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

摘要

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. The capacity for minerals to incorporate Fe~(3+) may also be a significant factor controlling the oxidation state of the mantle, 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 fuga-city. 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. 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.