Microinclusions in monocrystalline octahedral diamonds and coated diamonds from Diavik, Slave Craton: Clues to diamond genesis

Detailed in-situ analyses of trace-element patterns, N contents, N aggregation state and δ13C define two populations of gem-quality diamonds in the A154 kimberlite; these probably formed at different times and from different media. Four octahedra with 500–1200 ppm N and 12–35% B aggregation represent an older population; flat REE patterns and low Ba + K indicate that they crystallised from fluids similar to those in many monocrystalline diamonds worldwide. The second population includes coated octahedra and uncoated etched octahedra. On two etched stones, abundant flat-based trigons bottom out 10–15 μm below the faces of the stones, in a layer with abundant microinclusions (0.5 μm diameter) enriched in LREE, Ba and HFSE. Four coated octahedra with 800–1200 ppm N and 11–20% B aggregation have trace-element patterns consistent with growth from a fluid with high LREE + Ba, similar to that in the microinclusion-rich layers of the etched octahedra; the contents of LREE and Ba, and Ba/La, increase toward the rims. The coats on these stones typically have higher Ba than the outer cores, but Ba/La may be either higher or lower. δ13C is relatively constant (− 5.5 to − 4.7‰) across one etched octahedron but drops to − 9.7‰ in the microinclusion layer. The core of a coated stone has δ13C like that of the microinclusion layer (− 9.8 to − 10.5‰), but δ13C drops to − 13.9‰ in the coat. In the coated stones, core-to-rim decreases in the degree of N aggregation are not correlated with N content, but are accompanied by increases in Ba and LREE. This pattern, the lack of δ13C zoning in the one stone analysed, and homogeneous CL images suggest that the coated stones have grown continuously from a range of evolving fluids. The opaque coats appear to represent a final late stage of accelerated growth in some diamonds, rather than random overgrowths on pre-existing diamonds. Zoning patterns and FTIR data are consistent with the growth of the coated and etched gem-quality stones at 1200–1250 °C in the deep lithosphere shortly before their entrainment in the kimberlite.