The redox state of the mantle during and just after core formation

Abstract

Siderophile elements are depleted in the Earth's mantle, relative to chondritic meteorites, as a result of equilibration with core-forming Fe-rich metal. Measurements of metal-silicate partition coefficients show that mantle depletions of slightly siderophile elements (e.g. Cr, V ) must have occurred at more reducing conditions than those inferred from the current mantle FeO content. This implies that the oxidation state (i.e. FeO content) of the mantle increased with time as accretion proceeded. The oxygen fugacity of the present-day upper mantle is several orders of magnitude higher than the level imposed by equilibrium with core-forming Fe metal. This results from an increase in the Fe 2 O 3 content of the mantle that probably occurred in the first 1 Ga of the Earth's history. Here we explore fractionation mechanisms that could have caused mantle FeO and Fe 2 O 3 contents to increase while the oxidation state of accreting material remained constant (homogeneous accretion). Using measured metal-silicate partition coefficients for O and Si, we have modelled core-mantle equilibration in a magma ocean that became progressively deeper as accretion proceeded. The model indicates that the mantle would have become gradually oxidized as a result of Si entering the core. However, the increase in mantle FeO content and oxygen fugacity is limited by the fact that O also partitions into the core at high temperatures, which lowers the FeO content of the mantle. (Mg,Fe)(Al,Si)O 3 perovskite, the dominant lower mantle mineral, has a strong affinity for Fe 2 O 3 even in the presence of metallic Fe. As the upper mantle would have been poor in Fe 2 O 3 during core formation, FeO would have disproportionated to produce Fe 2 O 3 (in perovskite) and Fe metal. Loss of some disproportionated Fe metal to the core would have enriched the remaining mantle in Fe 2 O 3 and, if the entire mantle was then homogenized, the oxygen fugacity of the upper mantle would have been raised to its present-day level.

Large oxygen excess in the primitive mantle could be the source of the Great Oxygenation Event

“…The removal of Fe 0 left the primitive lower mantle with a relative Fe 3+ excess (Frost et al, 2008), and thus a relative excess of oxygen, compared to the present day mantle. The true oxygen excess in the primitive lower mantle was controlled by two main parameters: (i) the amount of Bg present in the lower mantle when the Fe 0 segregation stopped.…”

“…However, the core-mantle segregation induced drainage of Fe 0 droplets from the molten mantle down into the core, which resulted in a large Fe 3+ excess in the primordial lower mantle (Frost et al, 2008). This excess became a potential source of oxygen for the shallow mantle and the Earth's surface.…”

“…When this mineral mixture migrates upwards to the upper mantle, the coexisting Fe 3+ and Fe 0 can eventually recombine with each other (i.e. 2Fe 3+ + Fe 0 → 3Fe 2+ ), leading to a dominantly Fe 2+ -bearing upper mantle with a true Fe 3+ excess of only 2-3 % (Frost et al, 2008). This ongoing cycle of Fe 2+ disproportionation followed by Fe 3+ plus Fe 0 recombination results in a neutral budget of the cation/oxygen ratio of the mantle.…”

Redox state of early magmas

“…Experiments have shown 13 that, at the pressure of the lower mantle, iron(II) oxide is converted into iron metal and iron(III) oxide -which means that large bodies such as Earth can self-oxidize their mantle, whereas smaller ones cannot (or do so to a lesser extent). In other words, a nominally dry mantle can be oxidized.…”

The Composition of the Deep Earth

“…This reaction, first proposed by Javoy (1995), which plays a key role on the estimation of the deep mantle compositions in the E-chondrites framework (Javoy et al 2010;Kaminski and Javoy 2013), is now acknowledged in core chemistry research (e.g., Frost et al 2008) and is explained in detail below.…”