Unveiling the Mysteries of Earth’s Mantle Through Diamonds

Deep within the Earth, the mantle operates as a relentless, enigmatic machine, driving volcanic activity, recycling the crust, and regulating the planet’s long-term evolution. However, one of its most elusive characteristics—the redox state—remains challenging to measure directly. In a new study led by Yael Kempe and Yaakov Weiss from the Institute of Earth Sciences at the Hebrew University, a rare glimpse into these deep processes has been captured within nano and micro inclusions in diamonds from the Finsch Mine in South Africa.

A Rare Discovery in the Depths

For decades, high-pressure models and experiments suggested that nickel-rich metal alloys should stabilize in the mantle at depths between 250-300 km. However, natural samples confirming these predictions have been exceedingly rare.

In collaboration with colleagues from the University of Nevada, the University of Cambridge, and the Nano Center at the Hebrew University, Weiss’s team has now identified nano-metallic inclusions of nickel and iron, along with nickel-rich carbon micro-inclusions preserved within diamonds formed between 280-470 km beneath the Earth’s surface. These inclusions represent the first direct evidence of nickel-rich alloys at the expected depth—a long-awaited confirmation of redox models in the mantle.

The diamond’s mineral cargo also includes coesite, an aluminum and potassium-rich phase, and solid molecular nitrogen inclusions, providing multiple pressure markers that constrain their origin to the deep upper mantle and the shallow transition zone.

Frozen Redox Snapshots in Carbon

The significance of the discovery extends beyond merely confirming theoretical models. The coexistence of nickel-iron alloy and nickel-rich carbonates suggests a metasomatic redox freezing interaction—a dynamic interaction where oxidized carbonatitic silicate magma infiltrated reduced mineral-bearing peridotite.

In this environment, preferential oxidation of iron relative to nickel led to the enrichment of the remaining alloy in nickel. Meanwhile, nickel-rich carbonates and diamonds crystallized from the magma. Essentially, the diamonds froze a fleeting geochemical moment: the transformation of reduced mantle rock into a more oxidized, volatile-rich domain and the reduction of carbonates to form diamonds.

Weiss states, “This is a rare snapshot of mantle chemistry in action. Diamonds act as tiny time capsules, preserving an interaction that would otherwise vanish as minerals re-equilibrate with their environment.”

Implications for Mantle Dynamics and Magmatic Activity

These findings have broad implications. If local metasomatic interactions periodically oxidize small portions of the mantle, they may help explain why some inclusions in deep diamonds unexpectedly record highly oxidized conditions.

Such processes also shed light on the origins of volatile-rich magmas. Enrichment of mantle peridotite with carbonates, potassium, and incompatible elements during these oxidation events may prime the mantle for the formation of kimberlite, lamprophyre, and some oceanic island basalts later on. In other words, the tiny inclusions in Finsch diamonds suggest broad links between subduction, mantle redox dynamics, and magma generation that shapes continents and brings diamonds to the surface.

Diamonds as Witnesses to the Mantle

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

The work of Kempe and Weiss marks a milestone: the first natural confirmation of nickel-rich alloys at theoretically predicted mantle depths, offering a vivid illustration of how the redox landscape evolves deep within the Earth through magma-rock interactions.

Conclusion

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