Hydrogen Bonding and Spin State of (Al,Fe)-bearing Phase H at High Pressure

Oxyhydroxide phases in the (Al,Fe)OOH-MgSiO2(OH)2 system have been found to coexist with bridgmanite and retain hydrogen in their crystal structures at lower-mantle pressures and temperatures in experiments [1, 2], in part due to the formation of strong, symmetric hydrogen bonds [3]. Accumulation of these phases as part of subducted slab material may contribute to the observed anti-correlation between negative shear-wave velocity anomalies and positive bulk sound velocity anomalies at the edges of large, low shear velocity provinces [4,5]. Fe-bearing phases are also affected by changes in the electronic structure of Fe3+, which can result in a high to low-spin crossover that influences elastic properties at lower mantle conditions. While both are important phenomena in oxyhydroxides at high pressure, the relationship between hydrogen bond symmetrization and the spin transition of Fe3+ has not been explored for compositions in this system which include Mg and Si cations.

Powder X-ray diffraction measurements were performed in a symmetric diamond anvil cell on (Al,Fe)-bearing phase H (Al0.84Fe3+0.07Mg0.02Si0.06OOH) to pressures up to 125 GPa at beamline 13-ID-D (GSECARS) and synchrotron Mössbauer spectra collected at beamline 3-ID-B of the Advanced Photon Source, Argonne National Laboratory. Synchrotron infrared absorption spectra were collected at beamline 22-IR-1, National Synchrotron Light Source II, Brookhaven National Laboratory. We fit the pressure-volume data determined from x-ray diffraction with a model that accounts for a compression-induced spin transition in Fe3+. Synchrotron Mössbauer spectra and synchrotron infrared spectra constrain the electronic environment of Fe3+ and describe the vibrational frequencies of infrared-active modes associated with the hydrogen bonds, respectively. We evaluate the consequences of the spin transition of Fe3+ in (Al,Fe)-bearing phase H and its relationship to changes in hydrogen bonding, with implications for the seismic signature of subducted oceanic crust in the lowermost mantle.

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