Iron is an abundant element in meteorites, where it is contained in various forms such as metal, oxides, silicates or sulfides. In these materials, investigation of the oxidation state of transition metal elements and Fe in particular is an indicator of the environmental conditions during the meteorite formation and evolution. The detailed quantification of the element distribution, concentration variation and the structure of defects are important for understanding the formation and transformation stages. Therefore the correlative investigation of defects structure and oxidation state variation within a meteoritic material at the atomic level can be essential to the understanding of the distinct formation mechanisms.

In this context, the current study focusses on the detailed investigation by STEM-EELS coupled to HAADF imaging of planar defects in Fe-rich olivine (Mg,Fe)₂SiO₄, present in the Allende meteorite [1]. The nature of the planar defects in olivine is still controversial in the literature: it can be associated to a deformation mechanism [2] or can be due to an aqueous alteration episode [3]. HAADF images of two types of planar defects, present in the Allende meteorite are given in Fig. 1 (a) and (b). Both planar defects are parallel to (100) planes of the olivine. The thicker ones, Fig. 1(a), are Cr-rich as seen on the EELS spectra given in Fig. 2. STEM-EELS mapping reveals an enrichment of Fe at the interface between the thick Cr-rich defects and olivine. The Fe-L_3,2 ELNES structures do not seem to indicate any change of the oxidation state in this Cr-rich defect. The thinner (100) defects, Fig. 1(b), are associated to considerable Fe enrichment and Mg depletion over 2 to 4 atomic planes. Even though these thinner defects are often linked to the Cr-rich defects, they do not contain Cr. The analysis of the atomically resolved Fe-L_3,2 fine structure reveals the appearance in certain cases of a change of oxidation state of iron and partial Fe³⁺ state within the thin Fe-rich planar defects. This study gives an important insight on the structure of planar defects in the olivine and the associated change of the Fe oxidation state. Based on this fine structure analysis, a possible formation mechanism constraining the evolution of the environment redox condition will be proposed.  

References

[1] H. Palme, B. J. Fegley, Earth and Planetary Science Letters (1990), vol. 101, p. 180–195.

[2] L. P. Keller, Meteoritics and Planetary Science (1998), Vol. 33, p. A83.

[3] Khisina, R. Wirth, S. Matsyuk, M. Koch-Müller, Eur. J. Mineral. (2008) vol. 20, p. 1067.

Acknowledgement

The CEA-METSA network is acknowledged for financial support. The TEM national facility in Lille (France) is supported by the Conseil Regional du Nord-Pas de Calais, the European Regional Development Fund (ERDF), and the Institut National des Sciences de l’Univers (INSU, CNRS).

Figures:

Fig. 1. HAADF images of (100) planar defects in the Fe-rich olivine (Mg,Fe)2SiO4. (a) Cr containing and (b) Fe rich.

Fig. 2. EELS spectra from the Cr containing defect (green line) given in Fig. 1(a) and the olivine (red line).

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