The Redox State of Earth's Mantle

Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo 2 with depth, which appears to result principally from the effect of pressure on the controlling Fe 3+ /Fe 2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo 2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe 2 O 3 -garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe 3+ /ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe 3+ into (Fe,Mg)SiO 3 perovskite results in very high perovskite Fe 3+ /ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle should contain approximately 1 weight% metallic Fe formed through FeO disproportionation, if the bulk oxygen content is the same as the upper mantle. Loss of disproportionated metallic Fe from the lower mantle during core formation could explain the higher Fe 3+ /ΣFe ratio of the present-day upper mantle when compared to that expected during core formation. The influence of pressure on mantle fo 2 has important implications for the speciation of C-O-H-S volatile phases in Earth today and during its early evolution.