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The lower mantle is believed to consist predominantly of (Mg,Fe)(Si,Al)O₃ perovskite and (Mg,Fe)O. We have studied the effect of aluminium on the oxidation state of iron in the silicate perovskite phase, and found that Fe³⁺/ Fe is controlled largely by the amount of aluminium (see Annual Report 1997). In a continuation of this study we have focused on electron energy loss spectroscopy (EELS) to determine Fe³⁺/ Fe in these perovskite phases. While Mössbauer spectroscopy is a well known and established method, EELS provides a new possibility to investigate Fe³⁺/ Fe with high spatial resolution.

The samples of (Mg,Fe,Al)(Si,Al)O₃ perovskite were synthesised from orthopyroxene in a multianvil press at conditions relevant to the lower mantle (26 GPa, ~1700°C, run time 2-8 h) at different oxygen fugacities produced by using Re and Fe capsules. The resulting run products were characterized by X-ray diffraction and chemical composition was analysed with an electron microprobe. The Fe³⁺/ Fe ratio was determined using both ⁵⁷Fe-Mössbauer spectroscopy and EELS.

A typical EELS spectrum is illustrated in Fig. 3.2-9, which shows the FeL₂ and FeL₃ edges. Fe³⁺/ Fe was determined using the 'small window' method developed by van Aken et al., where the integral intensity ratio of the grey shaded areas is calculated and Fe³⁺/ Fe is determined from a calibration curve. Our results indicate that EELS and Mössbauer data are in good agreement. The high spatial resolution of EELS (on the scale of nm) enables determination of Fe³⁺/ Fe of single grains, and we are applying the method to multi-phase assemblages relevant to the lower mantle.
 

Fig. 3.2-9: Electron energy-loss spectrum of (Mg0.901Fe0.107)Si0.985O3 perovskite, where the shaded areas are used to determine Fe3+/Fe.