Solving the paradox of the Fe-O system to understand the mineralogy of deep planetary interiors

岩石行星（如地球）表面的过程与它们内部的化学和动力学直接相关。大气的存在及其组成、地球发电机的形成和功能、板块构造——行星宜居性的最重要条件——都与行星主体的矿物物理和地球化学直接相关。构成地球地幔的关键矿物和许多被提出构成地外岩石行星地幔的相都是与氧化铁同构的（如环woodite与Fe3O4同构，akimotoite与赤铁矿、Fe2O3具有相同的晶体结构，方长石和无垢石也具有相同的晶体结构）。合成不可淬灭的高p - t硅酸盐通常需要极端p - t条件，目前阻碍了对其在mbar条件下的结构-性能关系的更深入了解。与硅酸盐不同，相应的氧化铁通常可以在更温和的条件下合成，但仍然具有挑战性，这使它们成为研究高p、T下行星地幔矿物复杂相和化学转化的完美模型化合物。然而，在过去的几年里，在高压下发现了十几种氧化铁，但还没有硅酸盐类似物的报道。这种所谓的“Fe-O系统悖论”是出乎意料和令人惊讶的，需要一个解释。目前，我们不能排除Fe-O系统和硅酸盐的行为可能确实根本不同，或者地球或超级地球内部的条件可能阻止复杂硅酸盐的形成，但这是极不可能的。相反，当前提案的工作假设是，迄今为止使用的实验方法以及迄今为止探索的p，T条件不适合检测复杂相。寻找“Fe-O系统悖论”的正确答案的一个主要瓶颈是缺乏关于行星材料在地幔和核心条件下（即压力高于100 GPa和温度高于2500 K）行为的实验数据。对于大多数被认为是形成超级地球内部的化合物来说，一个关键参数——它们的晶体结构——是未知的。我和同事们一直在积极开发金刚石砧细胞中的单晶x射线衍射（SCXRD），它不仅开辟了在极端条件下确定化合物晶体结构的道路，而且还允许改进它们的化学成分。在这个项目的框架内，我将开发在高p和高t条件下收集SCXRD的技术，并探索将该方法的当前限制扩展到250 GPa以上的方法。我将运用已开发的技术解决“Fe-O系统悖论”，研究地球和行星材料（主要是Fe-Mg-Al-Si-O系统）在100gpa以上压力和2500k以上温度下的化学和晶体化学。

Processes on the surfaces of rocky planets, such as Earth, are directly connected to the chemistry and dynamics of their interiors. The presence of an atmosphere and its composition, the formation and function of a geodynamo, plate tectonics – the most important conditions for a planet’s habitability – are all directly linked to mineral physics and geochemistry of the bulk planet. The key minerals constituting the Earth’s mantle and many phases proposed for mantles of the extra-terrestrial rocky planets are isostructural to iron oxides (e.g. ringwoodite is isostructural to Fe3O4, akimotoite has the same crystal structure as hematite, Fe2O3, and periclase and wuestite are also isostructural). The extreme p,T-conditions generally required for the synthesis of non-quenchable high-p,T-silicates are currently preventing a deeper understanding of their structure-property relations at Mbar-conditions. Unlike their silicate counterparts, the corresponding iron oxides generally can be synthesized at much milder, but still challenging, conditions, making them perfect model compounds for studying complex phases and chemical transformations of planetary mantle minerals at high p,T. However, for more than a dozen iron oxides that have been discovered at high pressures in the last few years, no silicate analogs have been reported yet. This so-called “Fe-O system paradox” is unexpected and surprising and requires an explanation. Currently, we cannot exclude that the behavior of the Fe-O system and silicates might be indeed fundamentally different or that conditions within the Earths or super-Earths might prevent complex silicates to form, but this is highly improbable. Instead, the working hypothesis of the current proposal is that the experimental methods used so far, and the p,T conditions explored up to now were inappropriate for the detection of complex phases. A major bottleneck in finding the correct answer for “Fe-O system paradox” is the lack of experimental data concerning the behavior of planetary materials at their mantle and core conditions i.e. pressures above 100 GPa and temperatures above 2500 K. A key parameter – their crystal structure – is unknown for most of the compounds thought to form e.g. the interior of Super-Earths. Single-crystal X-ray diffraction (SCXRD) in diamond anvil cells, which I and co-workers have been actively developing, opens the way to determine not only the crystal structures of compounds at truly extreme conditions, but also allows to refine their chemical composition. In the frame of the project I’m going to develop technologies for collecting SCXRD both at high-p and high-T conditions and explore the ways to extend the current limits of the method beyond 250 GPa. I’m going to apply the developed techniques to resolve the “Fe-O system paradox”, and study the chemistry and crystal chemistry of geo- and planetary materials (mainly in the Fe-Mg-Al-Si-O system) at pressures above 100 GPa and temperatures above 2500 K.