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

Author:

Strozewski Benjamin1ORCID,Buchen Johannes12ORCID,Sturhahn Wolfgang1ORCID,Ishii Takayuki34ORCID,Ohira Itaru5ORCID,Chariton Stella6,Lavina Barbara67ORCID,Zhao Jiyong7,Toellner Thomas S.7,Jackson Jennifer M.1ORCID

Affiliation:

1. Seismological Laboratory California Institute of Technology Pasadena CA USA

2. Now at Department of Earth Sciences University of Oxford Oxford UK

3. Bayerisches Geoinstitut Universität Bayreuth Bayreuth Germany

4. Now at Center for High Pressure Science and Technology Advanced Research Beijing China

5. Department of Chemistry Gakushuin University Tokyo Japan

6. Center for Advanced Radiation Sources The University of Chicago Chicago IL USA

7. Advanced Photon Source Argonne National Laboratory Argonne IL USA

Abstract

AbstractThe 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: Al0.84Fe3+ 0.07Mg0.02Si0.06OOH, 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.

Funder

National Science Foundation

Publisher

American Geophysical Union (AGU)

Subject

Space and Planetary Science,Earth and Planetary Sciences (miscellaneous),Geochemistry and Petrology,Geophysics

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