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

地球的氧含量是在大约45亿年前从小行星和行星胚胎中吸积出来的。地球的铁金属核心与吸积同时形成，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，因此我们预计地幔中的所有铁都以FeO（二价氧化态的铁）的形式存在。然而，地球的上地幔比这氧化得多，因此它不可能与地核平衡。有趣的是，上地幔显然早在太古代（约40亿年）就获得了氧化状态，这意味着与原始过程的联系。地幔氧化态是一个长期存在的地球化学之谜，其解答对地球的形成和演化具有重要意义。大多数以前的地幔氧化模型都采用了增生物质的成分。例如，也许后期的物质比早期形成核心的物质氧化性更强。又或者是增生物质中的氢与铁发生反应，氧化了地幔。这两种情况基本上是不可能测试的，因为我们无法追踪形成地球的物质的起源。最近，一个新的和可测试的机制已被推广。镁钙钛矿矿物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。事实证明，当铝（Al 3+）取代到钙钛矿结构中时，它在能量上非常有利于它与Fe 3+阳离子偶联以实现电荷平衡。这种取代反应显然甚至在还原条件下如在芯偏析期间也起作用。显然，Fe 3+的来源是由钙钛矿中的自氧化还原反应提供的：3FeO = Fe 2 O 3 + Fe（金属）这个简单的FeO还原反应具有深远的意义。如果这种反应在地核形成期间发生，那么当大量的增生物质的底辟进入地核时，一些歧化的金属可能已经从地幔中移走了。在这种情况下，地幔会逐渐氧化。这种晶体化学机制不仅为氧化之谜提供了一个解决方案，而且显然也可以满足长期存在的关于地幔亲铁体和同位素组成的悖论。这个模型还需要进一步测试。自氧化反应仅在下地幔最浅处的压力下观察到。该提案中解决的基本问题是压力如何影响钙钛矿中各种Al和Fe取代机制之间的能量竞争。如果氧化铝可以在高压下进行不同的替代，而不需要Fe 3+，则自动氧化机制将关闭。在这里，我们提出了一个实验研究的主要目标，确定如果这个重要的FeO反硝化反应发生在整个下地幔的压力。

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.