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3.2 d. A study of the oxidation state of iron in (Mg,Fe)(Si,Al)O3 perovskite (S. Lauterbach, C.A. McCammon, F. Langenhorst and F. Seifert, in collaboration with P.A. van Aken/Darmstadt)

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.

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