Evidence for complex iron oxides in the deep mantle from FeNi(Cu) inclusions in superdeep diamond

The recent discovery in high-pressure experiments of compounds stable to 24-26 GPa with Fe₄O₅, Fe₅O₆, Fe₇O₉, and Fe₉O₁₁ stoichiometry has raised questions about their existence within the Earth's mantle. Incorporating both ferric and ferrous iron in their structures, these oxides if present within the Earth could also provide insight into diamond-forming processes at depth in the planet. Here we report the discovery of metallic particles, dominantly of FeNi (Fe_0.71Ni_0.24Cu_0.05), in close spatial relation with nearly pure magnetite grains from a so-called superdeep diamond from the Earth's mantle. The microstructural relation of magnetite within a ferropericlase (Mg_0.60Fe_0.40)O matrix suggests exsolution of the former. Taking into account the bulk chemistry reconstructed from the FeNi(Cu) alloy, we propose that it formed by decomposition of a complex metal M oxide (M₄O₅) with a stoichiometry of (Fe³⁺_2.15Fe²⁺_1.59Ni²⁺_0.17Cu⁺_0.04)_Σ=3.95O₅ We further suggest a possible link between this phase and variably oxidized ferropericlase that is commonly trapped in superdeep diamond. The observation of FeNi(Cu) metal in relation to magnetite exsolved from ferropericlase is interpreted as arising from a multistage process that starts from diamond encapsulation of ferropericlase followed by decompression and cooling under oxidized conditions, leading to the formation of complex oxides such as Fe₄O₅ that subsequently decompose at shallower P-T conditions.