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Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle
E Boulard1, M Harmand1, F Guyot1
1Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, IRD Institut de Minéralogie, Physique des Matériaux et Cosmochimie - IMPMC 4 Place Jussieu 75005 Paris France.
Geophysical Research Letters
|April 23, 2019
Summary
New iron oxides discovered at deep Earth conditions reveal iron changing from Fe3+ to Fe2+ and oxygen oxidizing. This finding impacts our understanding of deep planetary interior chemistry.
Area of Science:
- Geochemistry
- Mineral Physics
- Planetary Science
Background:
- High-pressure and temperature experiments reveal new iron oxide phases.
- Pyrite-type FeO2 and FeO2Hx phases are stable at deep terrestrial lower mantle conditions.
Purpose of the Study:
- Investigate the iron oxidation state in high-pressure transformation products of Fe3+OOH goethite.
- Determine the valence state of iron under deep mantle conditions.
Main Methods:
- In situ X-ray absorption spectroscopy.
- Laser-heated diamond-anvil cell experiments.
- Probing iron oxidation state at ~91 GPa and 1,500–2,350 K.
Main Results:
- Iron transformed from Fe3+ to Fe2+ at ~91 GPa and 1,500–2,350 K.
- A shift of -3.3 ± 0.1 eV in the Fe K-edge confirmed the valence change.
- Observed reductive valence change of iron suggests concomitant oxidation of oxygen from O2- to O-.
Conclusions:
- The study confirms reductive iron valence change in deep Earth conditions.
- This peculiar chemistry challenges existing models of crystal chemistry in planetary interiors.
- Findings support previous structural suggestions for pyrite-type FeO2 and FeO2Hx phases.
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