The oxidation state of the Earth is an area of great interest in petrology and mineral physics, as it plays a key role in governing mantle mineralogy and determining mineralogical host of elements with multiple valence states, such as iron and carbon. The amount of oxygen available to drive reactions in a system, as measured by oxygen fugacity, dictates a system’s mineralogy, as controlled by reactions with oxygen, including the oxidation of iron to form wüstite: Fe + 1/2O2 = FeO (iron wüstite buffer, IW), or the simultaneous oxidation of iron and diamond to form siderite: Fe + C + 3/2O2 = FeCO3 (siderite diamond iron buffer, SDI). The degree of oxidation of the lower mantle has been the subject of recent interest, particularly in light of the recently reported crystal-chemically controlled, pressure-induced auto-oxidation-reduction reaction in iron (Frost et al., 2004) and debates on the oxidation state of carbon in the mantle (Brenker et al., 2007; McCammon et al., 2004). In that pressure-induced iron self-reduction is independent of oxygen fugacity, it is likely that the coexistence of metallic iron and wüstite buffers mantle redox state at or near IW, as well as determines whether the host of carbon is either diamond or a carbonate. Therefore, knowledge of the relationship between the buffer assemblage containing both reduced and oxidized carbon (SDI buffer) and that containing both reduced and oxidized iron (IW buffer) is critical to knowing the mineralogical host of carbon throughout the mantle as a function of redox state.
Thermodynamic modeling of iron, carbon, wüstite, and siderite suggests the IW buffer lies between 1.5 and 2.5 log units above the SDI buffer across the pressure and temperature range of Earth’s mantle, suggesting that FeCO3 (siderite) will reduce to diamond. This model is supported by high-pressure, high-temperature experiments carried out in the laser-heated diamond anvil cell from 21-62 GPa and 2100-2300K, with starting material: Fe metal, FeCO3, and FeO. Diamond was detected by x-ray diffraction and Raman spectroscopy, as well as STEM-EDX on a thin foil prepared by focused ion beam milling (FIB). These findings suggest that in the more reducing regions of a laterally and axially heterogeneous mantle, carbonates will be reduced to diamond and/or iron carbide(s), with the greatest reduction potential occurring just before the siderite spin transition. In the more oxidizing regions, such as those near subduction zones and below D”, carbonate will be the stable host of carbon. If carbon is a major light element of the core, it is likely that it would have to have been sequestered prior to the formation of the post-perovskite phase and the D” region.