Evidence for complex iron oxides in the deep mantle from FeNi(Cu) inclusions in superdeep diamond
Evidence for complex iron oxides in the deep mantle from FeNi(Cu) inclusions in superdeep diamond
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DOI：
10.1073/pnas.2004269117
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发表时间：
2020-09-01
影响因子：
11.1
通讯作者：
Nestola, Fabrizio
中科院分区：
文献类型：
--
作者：
Anzolini, Chiara;Marquardt, Katharina;Nestola, Fabrizio
最近在高压实验中发现了含有Fe4O5、Fe5O6、Fe7O9和Fe(9)O(11)化学计量比的稳定在24-26 Gpa的化合物，这让人们对它们在地幔中的存在提出了疑问。这些氧化物在其结构中同时包含了铁和亚铁，如果存在于地球内部，也可以提供对地球深处钻石形成过程的洞察。在这里，我们报告了金属颗粒的发现，主要是FeNi(Fe0.71Ni0.24Cu0.05)，与来自地幔的所谓超深钻石中的近纯磁铁矿颗粒有密切的空间关系。铁方镁石(Mg_(0.60)Fe_(0.40))O基质中磁铁矿的微结构关系表明前者出溶。考虑到FeNi(Cu)合金的体化学重构，我们认为它是由化学计量比为(Fe2.153+Fe1.592+Ni0.172+Cu0.04+)(Sigma)O-=3.92(5)的复杂金属氧化物(M4O5)分解形成的。我们进一步提出了该相与超深金刚石中常见的可变氧化铁方镁石之间的可能联系。FeNi(Cu)金属与铁方镁石析出的磁铁矿的关系被解释为一个多阶段过程，该过程始于钻石包裹方镁铁石，然后在氧化条件下减压和冷却，导致形成Fe(4)O(5)等复杂氧化物，然后在较浅的P-T条件下分解。
The recent discovery in high-pressure experiments of compounds stable to 24-26 GPa with Fe4O5, Fe5O6, Fe7O9, 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 (Fe0.71Ni0.24Cu0.05), 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 (Mg0.60Fe0.40)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 (Fe2.153+Fe1.592+Ni0.172+Cu0.04+)(Sigma) O-= 3.92(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.