Dense Iron Oxyhydroxides May Store Water Deep in Earth’s Lower Mantle

Deep beneath our feet, roughly 2,900 kilometers down, the boundary between Earth’s molten iron core and its silicate mantle remains one of the most enigmatic regions of the planet. Seismologists have long detected peculiar patches there where earthquake waves slow down dramatically, so-called ultralow-velocity zones, whose origin has been debated for decades. Now, a team of researchers reports the synthesis of two previously uncharacterized dense iron oxyhydroxide phases under the extreme pressures and temperatures of the lowermost mantle, offering a compelling new candidate for what those mysterious zones might be made of—and, in the process, sketching out a possible hiding place for water at the very bottom of Earth’s interior.

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