The influence of low-spin ferrous iron on the oxidation state of the Earth's mantle

The Earth's mantle has elevated Fe³⁺ contents relative to those of other telluric bodies, a property thought to reflect the disproportionation of ferrous iron into its metallic and ferric counterparts during core formation. However, how the oxidation and electronic state of iron change as a function of pressure in compositions relevant to that of Earth's mantle are not fully understood. In this study, we present in-situ energy domain synchrotron Mössbauer spectra of ⁵⁷Fe-enriched peridotitic- and basaltic glasses at 298 K compressed from 1 bar to 174 GPa in a diamond anvil cell. Glasses were synthesised with different Fe³⁺/[Fe³⁺ + Fe²⁺] ratios, 0.02 ± 0.02 and 1.00 ± 0.02, respectively, as determined by colorimetry. At 1 bar, the spectrum of the Fe³⁺-basaltic glass is well fit by a single doublet. In contrast, the spectra of both Fe²⁺-rich peridotitic and basaltic glass are fit by two doublets, D₁ (~92 %) and D₂ (~8 %) at 1 bar. As pressure increases, the integral area of the D₂ doublet increases at the expense of D₁ to reach a D₂/(D₁ + D₂) ratio of 0.65 by 172 GPa. Because this transition is reversible with pressure and no metallic iron is detected, the D₂ feature is ascribed to Fe²⁺ in its low spin (LS) state, whereas D₁ is consistent with Fe²⁺ high spin (HS). This assignment resolves a long-standing controversy on the interpretation of the Mössbauer spectra of basaltic glasses. As a consequence of the stabilisation of Fe²⁺ with pressure, terrestrial planets more massive than Earth likely do not host increasingly oxidising mantles.