High-pressure phase relations in the Mg–Fe–O system and the conditions at which ferropericlase inclusions in diamonds are formed

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Melai, Caterina ; Frost, Daniel J. ; Boffa Ballaran, Tiziana ; McCammon, Catherine ; Miyajima, Nobuyoshi ; Wang, Lin:
High-pressure phase relations in the Mg–Fe–O system and the conditions at which ferropericlase inclusions in diamonds are formed.
In: Contributions to Mineralogy and Petrology. Bd. 181 (2026) . - 62.
ISSN 1432-0967
DOI: https://doi.org/10.1007/s00410-026-02341-w

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Abstract

Constraints can be placed on the formation conditions of (Mg,Fe)XO ferropericlase inclusions in natural diamonds based on their Fe3+ contents, given suitable knowledge of how these contents change with pressure, temperature, composition and oxygen fugacity. To achieve this, experiments have been conducted to determine the maximum Fe3+/Fetot ratio in ferropericlase, controlled by its coexistence with magnetite–magnesioferrite (Fe₃O₄–MgFe₂O₄) up to pressures of ~ 6 GPa, and with the high-pressure [Fe,Mg]₂Fe₂O₅ phase between 10 and 30 GPa. The experiments were performed between 1200 and 1800 °C and across a range of bulk Fe/(Fe + Mg) ratios. Mössbauer and electron energy loss spectroscopy were used to determine ferropericlase Fe3+/Fetot ratios, a task that is complicated by exsolution of nanocrystalline magnetite–magnesioferrite during quenching. Using these data, a thermodynamic model was developed that describes the entire range of ferropericlase compositions in the Mg–Fe–O system between 1 atmosphere and 30 GPa. It is shown that the Fe3+/Fetot ratio of ferropericlase within diamond-forming assemblages decreases strongly above 10 GPa, and that the majority of ferropericlase inclusions in diamonds with measured Fe3+/Fetot, have ratios too high for them to have formed in the lower mantle. Most of these ferropericlase inclusions have maximum formation depths in the transition zone, but a few of the most oxidised could only have formed in the upper mantle from oxidised precursors such as carbonates. We also show that high pressure (Fe,Mg)2Fe2O5 and (Fe,Mg)3Fe2O6 phases can become stable during diamond formation towards the base of the transition zone, when bulk Fe/(Fe + Mg) ratios are > 0.5.