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

• Published in: Journal of geophysical research. Solid earth 2023-04, Vol.128 (4), p.n/a
• Main Authors: Strozewski, Benjamin, Buchen, Johannes, Sturhahn, Wolfgang, Ishii, Takayuki, Ohira, Itaru, Chariton, Stella, Lavina, Barbara, Zhao, Jiyong, Toellner, Thomas S., Jackson, Jennifer M.
• Format: Article
• Language: English
• Subjects: Acoustic velocity; Analytical methods; Compressibility; Crystal structure; Earth; Earth mantle; Elastic properties; Electronic structure; Equations of state; Geophysics; GEOSCIENCES; High temperature; Hydrogen; Hydrogen bonding; Hydrogen bonds; Iron; Lower mantle; Mossbauer spectroscopy; Oceanic crust; P-waves; Phases; Pressure effects; Scattering; Seismic activity; Seismic stability; Seismic velocities; Seismic waves; Solid solutions; Sound velocity; Spectroscopy; Spectrum analysis; Subduction (geology); Synchrotrons; Transport; Velocity; X-ray diffraction
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB026291

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: 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. Plain Language Summary Hydrogen can be transported into the interior of the Earth by phases which contain hydrogen in their crystal structure. Interpretation of measured seismic velocities relies on understanding the elastic properties of minerals, including their equation of state. In this study, we use X‐ray diffraction and Mössbauer spectroscopy to constrain the equation of state for a particular hydrous phase known as (Al, Fe)‐phase H and understand pressure‐induced related effects due to changes in the electronic structure of Fe atoms. Our results imply that only a small amount of this phase needs to be transported to the deep mantle to reproduce observed seismic velocities at the edges of large structures known as low seismic‐velocity provinces. The presence of (Al, Fe)‐phase H in subducted oceanic crust may also