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Published April 2023 | metadata_only
Journal Article Embargoed

Equation of State and Spin Crossover of (Al, Fe)-Phase H


The transport of hydrogen into Earth's deep interior may have an impact on lower mantle dynamics as well as on the seismic signature of subducted material. Due to the stability of the hydrous phases δ-AlOOH (delta phase), MgSiO₂(OH)₂ (phase H), and ε-FeOOH at high temperatures and pressures, their solid solutions may transport significant amounts of hydrogen as deep as the core-mantle boundary. We have constrained the equation of state, including the effects of a spin crossover in the Fe³⁺ atoms, of (Al, Fe)-phase H: Al_(0.84)Fe³⁺_(0.07)Mg_(0.02)Si_(0.06)OOH, using powder X-ray diffraction measurements to 125 GPa, supported by synchrotron Mössbauer spectroscopy measurements on (Al, Fe)-phase H and δ-(Al, Fe)OOH. The changes in spin state of Fe³⁺ in (Al, Fe)-phase H results in a significant decrease in bulk sound velocity and occurs over a different pressure range (48–62 GPa) compared with δ-(Al, Fe)OOH (32–40 GPa). Changes in axial compressibilities indicate a decrease in the compressibility of hydrogen bonds in (Al, Fe)-phase H near 30 GPa, which may be associated with hydrogen bond symmetrization. The formation of (Al, Fe)-phase H in subducted oceanic crust may contribute to scattering of seismic waves in the mid-lower mantle (∼1,100–1,550 km). Accumulation of 1–4 wt.% (Al, Fe)-phase H could reproduce some of the seismic signatures of large, low seismic-velocity provinces. Our results suggest that changes in the electronic structure of phases in the (δ-AlOOH)-(MgSiO₂(OH)₂)-(ε-FeOOH) solid solution are sensitive to composition and that the presence of these phases in subducted oceanic crust could be seismically detectable throughout the lower mantle.

Additional Information

© 2023. American Geophysical Union. This work was supported by the National Science Foundation's (NSF) Collaborative Studies of the Earth's Deep Interior (CSEDI) under EAR-2009935. Portions of this work were performed at sectors 3 and 13-GeoSoilEnviroCARS of the Advanced Photon Source (APS), a United States Department of Energy (DOE) facility. Use of the APS is supported by the U.S. DOE Office of Science (DE-AC02-06CH11357). Sector 3 operations are partially supported by COMPRES (EAR-1661511). GeoSoilEnviroCARS is supported by NSF – Earth Sciences (EAR-1634415) and DOE-Geosciences (DE-FG02-94ER14466). Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR-1606856. The authors declare no competing financial interests. Colormaps used in this article were generated using the tools created by Dr. David Nichols: http://www.davidmathlogic.com. The authors thank two anonymous reviewers for their comments, which significantly improved the quality of the manuscript.

Additional details

August 22, 2023
August 22, 2023