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, yet the oxidation- and electronic state of iron at extreme pressures is poorly known. We present in-situ energy-domain synchrotron Mössbauer 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 $\pm$ 0.02 to 1.00 $\pm$ 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 $\sim$92 % and $\sim$8 %, respectively, but the integral area ratio, D${}_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 $\sim$40 GPa, before decreasing at higher pressures due to the stabilisation of Fe${}_{\mathrm{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.

33 pages, 7 figures, 1 table

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