The oxidation state recorded by rocks from the Earth's upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals1,2,3. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle4,5, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle5,6. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1 wt% of a metallic iron-rich alloy. The mantle's oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.
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We thank H. Fischer, G. Herrmannsdörfer, D. Krausse and H. Schulze for technical assistance. The German Science Foundation (DFG) and the EU Access to Research Infrastructures Programme supported this research.
The authors declare that they have no competing financial interests.
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Frost, D., Liebske, C., Langenhorst, F. et al. Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle. Nature 428, 409–412 (2004). https://doi.org/10.1038/nature02413
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