THE HIGH-PRESSURE AND HIGH-TEMPERATURE PHASE STABILITY, DENSITY, AND THERMOELASTICITY OF NI, FE, AND CO OXIDES AND SILICATES AND IMPLICATIONS FOR A DEEP EARTH ELECTROCHEMICAL SERIES

The distribution of transition metals among the oxides, silicates, and metals of the Earth’s interior provides a record of the whole Earth’s physical and chemical evolution, and their abundance helps govern mechanical, electrical, and thermal transport properties of the Earth’s interior Understanding the abundance and distribution of these elements requires knowing their thermodynamic behavior, including phase stability, partitioning, and thermoelasticity at the high pressures and temperatures relevant to the Earth’s deep interior. Here we present measurements of the thermoelastic properties and phase stabilities of a suite of transition metal ringwoodites at pressures and temperatures corresponding to the Earth’s mid-mantle. X-ray diffraction measurements were performed using synchrotron X-ray diffraction in conjunction with in situ laser heating in the diamond anvil cell at GSECARS and HPCAT at the Advanced Photon Source. The relative thermoelastic properties of Fe₂SiO₄, Ni₂SiO₄, and Co₂SiO₄ ringwoodite are examined, and compared with relative thermoelastic parameters of FeO, NiO, and CoO to assess the degree to which the metal cation controls the thermoelastic properties of the silicate and oxide phases. Using our new high P,T thermoelastic data combined with existing measurements of high pressure thermoelasticity of transition metals and their oxides, the differences between free energies for transition metal ringwoodites and their corresponding metals are calculated as a function of pressure and temperature. The calculations of relative redox potential for Fe, Ni, and Co predict a crossover in the relative redox potential of Ni and Co at pressures corresponding to the top of the lower mantle. This crossover occurs in both in the oxide and the silicate, at 40 GPa and 60 GPa respectively. This qualitatively consistent behavior for the oxide/metal series and the silicate/metal series implies that the relative redox behavior of Ni and Co is insensitive to small variations in metal and oxide composition. Using this information, we present a pressure- and temperature-dependent electrochemical series for Ni, Co, and Fe, at conditions relevant for the Earth’s interior.