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…