1,2,1,- 1Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai, China (hongsheng.yuan@hpstar.ac.cn; qingyang.hu@hpstar.ac.cn)
- 2Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland (lianjie.man@eaps.ethz.ch)
- 3Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, Germany (dan.frost@uni-bayreuth.de; leonid.Dubrovinsky@Uni-Bayreuth.de)
The chemical evolution of Earth’s mantle is governed by the interplay between primordial reservoirs formed during accretion and recycled components introduced by subduction. Although geochemical evidence indicates the persistence of deep primordial water1, viable mineralogical hosts within the iron-rich2-4, high-temperature residues of a crystallized basal magma ocean (BMO) remain elusive. Here we report the synthesis and crystal structures of two hexagonal iron oxyhydroxides, Fe5O12Hx (x≥9) and Fe7O12Hx (x≥3) at 78–198 GPa and 2,400–2,800 K, using in situ single-crystal X-ray diffraction in a laser heated diamond anvil cell. We identify the enigmatic "H-phase"5—a controversial feature in deep-mantle mineralogy—as our Fe5O12Hx oxyhydroxide. We show that its formation is triggered by trace adsorbed moisture even in nominally anhydrous systems, resolving long-standing debates regarding the stability of iron-bearing bridgmanite. Unlike previous candidates, these dense oxyhydroxides coexist with major lower-mantle minerals under conditions representative of BMO crystallization and the margins of large low shear velocity provinces. This finding identifies these oxyhydroxides as solid compounds that chemically anchor primordial water, reconciling early Earth solidification with the genesis of ultralow velocity zones and potentially serving as deep sources for volatile-rich mantle plumes.
Reference
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How to cite: Yuan, H., Man, L., Hu, Q., Frost, D., and Dubrovinsky, L.: Deep-mantle iron oxyhydroxides as reservoirs of primordial and recycled water, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15371, https://doi.org/10.5194/egusphere-egu26-15371, 2026.
