title: Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle.
abstract: 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 minerals. The capacity for minerals to incorporate Fe3+ 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 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 mantle. 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.
authors.affiliation: Bayerisches Geoinstitut, University of Bayreuth, D-95440 Bayreuth, Germany. dan.frost@uni-bayreuth.de
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Frost
authors.forename: Christian
authors.initials: C
authors.lastname: Liebske
authors.forename: Falko
authors.initials: F
authors.lastname: Langenhorst
authors.forename: Catherine A
authors.initials: CA
authors.lastname: McCammon
authors.forename: Reidar G
authors.initials: RG
authors.lastname: Trønnes
authors.forename: David C
authors.initials: DC
authors.lastname: Rubie