Synthesis and Stability of an Eight-Coordinated Fe3O4 High-Pressure Phase: Implications for the Mantle Structure of Super-Earths

1C. C. Zurkowski,1J. Yang,1S. Chariton,2V. B. Prakapenka,1Y. Fei
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2022JE007344]
1Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, USA
2Center for Advanced Radiation Sources, The University of Chicago, Lemont, IL, USA
Published by arrangement with John Wiley & Sons

Super-Earths ranging up to 10 Earth masses (ME) with Earth-like density are common among the observed exoplanets thus far, but their measured masses and radii do not uniquely elucidate their internal structure. Exploring the phase transitions in the Mg-silicates that define the mantle-structure of super-Earths is critical to characterizing their interiors, yet the relevant terapascal conditions are experimentally challenging for direct structural analysis. Here we investigated the crystal chemistry of Fe3O4 as a low-pressure analog to Mg2SiO4 between 45–115 GPa and up to 3000 K using powder and single crystal X-ray diffraction in the laser-heated diamond anvil cell. Between 60–115 GPa and above 2000 K, Fe3O4 adopts an 8-fold coordinated Th3P4-type structure (I-43d, Z = 4) with disordered Fe2+ and Fe3+ into one metal site. This Fe-oxide phase is isostructural with that predicted for Mg2SiO4 above 500 GPa in super-Earth mantles and suggests that Mg2SiO4 can incorporate both ferric and ferrous iron at these conditions. The pressure-volume behavior observed in this 8-fold coordinated Fe3O4 indicates a maximum 4% density increase across the 6- to 8-fold coordination transition in the analog Mg-silicate. Reassessment of the FeO—Fe3O4 fugacity buffer considering the Fe3O4 phase relationships identified in this study reveals that increasing pressure and temperature to 120 GPa and 3000 K in Earth and planetary mantles drives iron toward oxidation.

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