How did Earth's Mantle Become Oxidized? The Role of Perovskite Crystal Chemistry in Earth's Evolution

地球的含氧量是在大约45亿年前从小行星和行星胚胎吸积而来的过程中确定的。地球的铁金属核与吸积作用同时形成，并剥离了硅酸盐地幔中的大部分铁。当地核最后一次与地幔平衡时，它必须在允许金属铁与硅酸盐稳定的条件下这样做，所以我们预计地幔中的所有铁都会以FeO(二价氧化态的铁)的形式出现。然而，地球上地幔的氧化程度要比这高得多，以至于它不可能与地核保持平衡。有趣的是，上地幔显然早在太古宙(~40亿年)就获得了氧化状态，这意味着与原始过程有关。地幔氧化状态是一个由来已久的地球化学之谜，其解决方案对地球如何形成和演化具有重要意义。大多数以前的地幔氧化模型都考虑了吸积物质的组成。例如，也许后期的物质比早期更易氧化，因为早期形成了大部分的核心。或者可能是吸积物质中的氢与铁反应并氧化了地幔。这两种情况基本上都无法测试，因为我们无法追踪形成地球的吸积物质的来源。最近，一种新的、可测试的机制得到了推广。镁钙钛矿构成了地球下地幔的大部分，使其成为地球上最丰富的矿物。结果表明，当Al(Al~(3+))取代钙钛矿结构时，Al~(3+)与Fe~(3+)阳离子偶合，在能量上非常有利于钙钛矿结构的电荷平衡。显然，这种取代反应即使在核心偏析这样的还原条件下也能进行。显然，Fe3+的来源是由钙钛矿中的自氧化还原反应提供的：3FeO=Fe2O3+Fe(金属)这个简单的FeO歧化反应具有深远的意义。如果这个反应是在地核形成期间进行的，那么当大量的吸积物质底辟进入地核时，一些分散的金属可能已经从地幔中移走了。在这种情况下，地幔将逐渐被氧化。这种晶体化学机制不仅为氧化之谜提供了解决方案，而且显然还可以满足长期以来关于地幔亲铁和同位素组成的悖论。这一模式还需要进一步测试。只有在下地幔最浅的压力下才能观察到自氧化反应。这一建议所涉及的基本问题是压力如何影响钙钛矿中各种Al和Fe替代机制之间的能量竞争。如果氧化铝可以在高压下以不同的方式替代，而不需要Fe3+，那么自动氧化机制就会关闭。在这里，我们建议进行一项实验研究，主要目的是确定这一重要的FeO歧化反应是否在整个下地幔的压力下发生。

The oxygen content of Earth was established during its accretion from planetesimals and planetary embryos some 4.5 billion years ago. Earth's iron metal core formed simultaneously with accretion, and stripped the silicate mantle of most of it iron. When the core was last equilibrated with the mantle, it must have done so at conditions that permit metal iron to be stable with silicate, and so we would expect all the iron in the mantle to occur as FeO (iron in a divalent oxidation state). However, Earth's upper mantle is much more oxidizing than this, such that it could not have equilibrated with the core. Interestingly, the upper mantle apparently obtained its oxidized state as far back as the Archean (~ 4 billion years), and this implies a link with primordial processes. The mantle oxidation state is a longstanding geochemical enigma, the solution to which has important implications for how the Earth formed and evolved. Most previous models for mantle oxidation enlist the composition of accreting materials. For example, perhaps late-stage materials were much more oxidizing than in early stages when the bulk of the core formed. Or perhaps hydrogen in accreting materials reacted with iron and oxidized the mantle. Both these scenarios are basically impossible to test because we cannot trace the origin of the materials that accreted to form Earth. Recently, a new and testable mechanism has been promoted. The mineral Mg-perovskite constitues most of Earth's lower mantle, making it the most abundant mineral in Earth. It turns out that when aluminium (Al3+) substitutes into the perovskite structure, it is energetically very favorable for it to couple itself with an Fe3+ cation to achieve charge balance. This substitution reaction apparently operates even at reducing conditions like during core segregation. Apparently, the source of the Fe3+ is provided by an auto-oxidation-reduction reaction in perovskite: 3FeO = Fe2O3 + Fe (metal) This simple FeO disproportionation reaction has far reaching implications. If this reaction operated during core formation, then some of the disproprtionated metal may have been removed from the mantle when large diapirs of accretionary material made their way to the core. In this case, the mantle would become progressively oxidized. Not only does this crystal-chemical mechanism provide a solution to the oxidation puzzle, but it apparently can satisfy long standing paradoxes concerning the siderophile and isotopic composition of the mantle as well. This model needs further testing. The auto-oxidation reaction has only been observed at pressures of the shallowest part of the lower mantle. The fundamental question addressed in this proposal is how pressure affects the energetic competition among the various Al and Fe substitution mechanisms in perovskite. If alumina can substitute differently at high pressures without the need for Fe3+, the auto-oxidation mechanism would shut down. Here, we propose an experimental study with the primary objective of determining if this important FeO disproportionation reaction occurs at pressures throughout the lower mantle.