Several hundred kilometers under our feet, the chemistry of depths remains largely out of reach, deduced from theoretical models more than direct observations. However, two diamonds extracted from the South African mine of Voorspoed come to shake up this framework. Locked in their crystalline structure: metallic alloys rich in nickel and oxidized carbonates – two normally incompatible compounds. This improbable coexistence constitutes physical proof of a supposed mechanism, but never observed in the terrestrial mantle: the reaction between oxidized and reduced materials at great depth.
Contradictory chemistry frozen in diamonds
The two diamonds studied therefore contain inclusions that immediately confused scientists. These tiny structures imprisoned in the crystal contain two types of theoretically incompatible compounds. Namely: reduced metal alloys rich in nickel and oxidized carbonates. Both trained more than 280 kilometers deep. In chemistry, a reduced environment is low in oxygen, while an oxidized environment is rich. The two normally do not coexist stable.
In diamonds, these inclusions are called nanoincusions and microinclusions. They measure between 20 and 80 nanometers for the first, and up to 1 micron for seconds. One of the alloys analyzed contains up to 85 % nickel. It is an exceptionally high ratio compared to the estimates of mantle composition models. Indeed, they provide alloys rather rich in iron. Besides these metals, the researchers identified a Gaspéite type nickel carbonate (Nico₃), with a clear oxidized signature.
The simple fact of finding these two phases in the same microscopic space surprised Yaakov Weiss and Yael Kempe, they entrust to Scientific American. They put aside these samples for a year, thinking of an error or an artifact. It was only after new analyzes, carried out with colleagues from the universities of Cambridge and Nevada, that they understood that they had captured a rare moment of unstable balance, frozen in the diamond during its growth. Mineral proof of a process of chemical transformation of the deep mantle, hitherto only modeled.
A chemical reaction deeply frozen
The coexistence of the oxidized and reduced phases in diamonds does not result from a geological coincidence. It is indeed an active process in the terrestrial mantle, now better understood thanks to this study. The authors describe a redox freezing reaction (Redox-Friezing). By this mechanism, an oxidized carbonate magma infiltrates a reduced mantle rock, typically a peridotitis containing metallic alloys.
During this interaction, the oxidized compounds (such as carbonates) react with the metal alloys present in the rock. They trap both the reagents and reaction products in the diamond matrix in formation. It is this rare configuration that researchers have observed. They notably identified inclusions of solid (Δ-N₂) and solid carbon dioxide (CO₂-I). They constitute two reliable indicators of the prevailing pressure during the formation of the diamond: between 9 and 16 GPA, or 280 to 470 kilometers deep.
The diamond thus acts as a sealed capsule, preserving an instantaneous from a deep and ephemeral chemical process. The latter would have otherwise disappeared by mineralogical rebalancing. The high nickel content of alloys results from this reaction. The iron present in the metal is oxidized in priority, which concentrates the residual nickel in the remaining alloy.
According to the authors, the oxidized magma comes from subducted tectonic plates, rich in carbonates. By melting at great depth, these materials form a carbonatitic fusion capable of going up and interacting with the surrounding mantle rocks. The diamond is then formed at the very heart of the reaction, trapping all the chemical witnesses of this transient exchange.
An unprecedented confirmation of deep mantle models
This discovery provides direct experimental validation to hypotheses formulated for several decades on the deep chemistry of the mantle. The thermodynamic models provided, without material evidence until then, that from around 250 km deep, the drop in free oxygen in rocks led to the formation of metal alloys rich in nickel. But until today, no natural sample confirmed their presence at these depths.
The inclusions identified in the diamonds provide precisely this missing link. Synchrotron X-ray diffraction analysis and transmission electron microscopy (TEM) made it possible to identify the metal phase as a Nickel-Fer alloy. But with a nickel level much higher than predictive values, as mentioned above. In addition, the association with carbonates rich in nickel and oxygen indicates that the mantle is not uniformly reduced, as supposed to. But it presents locally oxidized areas, due to fluids or magmas from subduction.
This upsets the classic vision of a homogeneous chemical coat in depth. Weiss and his colleagues demonstrate that oxidizing metasomatosis, that is to say local alteration by fluids, can modify the chemical composition of a mantle area. This rare, but significant phenomenon could explain why certain ultra-profinity diamonds contain highly oxidized signature inclusions. Even though they come from overall reduced areas.
Finally, the study sheds light on a mysterious point. The presence of nickel atoms in the crystalline structure even of certain diamonds, despite the high atomic weight of nickel. According to Maya Kopylova, for Live sciencethis anomaly could be directly linked to the specific depth and chemical conditions of their training, revealed here for the first time.
Implications for the formation of magmas and earthly dynamics
Beyond the formation of diamonds, this discovery makes it possible to rethink the role of deep chemical reactions in the production of magmas rich in volatiles, like the Kimberlites-volcanic rocks carrying diamonds. These very oxidized magmas have long been considered formed in more superficial areas of the coat. However, new data show that oxidized environments can also exist well below 300 km. Which makes a deeper origin plausible for these eruptions.
The mantle areas altered by carbonate fluids are not content to produce diamonds. They can also be enriched with potassium, co₂ and incompatible elements. In other words, chemical elements that do not easily enter the structure of major minerals. This type of enrichment prepares the rock to melt more easily later, under the effect of a temperature rise or decompression. This could generate not only kimberlites, but also other types of magmas such as lamprophyres or certain basalts of ocean islands.
The discovery therefore makes it possible to make the link between local chemistry of the mantle, subduction, and magmatic dynamics. It shows that diamonds are not just passive geochemical witnesses. They crystallize in contexts where deep and decisive transformations are played out for the planet. As Weiss indicates, these micro-inclusions reveal certainly invisible mechanisms on a large scale. However, they are essential to understand the carbon cycle, the evolution of the mantle and the genesis of mineral resources.
These results recall that the terrestrial depths, although inexpensive, can be read in hollow in the solid matter, such as a mineral archive of a moving land. And that each diamond, far from the frozen symbol it represents, can be the active vestige of a dynamic process still in progress.
Source: Kempe, Y., Remennik, S., Tschauner, O. et al. “Redox State of the Deep Upper Mantle Recorded by Nickel-Rich Diamond Includes”. Nat. Geosci. (2025)
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