Stabilities and properties of Mg, Al and Cr-bearing solid solutions of newly discovered Fe3+-Fe2+ oxides at transition zone conditions: approaching geologically relevant compositions

具有AB2O4化学计量比的尖晶石结构相在地球上很常见，可以有多种成分。例如，(Mg，Fe)2SiO4环木石相是地球过渡带下半部的优势相。无硅氧化物如磁铁矿(FeFe2O4)和镁铁氧体(MgFe2O4)也以包裹体的形式存在于金刚石中，包括那些被认为在过渡带甚至下地幔具有超深成因的包裹体。因此，这种氧化物的相关系对于解释这种现象很重要，特别是当质地证据表明它们是由随后转化的前体相形成的时候。受冲击的陨石也可能保存着在高压下形成的后尖晶石相的证据。尖晶石后镁铁氧体组合的稳定性是之前一项研究的主题，该研究说明了这些信息如何限制了对自然样品的解释。这个项目是前一个项目的延伸，在前一个项目中，我们的目标是研究更复杂的成分的相关系，这些成分接近于在自然界中遇到的那些。一个主要的目标是确定Al和Cr对P和T的后尖晶石范围内Fe-Mg氧化物相关系的影响。在这种条件下，含有Fe4O5、Fe5O6或hp-Fe3O4型相的组合是可望的。根据以往的经验，也可能会遇到具有其他化学计量比的氧化相(如：镁铁氧体)。将描述这些阶段的特征，并将其组合整合成适用于上地幔深部和过渡带条件的相图。我们将进一步关注镁、铝和铬在Fe5O6中的固溶体行为，因为对这一相的了解很少，初步热力学计算表明，它的氧化还原稳定性使其比Fe4O5更有可能成为地球深部地幔预期条件下的候选相。还将测试其与地幔硅酸盐的潜在稳定性。为了建立O5和O6相的热力学模型，将研究它们的压缩行为

Spinel-structured phases with AB2O4 stoichiometry are common in the Earth and can have a variety of compositions. For example, (Mg,Fe)2SiO4 ringwoodite is a dominant phase in the lower half of the Earths transition zone. Si-free oxides such as magnetite (FeFe2O4) and magnesioferrite (MgFe2O4) also occur as inclusions in diamond, including those considered as having a super deep origin in the transition zone or even lower mantle. Thus, the phase relations of such oxides are important for interpreting such occurrences, especially when textural evidence suggests that they formed from a precursor phase that has subsequently transformed. Shocked meteorites may also preserve evidence for post-spinel phases that formed at high pressures. The stability of post-spinel assemblages of MgFe2O4 was the subject of a previous study that illustrated how such information can constrain the interpretation of natural samples. This project is an extension of the preceding project where we aim to investigate the phase relations of more complex compositions that approach those encountered in nature. One primary goal is to determine the effect that Al and Cr have on the phase relations of Fe-Mg oxides in the post-spinel range of P and T. Under these conditions, assemblages containing solid solutions of Fe4O5, Fe5O6 or hp-Fe3O4-type phases are expected. Based upon previous experience, oxide phases with other stoichiometries may also be encountered (i.e. Mg3Fe4O9). These phases will be characterised and their assemblages integrated into phase diagrams applicable to the conditions of the deep upper mantle and transition zone. We will further focus on the solid solution behavior of Mg, Al and Cr in Fe5O6, since very little is known about this phase and preliminary thermodynamic calculations suggest its redox stability makes it a more likely candidate phase for the expected conditions in the Earths deep mantle than Fe4O5. Its potential stability with mantle silicates will also be tested. In order to develop thermodynamic models for the O5 and O6 phases, their compression behavior will be investigated