We present ab initio (LDA + U
Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3bridgmanite (FA50) with the highest Fe3+‐Al3+coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yields
- NSF-PAR ID:
- 10374450
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 125
- Issue:
- 7
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The amount of ferric iron Fe3+in the lower mantle is largely unknown and may be influenced by the disproportionation reaction of ferrous iron Fe2+into metallic Fe and Fe3+triggered by the formation of bridgmanite. Recent work has shown that Fe3+has a strong effect on the density and seismic wave speeds of bridgmanite and the incorporation of impurities such as aluminum. In order to further investigate the effects of ferric iron on mineral behavior at lower mantle conditions, we conducted laser‐heated diamond‐anvil cell (LHDAC) experiments on two sets of samples nearly identical in composition (an aluminum‐rich pyroxenite glass) except for the Fe3+content; with one sample with more Fe3+(“oxidized”: Fe3+/ΣFe ~ 55%) and the other with less Fe3+(“reduced”: Fe3+/ΣFe ~ 11%). We heated the samples to lower mantle conditions, and the resulting assemblages were drastically different between the two sets of samples. For the reduced composition, we observed a multiphase assemblage dominated by bridgmanite and calcium perovskite. In contrast, the oxidized material yielded a single phase of Ca‐bearing bridgmanite. These Al‐rich pyroxenite samples show a difference in density and seismic velocities for these two redox states, where the reduced assemblage is denser than the oxidized assemblage by ~1.5% at the bottom of the lower mantle and slower (bulk sound speed) by ~2%. Thus, heterogeneities of Fe3+content may lead to density and seismic wave speed heterogeneities in Earth's lower mantle.
