Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle


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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Figure 1: Scanning (ac) and transmission (d) electron microscope images of experimental run products.
Figure 2: The variation of Al3+ and Fe3+ in silicate perovskite reported as atoms per two-cation formula unit.


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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.

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Correspondence to Daniel J. Frost.

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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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