The Earth’s deep interior holds many secrets that have long eluded direct observation, especially regarding its redox state—the measure of oxidation and reduction conditions that govern elemental speciation and physical properties deep within our planet. A groundbreaking new study published in Nature Geoscience by Kempe et al. sheds unprecedented light on this mysterious environment by uncovering naturally occurring nickel-rich metallic nanoinclusions in diamonds from the mantle transition zone. These findings not only confirm long-standing theoretical predictions about redox conditions at great depths but also reveal compelling insights into the dynamic processes shaping the deep upper mantle’s chemistry and mineralogy.
The redox state of Earth’s mantle is a critical parameter that controls a wide range of geochemical and geophysical processes, influencing not only the speciation of iron and carbon but also mantle melting, volatile cycling, and the generation of magmas that reach Earth’s surface, such as kimberlites and alkali basalts. Prior indirect evidence had suggested a gradient in oxygen fugacity—a measure of oxygen availability and chemical potential—where redox conditions decrease systematically with depth, at least to around 250 kilometers beneath the surface. This trend has significant implications for the stability of various mineral phases and elemental partitioning in the mantle.
Beyond approximately 250 kilometers, models predicted a further, albeit more modest, drop in oxygen fugacity linked to the stabilization of nickel-rich metallic alloys. According to geochemical theory and experimental petrology, the deeper mantle environment favors the segregation of metallic phases enriched in nickel and iron. However, the direct natural detection of these phases at relevant depths remained elusive. While garnets isolated from 250 to 500 kilometers depth showed evidence of increased oxidation states, consistent nickel-rich alloys found within natural mantle samples had not been identified, presenting a conundrum between predicted mantle redox conditions and the mineralogical record.
The recent study addresses this gap through the remarkable discovery of nanoscale metallic inclusions within diamonds extracted from the Voorspoed kimberlite, South Africa. These diamonds host minute grains of nickel-iron metal, as well as microinclusions of Ni-rich carbonate, offering a rare glimpse into deep mantle redox reactions at depths of roughly 280 to 470 kilometers. The depth estimates are robustly constrained by various high-pressure mineral barometers, firmly situating the diamonds’ provenance in the deep upper mantle and across the shallow mantle transition zone.
These nanoscale metallic inclusions are not mere curiosities but rather reveal a reactive interplay between oxidized melts and reduced mantle rocks. The coexistence of nickel-rich metal and carbonate inclusions reflects a metasomatic environment where carbonatitic melts rich in oxidized carbon species interacted with reduced, metal-bearing peridotitic sources. This interaction drove localized nickel enrichment and triggered the growth of diamonds, essentially capturing a dynamic snapshot of deep mantle fluid-rock exchanges that produce transient intermediate phases, which may persist or further evolve with progressive reactions.
From a geochemical perspective, this dual presence of a reduced metallic phase alongside an oxidized carbonate phase illustrates the highly heterogeneous and dynamic nature of mantle redox conditions. It reveals that oxidation states do not vary strictly with depth but also depend sensitively on localized melt-rock reactions, involving episodes of oxidation driven by ascending carbonatitic melts. These melts, enriched in oxidized carbon, have long been hypothesized to play critical roles in transporting carbon and other volatiles from the deep mantle upward, but their natural evidence at such depths has been scarce until now.
The implications of the discovery extend to our understanding of mantle metasomatism—chemical alteration of mantle rocks by migrating melts or fluids. The reaction preserving both nickel-rich metal and carbonate within the diamond matrix implies that mantle metasomatism occurs in pulses that can shift redox conditions on a fine spatial scale, periodically oxidizing the surrounding peridotite. This oxidation facilitates the mobilization of nickel and other compatible elements, subsequently influencing the genesis of diamond-forming fluids and melts. Consequently, these processes also bear on the composition and evolution of kimberlite magmas that uniquely transport diamonds and mantle xenoliths to Earth’s surface.
Importantly, this investigation provides the first unambiguous natural record for nickel-rich metallic alloy phases at their predicted mantle depths—a crucial missing link that validates decades of theoretical and experimental work. It confirms that nickel, commonly assumed to reside in silicate or sulfide phases, indeed partitions into a metallic phase under certain deep mantle redox regimes, indicating that Earth’s interior comprises chemically diverse and complex reservoirs. Such distributions of nickel and iron have profound implications for understanding mantle melting, metal transport, and the deep carbon cycle.
The presence of Ni-rich carbonate microinclusions alongside metallic nanophases also draws attention to the deep carbon cycle’s complexity. Carbonates play an essential part as carbon carriers deeply buried in the Earth, gradually releasing carbon to shallower regions through melting and fluid exsolution. Their observed association with metallic phases in a single diamond sample reflects a highly localized chemical milieu where carbon speciation, and thus redox state, can shift dramatically due to fluid-rock interaction, local melting, and pressure-temperature conditions.
From an experimental standpoint, these natural nanoscale inclusions offer valuable new constraints for calibrating high-pressure and temperature experiments designed to simulate mantle conditions. The coexisting Ni-rich metal and carbonate phases retrieved from nature provide precise mineralogical assemblages that can be targeted and reproduced in laboratories, thereby enhancing our quantitative understanding of oxygen fugacity gradients, metal alloys’ stability fields, and carbonate phase behavior at extreme mantle depths.
Moreover, these results open novel avenues for interpreting the diamond record as a geochemical archive not only of mantle composition but of redox and metasomatic processes as well. Diamonds are exceptional time capsules that lock in information from their growth environment for billions of years. The inclusions they host are integral to reconstructing the chemical and physical conditions of their formation. Therefore, the discovery of nickel-rich metallic inclusions embedded in diamonds signals a paradigm shift in how geoscientists approach mantle oxidation and carbon cycling.
The study further highlights kimberlites’ role as volcanoes capable of sampling and bringing to the surface fragments from the deepest parts of the upper mantle and transition zone. The nature of the melts implicated here—carbonatitic and silicic—confirms their episodic ascent and oxidation capacity. This periodic injection of oxidized carbon-bearing melts could partly explain surface volcanic diversity and the genesis of economically vital mineral deposits associated with kimberlitic magmatism.
In a broader planetary context, these insights contribute to our understanding of Earth’s differentiation and evolution. The mantle’s redox stratification affects core formation, mantle convection, and volatile trafficking, profoundly shaping the Earth’s geological and atmospheric history. Observing natural nickel-rich metal alloys at these depths suggests that Earth’s internal redox environment has evolved heterogeneously and remains chemically dynamic, influencing long-term planetary habitability and geodynamic behavior.
This discovery marks a pivotal step in mantle geochemistry, bridging theoretical predictions with tangible mineralogical evidence. The team’s innovative use of state-of-the-art imaging and analytical tools to detect nanoinclusions sets a new benchmark for high-precision mantle studies. As analytical techniques continue to improve, we can anticipate further revelations about the Earth’s inaccessible interior and the complex interplay of oxidation, carbon, and metal chemistry lurking at depths that feed our surface magmatic and tectonic processes.
In conclusion, the work of Kempe and colleagues not only resolves a longstanding mystery by revealing nickel-rich metallic nanoinclusions tied to mantle redox state at 280-470 km depth but also enriches our understanding of deep carbonatitic melts’ role in oxidizing the deep mantle and facilitating diamond formation. This landmark discovery integrates mineral physics, geochemistry, and petrology, showcasing the power of diamonds as high-pressure messengers and the intricate, dynamic nature of Earth’s interior chemistry.
As we uncover more about how carbon and metal cycles interconnect deep in the mantle, future research will inevitably explore the implications for mantle plume generation, kimberlite eruption mechanisms, and the global carbon budget, moving us closer to a unified picture of Earth’s deep carbon system and mantle redox evolution.
Subject of Research: The redox state and chemical environment of the deep upper mantle and mantle transition zone, revealed through nickel-rich metallic and carbonate inclusions within deep-origin diamonds.
Article Title: Redox state of the deep upper mantle recorded by nickel-rich diamond inclusions
Article References:
Kempe, Y., Remennik, S., Tschauner, O. et al. Redox state of the deep upper mantle recorded by nickel-rich diamond inclusions. Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01791-4
Image Credits: AI Generated
