Nickel-rich metallic and carbonate inclusions in diamonds: snapshots of redox-driven metasomatism in the deep upper mantle

Diamonds provide unique snapshots of otherwise inaccessible regions of Earth’s mantle, preserving mineral, melt, and fluid inclusions that record pressure, temperature, and redox conditions at the time of entrapment. Redox conditions are a key control on mantle mineralogy, carbon speciation, and melt generation, yet natural constraints at depths greater than ~250 km remain scarce.

Here we report diamond-hosted inclusions that document an active redox-driven metasomatic process in the deep upper mantle. Two diamonds from the Voorspoed mine (Kaapvaal craton, South Africa) contain coexisting nickel-rich metallic and carbonate inclusions, accompanied by silicate and oxide phases. Integrated infrared and Raman spectroscopy, electron microprobe analysis, transmission electron microscopy, and synchrotron micro-diffraction reveal the presence of a metallic Ni–Fe alloy with Ni# = 100 × Ni/(Ni + Fe) ≈ 85 and a Ni-rich carbonate with Ni# ≈ 89 within the same growth zones of the host diamonds. The extreme Ni enrichment of both phases indicates a genetic link and disequilibrium conditions during diamond formation.

Pressure-sensitive Raman and infrared signatures of coesite, N₂ and CO₂, together with the presence of high-pressure silicates including a K-rich NAL phase and Na–Al–rich pyroxene, constrain diamond formation to depths of ~280–470 km. These depths overlap with conditions where subducted carbonated oceanic crust is expected to intersect its melting curve.

We interpret the inclusion assemblage as a direct record of interaction between an oxidized carbonatitic–silicic melt derived from subducted material and a reduced, metal-bearing peridotitic mantle. Infiltration of this melt into reduced mantle lithologies triggered oxidation reactions, the redistribution of Ni between the alloy, olivine, and carbonate, and the growth of diamonds. Rather than recording equilibrium conditions, the high Ni# values of the alloy and carbonate reflect transient chemical exchange during metasomatism.

These observations provide rare natural evidence for deep mantle redox reactions involving carbonatitic–silicic melts and demonstrate the power of diamond-hosted inclusions to capture dynamic mantle processes. Such metasomatism may represent an important mechanism for mantle oxidation and enrichment, with implications for the generation of enriched alkalic magmas, including kimberlites and some ocean island basalts.