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 Fe 2+ , but the small amount of Fe 3+ that is present significantly affects properties that are sensitive to defect chemistry, including electrical conductivity, diffusivity and hydrogen solubility. Fe 3+ also determines the oxygen fugacity, where the upper mantle is relatively oxidised due to the high Fe 3+ /ΣFe ratio in spinel, even though the overall Fe 3+ concentration in the upper mantle is low due to its concentration in modally minor phases. The transition zone contains a similar amount of Fe 3+ , 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)O 3 perovskite involves the creation of significant Fe 3+(approximately 50% of available iron), even at reducing conditions, which is likely balanced in the bulk lower mantle by disproportionation of Fe 2+ to form Fe 3+ and metallic iron, and potentially in subducting slabs through reduction of oxidised species in the slab. The nature of Fe 3+ charge balance in (Mg,Fe)(Si,Al)O 3 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 Fe 2+ is more stable in the post perovskite phase relative to (Mg,Fe)(Si,Al)O 3 perovskite as suggested by experiments, both Fe 3+ /ΣFe 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)O 3 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.

Composition dependence of spin transition in (Mg,Fe)SiO3bridgmanite

“…3a and 3b). Fe in bridgmanite is expected to be high-spin at 1 bar (McCammon 2006;McCammon et al 2008;Lin et al 2012). In a previous XRD study of bridgmanite synthesized from these compositions, the bridgmanite structure was not observed at ambient conditions, possibly due to amorphization (Dorfman et al 2013).…”

High-pressure X-ray diffraction and X-ray emission studies on iron-bearing silicate perovskite under high pressures

“…Recent studies showed that silicate perovskite transforms to a post-perovskite structure just above the core-mantle region, the D layer [2]. Current consensus for the iron abundance and valence states in the lower-mantle minerals is that iron exists mainly as ferrous iron (Fe 2+ ) with a concentration level of approximately 20% (Fe/(Fe + Mg)) in ferropericlase, whereas iron exhibits two main valence states, Fe 2+ and ferric iron (Fe 3+ ), with a total concentration level of approximately 10% in silicate perovskite [3]. Enrichment of iron in silicate perovskite and post-perovskite may occur at the core-mantle boundary region [4].…”

Oxidation state of Fe in olivine in a lherzolite xenolith from Oku district, Oki-Dogo Island, Shimane Prefecture, Japan

“…Although McCammon (2006) stated that olivine, the most abundant mineral in the upper mantle, contributes zero Fe 3+ to the total budget due to its negligible Fe 3+ content, a small amount of Fe 3+ was confirmed in olivine from the Dish Hill mantle xenolith, California, USA (Banfield et al, 1992). In this study, we examined the oxidation state of Fe in olivine in a lherzolite xenolith from the Oku area in Saigo -cho, Oki -Dogo Island, Shimane Prefecture, Japan, and investigate the occurrence of Fe 3+ in that olivine.…”

“…McCammon (2006) concluded that Fe 3+ -bearing phases exist in the upper mantle and the transition zone, and that, even in the lower mantle, the transition to (Mg,Fe)(Si,Al)O 3 perovskite involves the creation of significant Fe…”