of “mantle” carbon and nitrogen. Here we have examined fibrous growth layers in five diamonds (as three rims or “coats” and two whole-crystal cuboids) from the Diavik Diamond Mine, Canada, using in situ C- and N-isotope and N-abundance measurements to investigate the origin and evolution of their parental fluids, and in particular, to test for isotopic variability within a suite of fibrous diamonds. High-resolution growth structure information was gleaned from cathodoluminescence (CL) imaging and, in combination with the isotopic data, was used to assess the nature of the transition from gem to fibrous growth in the coated diamonds. The two cuboids are characterized by fine concentric bands of fibrous and/or milky opaque diamond, with one sample (S1719) having intermittent gem-like growth layers that are transparent and colourless. The three coated diamonds comprise octahedral gem cores mantled by massive or weakly zoned fibrous rims, with sharp and well-defined gem–fibrous boundaries. For the two cuboid samples, δ 13 C and δ 15 N values were −7.7 to −3.2 ‰ (mean −6.3 ± 1.3 ‰; 1 SD; n = 84) and −5.6 to −2.1 ‰ (mean −4.0 ± 0.8 ‰; 1 SD; n = 48), respectively. The three fibrous rims have combined δ 13 C values of −8.3 to −4.8 ‰ (mean −6.9 ± 0.7 ‰; 1 SD; n = 113) and δ 15 N values of −3.8 to −1.9 ‰ (mean −2.7 ± 0.4 ‰; 1 SD; n = 43). N-abundances of the combined cuboid–fibrous rim dataset range from 339 to 1714 at. ppm. The gem cores have δ 13 C and δ 15 N values of −5.4 to −3.5 ‰ and −17.7 to +4.5 ‰, respectively, and N-abundances of 480 to 1699 at. ppm. Broadly uniform C- and N-isotope compositions were observed in each of the gem cores (variations of ~