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

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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