Compositional recycling structure of an Archean super-plume: Nb–Th–U–LREE systematics of Archean komatiites and basalts revisited

The volcanic stage of the 2.7-Ga Abitibi greenstone belt, Canada, is dominated by bimodal arc magma series and komatiite-basalt sequences. The latter represents an aerially extensive oceanic plateau erupted from an anomalously hot super-plume. Komatiites define a linear array of Nb/Th vs. Nb/U, extending from Nb/Th=8-20, and Nb/U=26-58, whereas basalts plot on a separate, but overlapping, field extending to higher Th/U but lower Nb/Th values. Inter-element ratios of Th, U, Nb, and LREE of komatiites and basalts plot with Phanerozoic and modern ocean plateau basalts. Th, U, Nb, and LREE are fractionated in subduction zones into low Nb/Th, Nb/U, and Nb/LREE arc crust, and complementary high Nb/Th, Nb/U, and Nb/LREE residual slab. Accordingly, the Archean komatiite-basalt association may be explained by a plume that likely originated from the core-mantle boundary with komatiites erupted from a hot axis containing recycled oceanic crust, and basalts erupted from the plume annulus that entrained upper mantle containing recycled oceanic and continental crust. High Nb/Th and Nb/U of plume-related volcanic sequences documented in Abitibi, Yilgarn, and Baltic Archean greenstone belts suggest that the extraction and recycling of continental crust may have occurred early in the Archean.