| Summary: | Parcels of diamond crystals from the A154 South kimberlite diatreme, Northwest Territories (n=281), and the Lynx kimberlite dyke, Quebec (n=6598) were examined in terms of colour, size, morphology, and UV fluorescence (A154 South samples only). A subset of stones from each parcel (A154 South: n=60, Lynx: n=20) were cut and polished to expose internal zonation and mineral inclusions. Exposed primary mineral inclusions were quantitatively analyzed for major elements by EMPA.
Diamond crystals from the Kelsey Lake kimberlite diatreme, Colorado (n=20), were cut into plates and analyzed for nitrogen aggregation states by FTIR. Twelve of these stones were then analyzed with further subsets from A154 South (n=18) and Lynx (n=16) for carbon isotope ratios and nitrogen abundances by SIMS. Every diamond crystal cut and polished had its internal zonation imaged with CL.
Mineral inclusion data from A154 South and Lynx show that the mantle keel of the Slave craton is slightly less depleted than that of the Superior craton, and both are less depleted than those of the Kaapvaal and Siberian cratons. Equilibration conditions plot on hotter geothermal gradients (surface heat flows ~42 mW/m2) than for those of typical Archean cratons (≤40 mW/m2). Equilibration temperatures (~1150-1250°C) are ~100-200°C hotter than previously reported from Kelsey Lake (~1020°C).
Kelsey Lake and A154 South samples have carbon isotope ratios and nitrogen contents typical of most diamond populations worldwide. Diamond crystals from Lynx are entirely different, consisting of mostly Type II diamond with δ13C (vs. PDB) values from approximately -3.6 ‰ to +1.7 ‰. These 13C-enriched samples are suggested to be the result of extreme Rayleigh fractionation of diamond from a carbonate fluid and possibly input of carbon sourced from subducted abiotic oceanic crust. Also notable is that growth trends (δ13C-[NT]) for most of the samples studied show little or no consistency with published fractionation models.
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