EBSD analysis of the Shergottite Meteorites: New developments within the technique and their implication on what we know about the preferred orientation of Martian minerals

What we know about the geology and mineralogy of the Martian surface has been characterised by both the use of remote sensing techniques and the analysis of Martian meteorites. Various techniques are employed to conduct these analyses including crystallographic, geochemical and spectral measurements, all of which enable us to infer a geological history for these rocks. Several references have been made to the potential for preferred orientation of crystals within the Shergottites [1] and their implication for the cooling history of the respective magmas on Mars [2]. We have already shown that a preferred orientation of the two pyroxenes, augite and pigeonite, can be seen in the Zagami meteorite using electron back-scatter diffraction (EBSD) analysis [3]. However, when compared to previous modal studies of the same meteorites [4], it becomes apparent that the current EBSD datasets for Martian meteorites are incomplete. Indexing of some minerals can be hampered by the lack of available matches within library databases for EBSD, or become difficult to resolve between minerals where crystallographic differences between similar minerals fall below the technical limitations of the instrument [3]. Recent advances in EBSD technologies combined with the simultaneous acquisition of energy-dispersive spectra (EDS) however now allow us to determine a more comprehensive set of analyses in a much shorter period of time, fully resolving even similar minerals where areas have been left with no indexing previously [5]. Preliminary investigations suggest that the new technology can successfully index >90% of the sample. The most recent EBSD analyses potentially reveals previously unseen fabrics in the meteorites alongside the EDS hyper-spectral imaging helping to resolve any unknown or questionable phases within them. In this study we will present new data from an investigation using EDS alongside EBSD analysis on 2 Shergottite meteorites, SAU 005 and Zagami, to further resolve their constituent minerals and any existing preferred orientations. [1] Stolper E. and McSween, H.Y. Jnr (1979) Geochemica et Cosomochimica Acta, 43, 1475-1498 [2] McCoy, T.J. et al. (1992) Geochimica et Cosomochimica Acta, 56, 3571-3582 [3] N.R. Stephen et al. (2010) Abstract #5008 73rd Annual Meeting of the Meteoritical Society [4] N.R. Stephen et al. (2010) Abstract #2367 Lunar & Planetary Science Conference XLI [5] J. Berlin et al. (2011) Abstract #2723 Lunar & Planetary Science Conference XLII