期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：35
页码：21088-21094
DOI：10.1073/pnas.2004269117
出版社：The National Academy of Sciences of the United States of America
摘要：The recent discovery in high-pressure experiments of compounds stable to 24–26 GPa with Fe 4 O 5 , Fe 5 O 6 , Fe 7 O 9 , and Fe 9 O 11 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 0.71 Ni 0.24 Cu 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 0.60 Fe 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 ( M 4 O 5 ) with a stoichiometry of (Fe 3 2.15 Fe 2 1.59 Ni 2 0.17 Cu 0.04 ) Σ = 3.95 O 5 . 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 Fe 4 O 5 that subsequently decompose at shallower P-T conditions.
关键词：diamond inclusions ; mantle dynamics ; iron oxides ; Earth’s deep interior ; Fe–Ni alloys
