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 Fe 3+ and Fe 2+ between the constituent minerals 1 , 2 , 3 . The capacity for minerals to incorporate Fe 3+ may also be a significant factor controlling the oxidation state of the mantle 4 , 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 Fe 3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe 3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe 3+ /total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle 5 , 6 . Consequently, the lower mantle must either be enriched in Fe 3+ or Fe 3+ must form by the disproportionation of Fe 2+ to produce Fe 3+ 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.