Redox state of the deep upper mantle recorded by nickel-rich diamond inclusions

Abstract

The redox state of Earth’s mantle is governed by the oxidation state of iron and carbon and influences key physical and chemical mantle parameters. Mantle xenoliths, and experimental and thermodynamic studies reveal a decrease in oxygen fugacity with depth, down to ~250 km. A further more modest drop is linked to the predicted stabilization of nickel-rich metallic alloy at 250–300 km. However, garnets from 250–500 km record more oxidized conditions, and no nickel-rich alloy has been reported from these depths to account as natural evidence for the predictions. Here we report nickel–iron metallic nanoinclusions and Ni-rich carbonate microinclusions in two diamonds from the Voorspoed mine, South Africa. Various pressure indicators confirm their origin in the deep upper mantle or the shallow transition zone (280–470 km). The coexistence of nickel-rich metal and carbonate indicates a reaction between oxidized carbonatitic melt and reduced metal-bearing peridotite that led to nickel enrichment and diamond growth. This reaction captures a snapshot of the dynamics of metasomatism, including the formation of intermediate products that may later react. The diamonds provide direct evidence for nickel-rich alloy at its predicted depth within the mantle. They also indicate the presence of deep carbonatitic–silicic melts, which episodically oxidize small volumes of the mantle and play a role in the formation of kimberlites and alkali basalts.