Abstract
Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe2+ in bridgmanite synthesized at 40-70 GPa and 2,000 K, while it is in mixed oxidation state (Fe3+/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe3+ in bridgmanite combined with the strong partitioning of Fe2+ into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant changes in bulk composition for most of the mantle, but possible changes in physical properties and processes (such as viscosity and mantle flow patterns) in the midmantle.
Highlights
Our current understanding of the electronic state of iron in lowermantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle
We report high-pressure experimental results in which Fe becomes dominantly Fe2+ in bridgmanite synthesized at 40–70 GPa and 2,000 K, while it is in mixed oxidation state (Fe3+/ Fe = 60%) in the samples synthesized below and above the pressure range
McCammon [4] reported a large amount of Fe3+ (Fe3+/ Fe = 60%; fraction of Fe3+ with respect to total Fe in a phase) in Al-bearing bridgmanite synthesized in a multianvil press, which was subsequently confirmed under reducing conditions [1, 5]
Summary
Our current understanding of the electronic state of iron in lowermantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). We calculated the volume change in charge disproportionation of Fe to such a configuration (SI Appendix, Fig. S8C and section S4.7) and found that the volume of the VIII[HSFe3+, Al3+]–VI[LSFe3+, Al3+] configuration is significantly smaller, supporting the proposed change in the substitution mechanism at pressures >70 GPa. Our results indicate that almost all of the iron in bridgmanite is ferrous (Fe2+), with little ferric (Fe3+) iron at 1,100- to 1,700-km depths (hereafter low ferric iron bridgmanite zone; LIBZ), whereas bridgmanite in the lower mantle regions above and below LIBZ contains >50% of iron in the ferric oxidation state (hereafter high ferric iron bridgmanite zone; HIBZ).
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