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 energy-domain 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 LS 2 + . This peak coincides with estimated core-mantle 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 2 -rich for Moon-sized bodies, to CO-rich for Earth-like planets and H 2 - and CH 4 -bearing around super-Earths and sub-Neptunes.
Girani et al. (Sun,) studied this question.