Low-spin ferrous iron suppresses mantle oxidation beyond Earth-like pressures

The Earth's mantle has elevated Fe 3+ relative to those of other rocky bodies, yet the oxidation- and electronic state of iron at extreme pressures is poorly known. We present in-situ energy-domain synchrotron M"ossbauer spectra of 57 Fe-enriched silicate glasses at 298 K from 1 bar to 174 GPa in a diamond anvil cell. Glasses were synthesised with Fe 3+ /[Fe 3+ + Fe 2+] from 0.02 plus or minus 0.02 to 1.00 plus or minus 0.02, as determined by colourimetry. While pure Fe 3+ -basaltic glass shows minimal changes 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 about 92 % and about 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 Fe 2+ low spin (LS), whereas D 1 is Fe 2+ high spin (HS). Consequently, the Fe 3+ /[Fe 3+ +Fe 2+] of planetary mantles reach a maximum near about 40 GPa, before decreasing at higher pressures due to the stabilisation of Fe 2+ LS. 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.