The study, led by Hongsheng Yuan of the Center for High Pressure Science and Technology Advanced Research in Shanghai, together with Lianjie Man and Leonid Dubrovinsky of the Bayerisches Geoinstitut at the University of Bayreuth and an international team of collaborators, is published in Nature Geoscience. Using laser-heated diamond anvil cells—devices that squeeze samples between two gem-quality diamond tips while laser beams raise temperatures to thousands of degrees—the researchers created conditions rivaling those near the core–mantle boundary, where pressures exceed one hundred gigapascals and temperatures climb above several thousand kelvin.

What emerged from these experiments were two hexagonal iron oxyhydroxides with the nominal compositions Fe5O12Hx and Fe7O12Hx, where x denotes a variable hydrogen content. These are not exotic compounds dreamed up in a computer simulation; they are crystalline phases whose structures were determined directly from in situ single-crystal X-ray diffraction measurements at synchrotron facilities, including the European Synchrotron Radiation Facility, PETRA III at DESY in Hamburg, and the Shanghai Synchrotron Radiation Facility. The crystallographic data underpinning the structure determinations have been deposited in the Cambridge Structural Database, and the broader experimental datasets are openly available through figshare, giving other researchers the means to scrutinize and extend the findings.

The significance of these phases lies in two properties highlighted by the team. First, they can form under water-undersaturated conditions, meaning they do not require an abundant free water reservoir to crystallize. This is crucial because the deep mantle is generally thought to be far drier than the shallow regions from which volcanic gases escape. Second, the new oxyhydroxides are remarkably dense. Density matters enormously in a planet whose interior is sorted by gravity: materials less dense than their surroundings tend to rise, while denser ones sink. The high densities of Fe5O12Hx and Fe7O12Hx mean that once formed, they would gravitationally settle and remain stably parked near the base of the mantle rather than escaping upward.

That combination of stability and density provides what the authors describe as a plausible mineralogical basis for the seismically observed ultralow-velocity zones. These zones, which can stretch for hundreds of kilometers horizontally but are only a few to tens of kilometers thick, reduce seismic shear and compressional wave speeds so severely that ordinary mantle minerals alone struggle to explain them. Hydrogen-bearing iron-rich phases, with their altered elastic properties and potentially partial molten character, have been increasingly invoked as candidates. Earlier work had identified a hydrogen-bearing iron peroxide and pyrite-type FeOOH as possible deep water hosts; the new hexagonal oxyhydroxides extend that family of compounds and demonstrate that iron, oxygen, and hydrogen can combine into dense crystalline architectures at the pressures and temperatures of the lowermost mantle.

The context for why water might be down there in the first place reaches back to the planet’s earliest history. Isotopic anomalies in helium and hydrogen recorded in deep-sourced volcanic rocks have long hinted that some primordial water—water inherited from the materials that accreted to form Earth—survives in isolated reservoirs deep in the mantle rather than having been lost entirely to space or mixed into the surface water cycle. A leading scenario involves a basal magma ocean: a dense, iron-rich melt that ponded at the base of the mantle during planetary differentiation and crystallized slowly over geological time. The question that has frustrated researchers is which minerals, if any, could have sequestered hydrogen during that crystallization. Most candidate hydrous silicates decompose at the relevant temperatures, and common mantle minerals such as bridgmanite hold very little water at depth. The new oxyhydroxides fill that gap, offering mineral hosts capable of locking hydrogen into the deep interior both from primordial sources and from water recycled downward by subducting slabs.

Subduction supplies the second pathway. Oceanic plates carry hydrated minerals down into the mantle, and laboratory studies in recent years have shown that some hydrous phases can survive transit to the lowermost mantle, potentially delivering water to the core–mantle boundary billions of years after Earth formed. If that arriving water reacts with iron-bearing mantle material—or with metallic iron from the core itself—the dense oxyhydroxides identified in this study could crystallize and accumulate. The result would be chemically distinct, hydrogen-rich lumps at the base of the mantle, exactly the kind of heterogeneity that seismic imaging keeps revealing.

The implications ripple outward from mineralogy into planetary evolution. The authors note that the stability of these phases provides a mechanism for transporting volatiles into Earth’s core, where hydrogen and oxygen could act as light elements dissolved in liquid iron alloy, or conversely for their episodic release back into the mantle. Mantle plumes—hot, buoyant upwellings that rise from the core–mantle boundary and feed surface hotspots such as Hawaii and Iceland—appear to be rooted in some of the same deep regions. If dense oxyhydroxide accumulations are destabilized or partially melted at the edges of plumes, the water they carry could be injected into the upwelling flow, eventually influencing the melting behavior, chemistry, and gas emissions of hotspot volcanoes at the surface. In this way, minerals formed nearly three thousand kilometers down could modulate the volatile budget of the planet across geological time.

The experimental achievement behind these conclusions should not be understated. Diamond anvil cell experiments at lowermost mantle conditions are notoriously difficult: samples are measured in volumes smaller than the width of a human hair, laser heating must be controlled with precision to avoid catastrophically melting the diamond anvils, and extracting single-crystal diffraction data from polycrystalline samples squeezed to megabar pressures demands sophisticated analysis software and high-brightness synchrotron beams. The team combined these techniques with electron microscopy and chemical analysis of recovered samples to characterize the new phases from multiple angles, including constraints on their hydrogen contents and densities at high pressure. Extended data on unit-cell parameters and refined atomic positions support the structural assignments, and the authors report that chemical analysis of recovered material found no evidence of contamination from the rhenium gasket or other experimental artifacts that might otherwise mimic the observed compositions.

Much remains to be tested. Whether the new oxyhydroxides can account quantitatively for the seismic properties of ultralow-velocity zones, how much water the deep mantle could realistically store in them, and how they interact with surrounding mantle minerals at the atomic scale are questions for future experiments and modeling. But the study reframes a long-standing problem in Earth science: the search for the planet’s missing deep water no longer needs to rely on sparse hydrous silicates that barely survive at depth. Iron, the most abundant element in Earth’s core and a major component of the deep mantle, combined with oxygen and hydrogen, may instead form its own family of dense reservoir minerals. If those reservoirs exist at the core–mantle boundary today, they could be simultaneously a vault preserving water from the planet’s birth, a sink for water recycled from its surface, and a chemical trigger for the plumes that reshape its surface—tying together, in a single crystal structure, processes that span the whole depth and history of our planet.

Subject of Research: Dense iron oxyhydroxide phases as potential water reservoirs in the deep lower mantle

Article Title: Dense iron oxyhydroxides as possible reservoirs of water under deep mantle conditions

Article References: Yuan, H., Man, L., Kupenko, I., Yin, Y., Aprilis, G., Pantousas, A., Pakhomova, A., Zhou, W., Pierru, R., Yang, K., Glazyrin, K., Rodrigues, J. E., Garbarino, G., Ball, J. A. D., Bykova, E., Hu, Q., Sossi, P. A., Frost, D. J., & Dubrovinsky, L. (2026). Dense iron oxyhydroxides as possible reservoirs of water under deep mantle conditions. Nature Geoscience. https://doi.org/10.1038/s41561-026-02067-1

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02067-1

Keywords: iron oxyhydroxides, deep mantle water, core–mantle boundary, ultralow-velocity zones, diamond anvil cell, high-pressure experiments, basal magma ocean, mantle plumes, subduction, hydrogen storage, volatile cycles, Nature Geoscience

Traditionally, these deep mantle anomalies have been linked to the relics of an ancient basal magma ocean (BMO), a global layer of partially molten silicates thought to have existed during Earth’s formative years. The crystallization and differentiation of this primordial magma ocean have been hypothesized to create compositional heterogeneities retained to this day. However, the existing models face a critical challenge: the predicted thick layers of iron-rich ferropericlase—expected from a crystallized BMO—do not align with seismic observations. Instead, tomography suggests that the lowermost mantle’s composition must be more varied and less dominated by ferropericlase cumulates.

Addressing this paradox, researchers Deng, Miyazaki, Yuan, and colleagues harnessed an innovative approach combining thermodynamic and geodynamic modeling to explain how contamination from the Earth’s core could modify the basal magma ocean’s crystallization process. Their model introduces the concept of a basal exsolution contaminated magma ocean, wherein oxides exsolved from the Earth’s core percolate upward and contaminate the crystallizing mantle material. This interaction fundamentally alters the mineralogy and dynamics at the planet’s bottommost layer, challenging previously held assumptions about the mantle’s composition.

Thermodynamic modeling demonstrated a key result: the presence of core-derived oxides suppresses the crystallization of ferropericlase in the basal magma ocean. Instead of forming overly thick layers of this iron-rich mineral, the contamination encourages the crystallization of alternative phases that better fit seismic velocity profiles observed beneath large low-velocity provinces. This insight provides a reconciliatory bridge between geochemical theory and seismic data, offering a more coherent picture of Earth’s deep interior structure.

The researchers’ geodynamic simulations further elucidate how this contaminated, crystallized mantle material evolves. As the basal magma ocean solidifies under the influence of continuous core exsolution, the resulting solid mantle layer takes on distinct physical and chemical properties consistent with LLVPs and ULVZs observed by seismic tomography. The simulations indicate that this layer is compositionally dense and thermomechanically distinct, creating conduits for dynamic interactions between mantle convection, core-mantle boundary processes, and deep Earth’s layered structure.

One particularly fascinating outcome of this study is the potential explanation for small-scale seismic scattering phenomena detected near core-mantle boundaries. The geodynamic models suggest that diapirs—buoyant blobs—of oxide-rich material exsolved from the core can become entrained within the solid mantle matrix. These diapirs, possessing unique chemical signatures, could act as localized heterogeneities responsible for the complex scattering patterns seen in seismic data across deep mantle regions.

Furthermore, the contaminated basal magma ocean’s preserved chemical distinctiveness could explain geochemical anomalies found in ocean island basalts (OIBs), which are volcanic rocks sourced from deep within the mantle. In particular, isotopic signatures for silicon, tungsten, and helium extracted from OIBs may trace their origin back to this basal melting and contamination process. Such signatures highlight a unified mechanism, uniting seismic, geochemical, and geodynamic observations within the framework of basal magma ocean evolution influenced by core exsolution.

This new model injects fresh vigor into longstanding debates about the core-mantle boundary region, which plays a pivotal role in Earth’s thermal and chemical evolution. The concept of a magma ocean continuously contaminated by core-derived oxides not only solves inconsistencies in the predicted mineralogical layering but also enriches our understanding of how Earth’s internal reservoirs interact over geological time. This complex interplay influences surface volcanism and, ultimately, the planet’s habitability.

By integrating insights from thermodynamic calculations with high-resolution geodynamic simulations, the research highlights how subtle chemical interactions at the core-mantle boundary can dramatically alter the mantle’s physical state and compositional heterogeneity. These heterogeneous reservoirs beneath the tectonic plates are key players in Earth’s mantle convection system, which governs the recycling of materials and the generation of the geomagnetic field.

Beyond the immediate geological implications, this work hints at broader planetary processes. The mechanisms proposed could be extrapolated to interpret the internal structures of other terrestrial planets or exoplanets with differentiated cores and mantles. Understanding how core exsolution influences basal magma oceans could provide a benchmark for planetary formation and evolution models in the broader cosmos.

At the heart of this story is Earth’s basal magma ocean, a vast and ancient feature once thought to be homogenously crystallizing into predictable layers. Instead, this ocean emerges as a chemically dynamic and evolving system, shaped by ongoing contamination from the core. This revised perspective not only offers explanations for the seismic and geochemical riddles but also exemplifies the intricate coupling between Earth’s core and mantle.

The implications of this research challenge the paradigm that the mantle is chemically uniform beneath the seismic boundaries and that LLVPs simply represent large blobs of early-formed mantle. They embrace a more nuanced vision, where the mantle is a mosaic of interacting reservoirs, continuously sculpted by geochemical fluxes arising from the core. Such a setting naturally explains the complex signatures observed in deep mantle tomography and geochemical fingerprints on the surface.

Moreover, the study elevates the importance of core exsolution — the process by which light elements and oxides segregate from the core — as a dominant factor influencing mantle evolution. This finding shifts the scientific conversation away from treating the lower mantle and the core as isolated reservoirs to considering their continuous and intricate exchange influencing Earth’s long-term dynamics.

As the scientific community digests these findings, future research will likely seek to refine the model through complementary seismic imaging and high-pressure mineral physics experiments. This will help further constrain the mineral phases predicted by the contaminated basal magma ocean and elucidate how such phases influence seismic wave propagation.

In conclusion, this pioneering work marries cutting-edge thermodynamic theory with detailed geodynamic processes, illuminating a previously unrecognized liaison between Earth’s core exsolution and deep mantle heterogeneity. It rewrites our understanding of the basal magma ocean’s fate and illuminates the geological record inscribed deep beneath the Earth’s surface, all while providing a cohesive framework connecting seismic anomalies, geochemical signatures, and planetary dynamics.

Article Title:
Deep mantle heterogeneities formed through a basal magma ocean contaminated by core exsolution

Article References:
Deng, J., Miyazaki, Y., Yuan, Q. et al. Deep mantle heterogeneities formed through a basal magma ocean contaminated by core exsolution.
Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01797-y

Image Credits: AI Generated