Effect of Fe3+ on Phase Relations in the Lower Mantle: Implications for Redox Melting in Stagnant Slabs

Ryosuke Sinmyo, Yoichi Nakajima, Catherine A. McCammon, Nobuyoshi Miyajima, Sylvain Petitgirard, Robert Myhill, Leonid Dubrovinsky, Daniel J. Frost

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Abstract

Recent studies have revealed that Earth's deep mantle may have a wider range of oxygen fugacities than previously thought. Such a large heterogeneity might be caused by material subducted into the deep mantle. However, high-pressure phase relations are poorly known in systems including Fe3+ at the top of the lower mantle, where the subducted slab may be stagnant. We therefore conducted high-pressure and high-temperature experiments using a multi-anvil apparatus to study the phase relations in a Fe3+-bearing system at 26 GPa and 1573–2073 K, at conditions prevailing at the top of the lower mantle. At temperatures below 1923 K, MgSiO3-rich bridgmanite, an Fe3+-rich oxide phase, and SiO2 coexist in the recovered sample. Quenched partial melt was observed above 1973 K, which is significantly lower than the solidus temperature of an equivalent Fe3+-free bulk composition. The partial melt obtained from the Fe3+-rich bulk composition has a higher iron content than coexisting bridgmanite, similar to the Fe2+-dominant system. The results suggest that strong mantle oxygen fugacity anomalies might alter the subsolidus and melting phase relations under lower mantle conditions. We conclude that (1) a small amount of melt may be generated from an Al-depleted region of a stagnant slab, such as subducted former banded-iron-formation, and (2) Fe3+ is not transported into the deep part of the lower mantle because of its incompatibility during melting.

Original languageEnglish
Pages (from-to)12484-12497
Number of pages14
JournalJournal of Geophysical Research: Solid Earth
Volume124
Issue number12
DOIs
Publication statusPublished - 1 Dec 2019

Keywords

  • bridgmanite
  • ferric iron
  • lower mantle
  • melting
  • redox state
  • stagnant slab

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    Sinmyo, R., Nakajima, Y., McCammon, C. A., Miyajima, N., Petitgirard, S., Myhill, R., Dubrovinsky, L., & Frost, D. J. (2019). Effect of Fe3+ on Phase Relations in the Lower Mantle: Implications for Redox Melting in Stagnant Slabs. Journal of Geophysical Research: Solid Earth, 124(12), 12484-12497. https://doi.org/10.1029/2019JB017704