Abstract The morphology and color of Diavik diamonds, their nitrogen concentrations and δ 13 C 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 Eu N 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.

We report sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages from high-grade gneisses of the Bondy gneiss complex, a volcano-plutonic arc and back-arc edifice hosting a Cu-Au-Fe oxides hydrothermal system in the Central Metasedimentary Belt, Grenville Province, Québec. Samples of quartzofeldspathic gneiss gave broadly similar results, with zircon cores indicating ages between 1.59 and 1.21 Ga and mantles or whole new zircon crystals giving Grenvillian metamorphic ages. A few analyses indicate minor involvement of Paleoproterozoic and Archean crustal material. Zircon cores are interpreted to have grown during crystallization of the quartzofeldspathic gneiss protoliths, but precise ages could not be determined for any of the samples. The spread of ages is attributed mainly to isotopic disturbance owing to hydrothermal alteration, high-pressure-temperature ( P-T ) metamorphism, and/or recent Pb loss. Only a sample of tonalitic gneiss yielded a well-defined, igneous crystallization age of 1386 ± 10 Ma. Younger zircons from the dated lithologies provide evidence for two episodes of Grenvillian metamorphism: a period of high- P-T granulite-facies metamorphism and partial melting at 1.21–1.18 Ga, and a younger, albeit localized, metamorphic overprint at ca. 1.15–1.13 Ga. The new SHRIMP U-Pb data indicate that the composite volcano-plutonic edifice likely formed, at least in part, at ca. 1.39 Ga. Together with recently published geochemical and Nd data, the new data (1) extend the known distribution of ca. 1.39 Ga arc-related magmatism in the Grenville Province, and (2) suggest that a major portion of the source region to the Bondy gneiss complex was produced by the addition of voluminous juvenile Mesoproterozoic material.