Low-spin ferrous iron suppresses mantle oxidation beyond Earth-like pressures
Abstract
The Earth's mantle has elevated Fe 3+ relative to those of other rocky bodies, a property thought to reflect the disproportionation of ferrous iron into its metallic and ferric counterparts during core formation at elevated pressures. However, whether more massive planets ('super-Earths') become increasingly oxidised is poorly known for lack of constraints on the oxidation-and electronic state of iron at extreme pressures. We present in-situ energydomain synchrotron Mössbauer spectra of 57 Fe-enriched peridotitic-and basaltic glasses at 298 K compressed from 1 bar to 174 GPa in a diamond anvil cell. Three glasses were synthesised with different Fe 3+ /[Fe 3+ + Fe 2+ ] ratios; 0.02 ± 0.02 (Fe 2+ -basaltic, peridotitic) and 1.00 ± 0.02 (Fe 3+ -basaltic), respectively, as determined by colourimetry. While the spectrum of pure Fe 3+ -basaltic glass shows minimal changes in its hyperfine parameters up to 174 GPa, the spectra of Fe 2+ -peridotitic and basaltic glasses are fit by two doublets, D 1 and D 2 . At 1 bar, their relative intensities are ∼92 % and ∼8 %, respectively, but the integral area ratio, D 2 /(D 1 + D 2 ), reaches 0.65 by 172 GPa. Because this transition is reversible with pressure and no metallic iron is detected, the D 2 feature is ascribed to Fe 2+ in its low spin (LS) state, whereas D 1 is consistent with Fe 2+ high spin (HS). The Fe 3+ /[Fe 3+ +Fe 2+ ] of planetary mantles at constant relative f O 2 increases to a maximum near ∼40 GPa, before decreasing at higher pressures due to the stabilisation of Fe 2+ LS . This peak coincides with estimated coremantle equilibrium on Earth, implying that its uniquely oxidised mantle and habitable state may result from core formation within a Goldilocks pressure range. Secondary atmospheres are predicted to transition from H 2rich for Moon-sized bodies, to CO-rich for Earth-like planets and H 2 -and CH 4 -bearing around super-Earths and sub-Neptunes.
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BibTeX
@article{girani2026lowspin,
title = {{Low-spin ferrous iron suppresses mantle oxidation beyond Earth-like pressures}},
author = {Alice Girani and Sylvain Petitgirard and Sergey Yaroslavtsev and Georgios Aprilis and James Badro and Antoine Bézos and Hugh St. C. O’neill and Paolo A Sossi},
journal = {Earth and Planetary Science Letters},
year = {2026},
month = sep,
doi = {10.1016/j.epsl.2026.120168},
url = {https://hal.science/hal-05740204v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Low-spin ferrous iron suppresses mantle oxidation beyond Earth-like pressures
AU - Alice Girani
AU - Sylvain Petitgirard
AU - Sergey Yaroslavtsev
AU - Georgios Aprilis
AU - James Badro
AU - Antoine Bézos
AU - Hugh St. C. O’neill
AU - Paolo A Sossi
PY - 2026
DA - 2026/09/15
JO - Earth and Planetary Science Letters
DO - 10.1016/j.epsl.2026.120168
UR - https://hal.science/hal-05740204v1
AB - The Earth's mantle has elevated Fe 3+ relative to those of other rocky bodies, a property thought to reflect the disproportionation of ferrous iron into its metallic and ferric counterparts during core formation at elevated pressures. However, whether more massive planets ('super-Earths') become increasingly oxidised is poorly known for lack of constraints on the oxidation-and electronic state of iron at extreme pressures. We present in-situ energydomain synchrotron Mössbauer spectra of 57 Fe-enriched peridotitic-and basaltic glasses at 298 K compressed from 1 bar to 174 GPa in a diamond anvil cell. Three glasses were synthesised with different Fe 3+ /[Fe 3+ + Fe 2+ ] ratios; 0.02 ± 0.02 (Fe 2+ -basaltic, peridotitic) and 1.00 ± 0.02 (Fe 3+ -basaltic), respectively, as determined by colourimetry. While the spectrum of pure Fe 3+ -basaltic glass shows minimal changes in its hyperfine parameters up to 174 GPa, the spectra of Fe 2+ -peridotitic and basaltic glasses are fit by two doublets, D 1 and D 2 . At 1 bar, their relative intensities are ∼92 % and ∼8 %, respectively, but the integral area ratio, D 2 /(D 1 + D 2 ), reaches 0.65 by 172 GPa. Because this transition is reversible with pressure and no metallic iron is detected, the D 2 feature is ascribed to Fe 2+ in its low spin (LS) state, whereas D 1 is consistent with Fe 2+ high spin (HS). The Fe 3+ /[Fe 3+ +Fe 2+ ] of planetary mantles at constant relative f O 2 increases to a maximum near ∼40 GPa, before decreasing at higher pressures due to the stabilisation of Fe 2+ LS . This peak coincides with estimated coremantle equilibrium on Earth, implying that its uniquely oxidised mantle and habitable state may result from core formation within a Goldilocks pressure range. Secondary atmospheres are predicted to transition from H 2rich for Moon-sized bodies, to CO-rich for Earth-like planets and H 2 -and CH 4 -bearing around super-Earths and sub-Neptunes.
SN - 0012-821X
ER -