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Earth's mantle is a restless, enigmatic engine that powers volcanism, recycles crust, and regulates the long-term evolution of the planet. But one of its most elusive characteristics—the redox state, or the balance of oxidized and reduced chemical species—remains difficult to measure directly.

A new study led by Yael Kempe and Yaakov Weiss, from the Hebrew University's Institute of Earth Sciences, offers a rare glimpse into these deep processes, captured within nano- and micro-inclusions in diamonds from South Africa's Voorspoed mine. The study is in the journal Nature Geoscience.

For decades, models and high-pressure experiments have suggested that nickel-rich metallic alloys should stabilize in the mantle at depths of roughly 250–300 km. Yet, natural samples confirming these predictions have been vanishingly scarce.

Working with colleagues from the University of Nevada, the University of Cambridge, and the Nanocenter at the Hebrew University, Weiss's team has now identified nickel-iron metallic nanoinclusions and nickel-rich carbonate microinclusions preserved inside diamonds that formed between 280–470 km below Earth's surface. These inclusions represent the first direct evidence of nickel-rich alloys at their predicted depth—a long-sought validation of mantle redox models.

The diamonds' mineral cargo also includes coesite, K-rich aluminous phases, and molecular solid nitrogen inclusions, providing multiple pressure markers that firmly constrain their origin to the deep upper mantle and shallow transition zone.

Redox snapshots frozen in carbon

The significance of the find goes beyond simple confirmation of theoretical models. The coexistence of nickel-iron alloy and nickel-rich carbonate points to a metasomatic redox-freezing reaction—a dynamic interaction in which an oxidized carbonatitic-silicic melt infiltrated reduced, metal-bearing peridotite. It joins earlier evidence from shallower depths that this is the main mode of formation of natural diamonds.

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In this environment, preferential oxidation of iron relative to nickel drove enrichment of the residual alloy in nickel. At the same time, nickel-rich carbonates and diamonds crystallized from the melt. In effect, the diamonds froze a fleeting geochemical moment: the conversion of a reduced mantle rock into a more oxidized, volatile-rich domain and the reduction of carbonates to form diamonds.

"This is a rare snapshot of mantle chemistry in action," says Weiss. "The diamonds act as tiny time capsules, preserving a reaction that would otherwise vanish as minerals re-equilibrate with their surroundings."

Implications for mantle dynamics and magmatism

These findings carry broad implications. If localized metasomatic reactions periodically oxidize small portions of the mantle, they may help explain why some inclusions in other superdeep diamonds record unexpectedly high oxidized conditions.

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Such processes also shed light on the origins of volatile-rich magmas. The enrichment of mantle peridotite in carbonate, potassium, and incompatible elements during these redox events could prime the mantle for the later formation of kimberlites, lamprophyres, and even some ocean island basalts.

In other words, the tiny inclusions in Voorspoed diamonds hint at large-scale links between subduction, mantle redox dynamics, and the generation of magmas that shape continents and bring diamonds to the surface.

Diamonds as mantle witnesses

The study underscores the scientific value of diamonds as more than just gemstones. Their inclusions—whether nanometer-scale alloys or high-pressure minerals—offer one of the only natural records of conditions hundreds of kilometers beneath our feet.

Kempe and Weiss's work marks a milestone: the first natural confirmation of nickel-rich alloys at mantle depths predicted by theory, and a vivid illustration of how Earth's redox landscape evolves through melt-rock interaction.

As researchers continue to probe these mineral time capsules, we may find that diamonds, once symbols of permanence, are also storytellers of change—bearing witness to the mantle's hidden chemistry and the processes that continue to shape our dynamic planet.