Diamonds from the Diavik Mine: From Formation Through Mantle Residence to Emplacement

Abstract

Abstract The morphology and color of Diavik diamonds, their nitrogen concentrations and δ13C values, and the composition of their mineral inclusions provide insights into the formation of diamonds and the evolution of the mantle root beneath the central Slave craton. The minerals which make up inclusions reflect a largely peridotitic mantle source region (77% peridotitic, 18% eclogitic, 1% ultradeep, and 4% ferropericlase bearing) predominantly composed of garnet-harzburgite. The major element geochemistry of the inclusions indicates that the degree of primary melt depletion during formation of the central Slave cratonic mantle root was distinctly lower than for other cratons worldwide—for example, beneath Yakutia and the Kaapvaal craton. The formation of peridotitic diamonds in the Paleoarchean was likely followed by lithosphere-scale cooling by about 150°C, based on differences in equilibration temperatures for touching and nontouching inclusion pairs and nitrogen aggregation-based residence temperature estimates for their respective diamond hosts. The protolith of the eclogitic diamond substrate likely was basaltic-gabbroic oceanic crust, as shown by trace element patterns that include weak positive EuN anomalies in one garnet and one clinopyroxene inclusion, and a normal mid-ocean ridge basalt (NMORB) signature for the calculated bulk-rock eclogite. The analysis of microinclusions in fibrous diamonds reveals the presence of high-density fluids that span a continuous compositional range between carbonatitic and saline end-members. The diamonds grew from continuously evolving and fractionating melts/fluids that moved through the mantle. Cathodoluminescence imaging of fibrous and clear monocrystalline diamonds indicates that they grew in pulses with intermittent periods of resorption.