Water transported into Earth’s interior by subduction strongly influences dynamics such as volcanism and plate tectonics1,2,3. Several recent studies have reported hydrous minerals to be stable at pressure and temperature conditions representative of Earth’s deep interior, implying that surface water may be transported as far as the core–mantle boundary4,5,6,7,8. However, the hydrous mineral goethite, α-FeOOH, was recently reported9 to decompose under the conditions of the middle region of the lower mantle to form FeO2 and release H2, suggesting the upward migration of hydrogen and large fluctuations in the oxygen distribution within the Earth system. Here we report the stability of FeOOH phases at the pressure and temperature conditions of the deep lower mantle, based on first-principles calculations and in situ X-ray diffraction experiments. In contrast to previous work suggesting the dehydrogenation of FeOOH into FeO2 in the middle of the lower mantle9, we report the formation of a new FeOOH phase with the pyrite-type framework of FeO6 octahedra, which is much denser than the surrounding mantle and is stable at the conditions of the base of the mantle. Pyrite-type FeOOH may stabilize as a solid solution with other hydrous minerals in deeply subducted slabs, and could form in subducted banded iron formations. Deep-seated pyrite-type FeOOH eventually dissociates into Fe2O3 and releases H2O when subducted slabs are heated at the base of the mantle. This process may cause the incorporation of hydrogen into the outer core by the formation of iron hydride, FeHx, in the reducing environment of the core–mantle boundary.
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We thank Y. Ohishi and N. Hirao for their assistance in the experiments at BL10XU, SPring-8 (proposal numbers 2014B1363 and 2016A1476). We are grateful to T. Irifune for technical supports and discussion. This work was supported by MEXT/JSPS KAKENHI (grant numbers JP15H05469, JP25220712 and JP15H05829 to M.N., JP16H06285 and JP26800274 to Y.K., JP26400516 to J.T., JP26287137 and JP15H05834 to J.T. and T.T.). This research was also supported in part by MEXT as “Exploratory Challenge on Post-K computer” (Frontiers of Basic Science: Challenging the Limits). This research used the computational resources of the K computer provided by the RIKEN Advanced Institute for Computational Science through the HPCI System Research project (Project ID: hp160251/hp170220).
The authors declare no competing financial interests.
Reviewer Information Nature thanks F. Guyot and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a, XRD patterns at 94–98 GPa after heating at 2,000 K and >2,400 K. Data were obtained using an X-ray flat-panel detector. Crystallization of CaCl2-type SiO2 from an SiO2 glass pressure medium was recognized at higher temperature. py, pyrite-type FeOOH; FeHx, dhcp-FeHx; SiO2, CaCl2-type SiO2; ppv, Fe2O3 post-perovskite; Au, gold. b, XRD patterns during heating at 100–110 GPa. XRD patterns are shown in order of increasing laser power since the temperature measurements at <1,500 K contain large uncertainties. The temperature was measured to be 1,500 K for a laser power of 35 W using the radiation spectrum. Data were obtained using an X-ray flat-panel detector. XRD peaks corresponding to dhcp-FeHx appeared before the growth of pyrite-type FeOOH. The rapid nucleation rate of Fe compared to FeOOH probably caused the metastable growth of dhcp-FeHx and released oxygen that may have diffused to the pressure medium.
Extended Data Figure 2 FeHx cell volumes (V/Z) derived from XRD patterns as a function of pressure at room temperature.
Filled circles indicate the volumes of dhcp-FeHx obtained in our experiment, which are located between those of hcp-Fe and dhcp-FeH (solid lines)16. Error bars reflect the standard deviations (1σ) derived from various equations of state for gold. The composition was estimated to be FeH0.7 on the basis of volume comparisons.
a, 40 GPa and 1,223 K. b, 40 GPa and 1,513 K. Two species of CaCl2-type hydroxide with different Fe/Al ratios were produced, as observed in colour contrast. Minerals in dark grey and light grey represent the Al-rich and Fe-rich compositions, respectively. White colour shows an Fe2O3 phase. At higher temperature, the contrast became weak owing to the wider solid-solution range.
a, 40 GPa and 1,223 K. b, 40 GPa and 1,513 K. Diffraction peaks from the Au capsule overlap with those from the sample. Red, Fe-rich hydroxide ε-FeOOH; blue, Al-rich hydroxide δ-AlOOH.
Extended Data Figure 5 Cell volumes of solid solution between ε-FeOOH and δ-AlOOH at ambient conditions as a function of the FeOOH component.
A wide solid-solution range was observed in the system FeOOH–AlOOH.
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Nishi, M., Kuwayama, Y., Tsuchiya, J. et al. The pyrite-type high-pressure form of FeOOH. Nature 547, 205–208 (2017) doi:10.1038/nature22823
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