Overview

• Nature Geoscience reports nickel–iron metallic nanoinclusions and nickel-rich carbonate microinclusions in Voorspoed mine diamonds formed 280–470 kilometers below the surface.
• The coexistence of metal and carbonate captures a metasomatic redox-freezing reaction in which oxidized carbonatitic–silicic melt infiltrated reduced, metal-bearing peridotite.
• Coesite, potassium-rich aluminous phases, and molecular solid nitrogen in the same diamonds provide independent pressure markers for the deep upper mantle and shallow transition zone.
• Preferential oxidation of iron over nickel enriched the residual alloy in nickel while diamonds and carbonate crystallized from the infiltrating melt.
• The authors suggest these localized redox events can enrich mantle domains in carbonate and potassium, potentially priming sources of kimberlites, lamprophyres, and some ocean-island basalts.