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

Abstract

Detailed in-situ analyses of trace-element patterns, N contents, N aggregation state and δ 13C 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. δ 13C 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 δ 13C like that of the microinclusion layer (− 9.8 to − 10.5‰), but δ 13C 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 δ 13C 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.