High-pressure experiments unveil new mineral hosts for water in the deep lower mantle -Dr. Hongsheng Yuan

Water is a critical component governing the dynamic evolution, magmatic melting, and volatile cycling of Earth's interior. Descending subduction slabs continually transport water from the surface into the deep Earth. Concurrently, isotopic anomalies in volcanic rocks, such as ocean island basalts, strongly point to the existence of a "primordial water" reservoir sequestered deep within the mantle since planetary accretion. This deep reservoir is thought to originate from the residual enrichment of volatile components during the fractional crystallization of an early-Earth basal magma ocean (BMO). However, a long-standing scientific puzzle remains: the storage capacity of water in the dominant lower-mantle minerals (bridgmanite, ferropericlase, anddavemaoite) is extremely limited; moreover, previously known high-pressure hydrous minerals generally break down under the high temperatures present at the core–mantle boundary. Could the primordial water hosted within a dense, iron-rich basal magma ocean crystallize into stable hydrous phases and survive at the base of the hot lower mantle over Earth's 4.6-billion-year history?

Recently, an international collaboration led by Associate Specialist Hongsheng Yuan and Prof. Qingyang Hu from the Center for High Pressure Science and Technology Advanced Research (HPSTAR, China), together with Dr. Lianjie Man, Prof. Leonid Dubrovinsky, and Prof. Daniel J. Frost from the Bayerisches Geoinstitut (BGI) at the University of Bayreuth, alongside researchers from ETH Zürich, Goethe University Frankfurt, the European Synchrotron Radiation Facility (ESRF), Deutsches Elektronen-Synchrotron (DESY), and the Shanghai Synchrotron Radiation Facility (SSRF), addressed this question. Employing laser-heated diamond anvil cells combined with in situ synchrotron X-ray diffraction at extreme pressures and temperatures simulating the lowermost mantle, the team successfully synthesized and identified two previously unknown hexagonal iron oxyhydroxides: Fe₅O₁₂H_x and Fe₇O₁₂H_x. Systematic experimental petrological investigations demonstrate that these dense, hydrogen-rich phases remain stable within complex mantle compositions, exhibiting dehydration temperatures well above those along the lowermost mantle geotherm. This discovery provides crucial mineralogical evidence for deciphering the formation and hosting mechanisms of water reservoirs near the core–mantle boundary, as well as the origin of geophysical anomalies like ultralow-velocity zones (ULVZs).

The study, titled “Dense iron oxyhydroxides as possible reservoirs of water under deep mantle conditions,” has been published in Nature Geoscience.

In Situ Extreme Experiments and Crystal Structure Determination

To simulate the high pressure–temperature regime of basal magma ocean crystallization in the deep lower mantle, the research team recreated conditions of 78–198 GPa and 2,400–2,800 K in laser-heated diamond anvil cells. Under these near-core–mantle-boundary conditions, iron-bearing silicates reacted under water-undersaturated conditions to two novel hexagonal iron oxyhydroxides belonging to the P6₃/m space group.

In situ single-crystal X-ray diffraction revealed that the two phases feature rigid frameworks constructed from rare, hyper-coordinated iron–oxygen polyhedra that construct channels running parallel to the -axis (Figure 1):

Figure 1: Crystal structure determination of the hexagonal iron oxyhydroxides. (a, b) Reconstructed reciprocal layers of reflections for Fe₅O₁₂H_x and F_e7O₁₂H_x; (c) Crystal structure of Fe₅O₁₂H_x at 100 GPa featuring the FeO₉ framework; (d) Crystal structure of Fe₇O₁₂H_x at 170 GPa featuring the FeO₈ framework. Both channel sites contain partially occupied iron cations.

Deep Geophysical Implications

Thermodynamic volume modeling combined with high-pressure equations of state indicates that Fe₅O₁₂H_x (x≥9) possesses a remarkable hydrogen storage capacity, with an equivalent water content reaching approximately 17 wt%. Consequently, only a tiny fraction of these phases is needed to accommodate large quantities of deep water. Moreover, both phases exhibit high thermal stability and calculated densities (6.08–6.98 g/cm³) substantially exceeding those of ambient mantle silicates under deep-mantle pressures. These physical attributes yield several major geodynamic implications:

Facilitating Deep Water Sequestration: Owing to their elevated density, once formed during BMO fractional crystallization or subduction, these phases settle gravitationally and accumulate above the core–mantle boundary, resisting entrainment by mantle convection.

Constraining Ultralow-Velocity Zones (ULVZs): Highly localized piles of dense, iron- and hydrogen-rich assemblages provide a viable mineralogical origin for the seismic anomalies characteristic of ULVZs rooted near the core–mantle boundary.

Linking Deep Reservoirs to Surface Volatiles: As deep-seated materials are entrained by ascending mantle plumes, decompression below approximately 70 GPa destabilizes the hydrogen-rich framework. The ensuing dehydration melting creates a direct conduit for sequestered deep volatiles to return to Earth's surface.

This work advances our understanding of water sources and storage mechanisms under extreme planetary conditions. It provides key mineralogical constraints for interpreting complex geophysical and geochemical signatures at the core–mantle boundary and establishes a physicochemical framework for tracing long-term deep volatile cycles in Earth and other rocky planets.

Related news report: 

The Earth’s puzzling deep water cycle. 

New iron-bearing compounds identified as potential water reservoirs near Earth’s core.                           

水，是地球生命的根基。我们熟知江河湖海等地表水体，但很少有人知道，地球内部还封存着体量巨大的水。长久以来，地球科学家一直猜想：在地幔最底部、地核之上的核幔边界，可能保存着远古遗留的原始水储库。但一道关键难题始终悬而未决：究竟何种矿物，能够在数千摄氏度高温、上百万大气压的极端环境下牢牢锁住水？近日，北京高压科学研究中心苑洪胜专业副研究员、胡清扬研究员，联合拜罗伊特大学巴伐利亚实验地球科学研究所（BGI）蔄廉洁博士，以及国内外多家顶尖科研机构的科学家，利用激光加热金刚石对顶压腔结合原位同步辐射 X 射线衍射技术，模拟下地幔底部极端温压条件，发现两种全新的高密度六方相铁羟基氧化物 —Fe₅O₁₂H_x （x≥9）与 Fe₇O₁₂H_x (x≥3)。这类致密富氢富铁氧化物可稳定存在于核幔边界极端环境，脱水温度远高于下地幔底部温度，既能储存地球原始水源，也可以容纳板块俯冲带入的再循环水，为解开深部水储库谜题提供了关键答案。相关研究成果以Dense iron oxyhydroxides as possible reservoirs of water under deep mantle conditions为题，发表于《自然-地球科学》。