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3.2 h. Ferric iron content of the lower mantle (C.A. McCammon)

The lower mantle, which constitutes more than half of the Earth's interior by volume, is believed to consist predominantly of (Mg,Fe)SiO3 perovskite with up to approximately 20% (Mg,Fe)O.

In the lower mantle system FeO-MgO-SiO2, the majority of iron occurs as Fe2+, and partitions preferentially into (Mg,Fe)O relative to the perovskite phase. Recent experiments by Wood and Rubie (Science, 273: 1522-24, 1996), however, have shown that Fe-Mg partitioning is drastically altered by the addition of 4-5 wt% Al2O3 to the system, resulting in an essentially equal partitioning of iron between the perovskite and (Mg,Fe)O phases. One explanation for this behaviour is a significant change in Fe3+/ Fe of the perovskite phase.

Multianvil experiments on quenched samples of (Mg,Fe)SiO3 perovskite synthesised at varying oxygen fugacities at 26 GPa and 1600°C show that Fe3+/Fe in the perovskite phase can reach approximately 20% under oxidising conditions. The addition of a small amount of Al2O3 to the system, however, dramatically increases the Fe3+ content: with addition of 3 wt% Al2O3 Fe3+/ Fe values of more than 50% in the perovskite phase were obtained. Figure 3.2-7

Fig. 3.2-7: Mössbauer spectra of (Mg,Fe)SiO3 perovskite: (a) Al-free; (b) containing 3.3 mol% Al2O3. The quadrupole doublets are shaded as follows: Fe2+ - no shading; Fe3+ - black; Fe2.5+ - grey. Note the dramatic increase in relative Fe3+ content of the perovskite phase in the presence of Al.

illustrates Mössbauer spectra showing the dramatic increase in Fe3+ content of (Mg,Fe)SiO3 perovskite when Al2O3 is added. The relative amount of Fe3+ is strongly correlated to the Al content, and the substitution Mg2+ + Si4+ -> Fe3+ + Al3+ appears to be instrumental in stabilising Fe3+. Based on the estimated Al content of the lower mantle, these results suggest that the majority of iron in the perovskite phase is Fe3+, which has significant implications for the physical and chemical properties of the lower mantle.

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