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Abstract 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 phasesδ‐AlOOH (delta phase), MgSiO2(OH)2(phase H), andε‐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 andδ‐(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 withδ‐(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.
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This content will become publicly available on December 7, 2026
Ferric Iron, Hydrogen, and Major Element Quantification of Amphibole Minerals Using Raman Spectroscopy and Multivariate Analysis
ABSTRACT Quantification of Fe redox state and hydrogen content of amphibole provides information regarding the relationship between oxygen and water concentrations in terrestrial and planetary materials. Raman spectroscopy is a powerful technique due to its ability to characterize both %Fe3+and H2O from single crystal measurements, in addition to other chemical, mineralogical, and structural properties. Raman spectral measurements of amphibole minerals are used here to estimate %Fe3+(relative to total Fe) and H2O (wt%) contents using partial least squares (PLS) multivariate modeling. The accuracy of our model for prediction of %Fe3+is ± 8.11% (absolute) expressed as root‐mean‐square error (RMSE) of the entire data set, covering the range from 0 to 100% with anR2value of 0.85. The model for prediction of H2O has an internal RMSE of ± 0.09 wt% over the range from 0.1 to 1.9 wt% with anR2value of 0.95. Additional compositional model variables for predicting FeO, Fe2O3, MgO, CaO, Cr2O3, Al2O3, and TiO2have highR2values above 0.82; theR2value for SiO2is 0.63. Reliable models could not be achieved for MnO, Na2O, and K2O. The successful creation of our compositional models along with detailed analysis of the PLS model coefficients indicates that Raman spectroscopy can be used as a quantitative compositional tool in characterizing the amphibole mineral group. Quantifying amphibole compositions is useful for evaluating repositories of hydrogen, constraining the water budget of the terrestrial crust and interior, developing geothermobarometers and geohygrometers, and quantifying magma ascent rates.
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- Award ID(s):
- 2042386
- PAR ID:
- 10654366
- Publisher / Repository:
- Journal of Raman Spectroscopy
- Date Published:
- Journal Name:
- Journal of Raman Spectroscopy
- ISSN:
- 0377-0486
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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