Microscopic properties to macroscopic behaviour: The influence of iron electronic state

Abstract

Iron is the only major element in the Earth with multiple electronic configurations (oxidation and spin state). In the upper mantle and transition zone iron is predominantly Fe2+, but the small amount of Fe3+ that is present significantly affects properties that are sensitive to defect chemistry, including electrical conductivity, diffusivity and hydrogen solubility. Fe3+ also determines the oxygen fugacity, where the upper mantle is relatively oxidised due to the high Fe3+/∑Fe ratio in spinel, even though the overall Fe3+ concentration in the upper mantle is low due to its concentration in modally minor phases. The transition zone contains a similar amount of Fe3+, but its distribution among all the major phases results in a significantly lower oxygen fugacity, near metal saturation. In the lower mantle the transition to (Mg,Fe)(Si,Al)O3 perovskite involves the creation of significant Fe3+ (approximately 50% of available iron), even at reducing conditions, which is likely balanced in the bulk lower 'mantle by disproportionation of Fe2+ to form Fe3+ and metallic iron, and potentially in subducting slabs through reduction of oxidised species in the slab. The nature of Fe3+ charge balance in (Mg,Fe)(Si,Al)O3 perovskite largely determines the influence on physical and chemical properties, where electrical conductivity is enhanced, diffusivity is reduced, and elasticity and hydrogen solubility vary depending on the substitution mechanism. If Fe2+ is more stable in the post-perovskite phase relative to (Mg,Fe)(Si,Al)O3 perovskite as suggested by experiments, both Fe3+/∑Fe and the nature of its influence on physical and chemical properties may be different in the post-perovskite phase. The influence of spin crossover on mantle properties remains unclear. Recent models show that the growth of an (Mg,Fe)(Si,Al)O3 perovskite-rich lower mantle during core formation would progressively oxidise the mantle to present levels as a portion of the disproportionated iron-rich metal phase became incorporated into the core, and the increase in oxygen fugacity during the later stages of Earth accretion would alter the partitioning behaviour of elements between mantle and core, resolving puzzles such as the siderophile element anomaly and the discrepancy between U-Pb and Hf-W systematics in the early Earth. Redox reactions involved in the movement of material across the lower mantle boundary could be related to the formation of deep diamonds, and potentially the rise of atmospheric oxygen through a mantle avalanche in the late Archean that disturbed the balance of volatile element cycles.