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
Final Report Abstract
In this project, we set out to experimentally determine the effect of Al and Cr on the phase relations of (Mg,Fe)Fe2O4. In addition we investigated the P-T stability range of Fe5O6 and the ability of this phase to incorporate other cations such as Mg as compared with Fe4O5. The stability field of Fe5O6 is quite large, beginning at ~9 GPa and extending to at least 28 GPa. At lower P, the stable assemblage is magnetite + wüstite. The phase boundary defined by equilibrium the Fe3O4 + 2 wüstite= Fe5O6 has only a small T-dependence and can to be described by: P (GPa) = 1.86 x 10-3 T (K) + 6.107. At higher P and relatively low T, the stability of Fe5O6 is limited by the equilibrium Fe5O6 = Fe4O5 + wüstite, whose position can be approximated by: P (GPa) = 27.7 x 10-3 T (K) - 18.48. We have refined the standard thermodynamic properties of Fe5O6 using our new constraints. The occurrence of wüstite in equilibrium with Fe5O6 in means that no additional phase with a composition lying between wüstite and Fe5O6, such as Fe6O7, should to be stable over the large P-T range that we investigated. On the other hand, Fe9O11 occurred in a few experiments, which has a composition intermediate between Fe5O6 and Fe4O5. The stability of Fe9O11 appears to be limited to pressures of 9-14 GPa. In terms of Mg incorporation in Fe5O6, we surprisingly found that not only is the Mg3Fe2O6 endmember unstable, but that solid solution along the Fe5O6-Mg3Fe2O6 binary is very limited to a maximum Mg content of XMg3Fe2O6 ≈ 0.22. Although the extent of Mg substitution in Fe9O11 has not been constrained, one experiment yielded essentially single phase Mg0.87(1)Fe2+4.13(1)Fe3+4O11, allowing us to obtain detailed information about its crystal structure via Rietveld refinement. Further experiments demonstrate that (Mg,Fe)2Fe2O5 can coexist with silicates with mantle relevant compositions including olivine, wadsleyite, ringwoodite, low-Ca pyroxene as well as majoritic garnet and phase B. This work is also the first report of Fe3+ incorporation in phase B. The interphase Mg-Fe2+ partitioning is such that O5-phase always has the highest Fe2+ content in the assemblage. The O6-phase did not appear in any of our experiments, supporting the lack of significant Mg solid solution in this phase. The influence of Al on the phase relations of the high-P oxides was investigated in the 2+Fe -Fe3+-Al-, Mg-Fe3+-Al-subsystems, as well as (Mg,Fe2+)(Fe3+,Al)2O4 compositions. For Mgfree compositions, the spinel phase breaks down to Fe2(Fe,Al)2O5 + (Fe,Al)2O3 at somewhat lower pressures that observed for Fe3O4. At ~16 GPa, Fe3(Fe,Al)4O9 coexists with an O5-phase and Fe3+-bearing corundum. This is a distinctly lower pressure for stabilisation of the O9-phase than observed in Mg-Fe3+ compositions. However, the stability of the O9-phase appears to be limited to a narrow range in P-T conditions, since at 18-20 GPa the run products exhibited breakdown textures indicating that another phase was stable, most likely a high-P O4-phase. For both the O5-phase and O9-phase, incorporation of Al is very limited; XAl =0.15 and 0.08 for hypothetical Fe2Al2O5 and Fe2+3(Al)4O9 endmembers, respectively. The same behavior is observed for Mg-bearing compositions. However, here the high-P O4-phase is quenchable. This stands in contrast to the appearance of an unquenchable phase (or phases) in Mg-Fe3+-Al compositions at lower P, before the O5-phase becomes stable. It is notable that the high-P O4- phase is capable of incorporating much more Al than the O5- and O9-phases. Although the phase relations of Al-bearing compositions broadly follow those for MgFe2O4, the presence of small amounts of Al does lead to significant shifts in the position of the phase boundaries, as well as depressing the P-T range of the phase field for the constituent oxides. In these more complex compositions, there is still no common phase boundary between a spinel-structured phase and its high-P polymorph; there are always intervening phase assemblages, particularly one containing an O5-phase. Unexpectedly, we find that the O5-phase is generally of much more relevance to the mantle assemblage than the O6-phase. An exception would be in those local environments that are anomalously Fe-rich, where Fe5O6 could be stable.
Publications
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(2017) New hp-phases of Fe-Mg spinels: Implications for the formation and uplift history of inclusions in diamonds, GeoBremen Conference, Bremen, Germany
Uenver-Thiele L, Woodland AB, Boffa Ballaran T, Miyajima N, Frost D
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(2018) Antiphase domain boundaries in magnetite: A key feature of unseen high-pressure mixedvalence iron oxides. International Mineralogical Association 22nd General Meeting, Melbourne, Australia, IMA2018-1666
Miyajima N, Uenver-Thiele L, Woodland AB, Sinmyo R, Ishii T, Boffa Ballaran T, Frost D
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(2018) Behavior of Fe4O5−Mg2Fe2O5 solid solutions and their relation to coexisting Mg-Fe silicates and oxide phases. Contributions to Mineralogy and Petrology 173:20
Uenver-Thiele L, Woodland AB, Miyajima N, Boffa Ballaran T, Frost D
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(2018) Synthesis and crystal structure of Mg-bearing Fe9O11: New insight in the complexity of Fe-Mg oxides at conditions of the deep upper mantle. American Mineralogist 103, 1873-1876
Ishii T, Uenver-Thiele L, Woodland AB, Alig E, Boffa Ballaran T
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(2019) Mg incorporation in Fe5O6 and its relevance as a possible mantle phase. EGU General Assembly, Vienna, Austria, EGU2019-15552
Uenver-Thiele L, Woodland AB, Boffa Ballaran T, Miyajima N
