Isotopic evidence for internal oxidation of the Earth's mantle during accretion
Highlights
► Fe isotope fractionation exists between Fe metal and Fe3 +-bearing perovskite. ► Disproportionation of Fe2 + by perovskite can explain mantle Fe3 + levels. ► Disproportionation may have taken place prior to or during the giant impact. ► The Earth's mantle was oxidised by the end of accretion. ► Mantle oxidation state is decoupled from the rise of atmospheric oxygen at 2.45 Ga.
Introduction
The Earth's mantle contains both ferrous and ferric iron. Chemical equilibrium between Fe3 + and Fe2 +-bearing minerals define the oxidation state of the upper mantle, which is currently several orders of magnitude (Wood et al., 1990) above the levels predicted for chemical equilibrium between the mantle and core and for the highly reducing conditions inferred for early core formation (Wade and Wood, 2005).
An understanding of how the Earth's mantle attained its current oxidation state is fundamental to understanding the development of the hydrosphere and atmosphere. Analyses of Archaean peridotites and komatiites (Berry et al., 2008, Canil, 2002) in addition to Hadean zircons (Trail et al., 2011) suggest that the mantle was oxidised early in Earth history. However, since core segregation during accretion must have taken place under reducing conditions it is necessary to identify a mechanism by which the mantle could have subsequently become oxidised. Processes that could have oxidised the Earth's mantle include the late accretion of oxidised, volatile-rich meteoritic material (Schönbachler et al., 2010) and internal oxidation mechanisms such as the dissolution of silicon (or carbon, or hydrogen) into the core (Javoy et al., 2010) and the disproportionation of mantle FeO into Fe3 + and metallic iron by magnesium silicate perovskite crystallisation in the lower mantle (Frost and McCammon, 2008, Wade and Wood, 2005). There is direct experimental evidence (Frost et al., 2004) of the latter mechanism, which can be represented in simplified form as:
The reaction above may have operated towards the end of accretion, when the Earth had grown large enough to stabilise perovskite. Continual core segregation would have removed metallic Fe to the core and the Fe3 + produced would have been released to the overlying mantle through dissolution and re-precipitation of perovskite (Wade and Wood, 2005). Disproportionation provides a means of reconciling initial core segregation under reducing conditions with the current oxidised state and high Fe3 + content (Canil et al., 1994) of the Earth's mantle. However, it is not clear whether disproportionation alone can account for current mantle Fe3 + levels, or whether additional processes are required. This specific question could be evaluated if the Fe3 + produced by disproportionation were to be traced and quantified. The goal of this study was, therefore, to test the Fe-disproportionation hypothesis using a suitable tracer, such as that potentially provided by Fe stable isotopes (Williams et al., 2006). Theoretical studies suggest that increasing pressure in planetary interiors will favour the incorporation of heavy Fe isotopes into silicate minerals relative to metal (Polyakov, 2009, Rustad and Yin, 2009), an effect which should be enhanced in the case of Fe3 +-bearing silicate perovskite, as theory (Polyakov and Mineev, 2000) predicts that the heavy isotopes of iron should also be concentrated in Fe3 +-bonds.
In order to test the disproportionation hypothesis, we synthesised samples of aluminous silicate perovskite coexisting with metallic Fe and measured the Fe isotopic compositions of the experimental run products to determine the Fe isotope fractionation factor between perovskite and metal. This fractionation factor was then used in conjunction with the Fe3 + contents of the product perovskites to calculate the proportion of the Earth's Fe3 + budget that can be accounted for by disproportionation.
Section snippets
Materials and methods
Aluminous pyroxene starting materials were synthesised in a piston cylinder press (1200 °C, 2 GPa) with 100:1 natural FeO to 54FeO and were subsequently mixed with 20 wt.% powdered metallic Fe and equilibrated in the stability field of silicate perovskite (1850 °C, 24 GPa). Full details of experimental procedures are provided as Supplementary information. Perovskite and metal were the only phases present in the run products (Fig. 1). The recovered samples were analysed by electron microprobe using
Attainment of isotopic equilibrium
The isotopic compositions of the separate metal and silicate phases are given in Table 1; a full dataset of the different leach fractions and leaching tests is given in Table 2. Isotope data are presented in permil notation where isotopic ratios (57Fe/54Fe and 56Fe/54Fe) are presented as deviations in parts per 1000 (δ57Fe and δ56Fe, respectively) from the measured ratios of the bracketing standard (the pure Fe standard IRMM014). The uncertainties on the isotope ratio measurements are the two
Discussion: test of the Fe-disproportionation model
The concentration of isotopically heavy Fe into Fe3 +-bearing silicate perovskite indicates that disproportionation should generate planetary mantles with isotopically heavy δ57Fe. While this mechanism could operate on any planet large enough to stabilise perovskite, it could not take place on a small planetesimal such as Vesta, as pressures sufficient to stabilise perovskite are not reached anywhere on this body. It is also unlikely that this process could operate to a significant extent on
Conclusions
The isotopic and experimental data presented here provide evidence for the partial dissolution of lower-mantle perovskite and redistribution of Fe3 + throughout the Earth's mantle during the Moon-forming giant impact. Later oxidation by subduction of surficial materials or late accretion of volatile-enriched material is thus not required to explain the oxidised nature of the Earth's interior. The oxidation state and Fe3 + content of the Earth's mantle was likely set by the end of accretion, in
Acknowledgements
The authors would like to acknowledge members of the Oxford Isotope Geochemistry group for helpful support, constructive discussions and technical assistance with mass spectrometry. Two anonymous reviewers are thanked for their thoughtful and constructive comments; Rick Carlson is thanked for editorial handling. Derek Preston and Dave Pinchin are thanked for their help in the construction and maintenance of the high-pressure lab at Oxford. Geoff Nowell and Chris Ottley are thanked for their
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