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3.7 b. Determination of Fe3+/ΣFe in perovskite/magnesiowüstite assemblages using electron energy loss spectroscopy (EELS) (S. Lauterbach, C.A. McCammon, F. Langenhorst and F. Seifert, in collaboration with P.A. van Aken/Darmstadt)

The oxidation state of iron in lower mantle perovskite and magnesiowüstite can affect many properties, including electrical conductivity, diffusivity and rheology. As determined by Mössbauer spectroscopy, the Fe3+/ΣFe ratio in the perovskite phase is relatively high and is controlled primarily by the aluminium concentration (see Annual Report 1997). In contrast, similar studies have shown that the Fe3+/ΣFe ratio is relatively low in single-phase magnesiowüstite. While study of the single-phase systems is important to establish the individual crystal chemistry of these phases, equilibrium studies of the two-phase assemblages are essential for characterising geophysically important variables such as partitioning of Fe2+ and Fe3+ between co-existing phases. For finely distributed perovskite/magnesiowüstite mixtures the limited spatial resolution provided by Mössbauer spectroscopy prevents an accurate determination of Fe3+/ΣFe in the separate phases. We therefore have extended the EELS technique reported in last year's Annual Report to determine the relative Fe3+ content in both phases with high spatial resolution.

The perovskite/magnesiowüstite assemblages are synthesised from pure oxide mixtures with a high initial Fe3+/ΣFe ratio. In order to achieve equilibrium during each run in a multianvil press at conditions relevant to the lower mantle (26 GPa, ˜1700°C), the run time is extended to 22-26 h. We use Re capsules, but do not add ReO2 to the starting mixture as in previous experiments in order to avoid contamination. The resulting run products are characterised by X-ray diffraction and the electron microprobe, while the Fe3+/ΣFe ratio is determined using electron energy loss spectroscopy.

Figure 3.7-3 shows EELS data for (Mg0.864,Fe0.155)(Si0.914,Al0.067)O2.953 perovskite coexisting with (Mg0.616,Fe0.380)O1.005 magnesiowüstite. The high resolution of the EELS technique (on the order of nanometers) enables the study of perovskite/magnesiowüstite assemblages relevant to the lower mantle bulk composition. We are able to derive the partition coefficients of Fe2+ and Fe3+ between the two phases using the derived Fe3+/ΣFe ratios in combination with the Fe content determined by the electron microprobe.

Fig. 3.7-3: EELS data collected for (Mg0.864,Fe0.155)(Si0.914,Al0.067)O2.953 perovskite coexisting with (Mg0.616,Fe0.380)O1.005 magnesiowüstite.

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