3 Minutes
Nickel-rich inclusions trapped in superdeep diamonds
Diamonds recovered from South Africa's Voorspoed mine contain nickel-iron metal and nickel-rich carbonate inclusions that record a suite of reactions operating nearly 300–470 km below Earth's surface. Scientists led by Yael Kempe and Yaakov Weiss (Hebrew University of Jerusalem) identified nano- and micro-sized metallic alloys and carbonate phases preserved inside diamonds that formed in the deep upper mantle and shallow transition zone. Scientists found nickel-rich alloys inside South African diamonds, confirming long-predicted mantle reactions. These tiny inclusions show how deep-Earth processes shape magmas and even the diamonds themselves.
Scientific background and methods
The Earth's mantle is chemically dynamic: it convects, transports volatiles, and exchanges material with the crust. One crucial but hard-to-measure parameter is the mantle's redox state—the balance between oxidized and reduced chemical species—which governs mineral stability, volatile speciation, and magma composition. High-pressure experiments and thermodynamic models have long predicted that nickel-rich metallic alloys can be stable at depths of a few hundred kilometers, but direct natural evidence has been scarce.
Weiss and colleagues used advanced electron microscopy and spectroscopy at the Nanocenter (Hebrew University) and partner labs (University of Nevada, University of Cambridge) to map and analyze nanoinclusions in Voorspoed diamonds. Pressure-sensitive inclusions such as coesite, K-rich aluminous phases, and molecular solid nitrogen constrained formation depths to about 280–470 km, supporting a deep-mantle origin for the trapped materials.
Key discovery: redox-freezing and reaction snapshots
The coexistence of nickel-iron metal and nickel-rich carbonates inside the same diamond indicates a localized metasomatic process called redox-freezing. In this scenario, an oxidized carbonatitic–silicic melt infiltrated reduced, metal-bearing peridotite. Iron preferentially oxidized and entered the melt, enriching the remaining metal in nickel; concurrently, nickel-rich carbonates and diamond crystallized from the melt or were reduced to form diamond.
"These diamonds act as microscopic time capsules," Weiss said, describing how both reactants and products were trapped before they could re-equilibrate with surrounding mantle rock. The result is the first natural confirmation of nickel-rich alloys at the depths predicted by theory, validating models of mantle redox behavior.
Implications for mantle dynamics and magmatism
These tiny inclusions carry broad implications. Localized oxidation events driven by melt infiltration could create pockets of volatile- and carbonate-rich mantle that later generate volatile-rich magmas such as kimberlites and lamprophyres. Such magmas can ascend rapidly from hundreds of kilometers and carry diamonds to the surface, linking deep redox processes to the formation and eruption of diamond-bearing magmas.
Broader geochemical links
If metasomatic oxidation is episodic and spatially patchy, it can explain why some superdeep diamond inclusions record higher oxygen fugacities than surrounding mantle. The enrichment of potassium, carbonates, and other incompatible elements during redox events may prime small mantle domains for explosive, volatile-rich eruptions.
Conclusion
The Voorspoed diamonds provide a rare, direct record of deep-mantle redox reactions and validate long-standing predictions about nickel-rich alloy stability at depth. By preserving both metal and carbonate phases, these diamonds illuminate how melt–rock interaction alters mantle chemistry and seeds the volatile reservoirs that produce kimberlites and other magmas. As mineral time capsules, diamonds continue to reveal the hidden processes shaping Earth’s interior.
Source: sciencedaily
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