instrumental techniques applied to mineralogy and geochemistry

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14 Elisabetta Mariani et al FIGURE 2. Amphibolite facies quartzo-feldspathic gneiss from the Lewisian Complex, NW Scotland. The section is cut parallel to lineation and perpendicular to foliation (kinematic x-z section) and show elongated aggregates dominated by K-feldspar but with subsidiary plagioclase, quartz and biotite. EBSD data was acquired on a Philips XL30 W-filament SEM, at 6.5 nA and 20 keV. The acquisition settings in Flamenco were 7 bands (the minimum needed to give a low level of misindexing –usually 8 are nedeed), 120 Hough resolution, 75 reflectors, band edges and 4x4 binning.
Electron backscatter diffraction (EBSD) in the SEM: applications to microstructures in minerals and rocks and recent technological advancements 15 One of the recent achievements in EBSD is successful automated analysis of very low symmetry minerals (triclinic and monoclinic) such as feldspars and some pyroxenes (e.g. omphacite). In Fig. 2 we report an EBSD case study of an amphibolite facies quartzo-feldspathic gneiss from the Lewisian Complex, NW Scotland (Pearce work in progress). In this specimen elongated aggregates dominated by K-feldspar with subsidiary plagioclase, quartz and biotite can be observed in the ‘tails’ of large porphyroclasts. Both K-feldspar and plagioclase were indexed using triclinic crystal symmetry. Lattice parameters for the two minerals are very similar only differing significantly in the angle. However this appears to be enough for the software to distinguish between the minerals in a statistically significant number of cases (Fig. 3). It is observed that misindexing in feldspars is more common than in other minerals. Orientations that are misoriented by 180° (pericline and albite twin laws) have very similar patterns that may differ only by one band. Thus certain orientations may be significantly affected by systematic misindexing problems. Some phase misidentification may also occur in feldspars. Misidentification and misindexing can be carefully processed out of the dataset in the post-processing stage thus allowing LPOs and boundary analysis for the whole mineral assemblage and interpretation the deformation/recrystallization mechanisms. EBSD combined with chemical analysis of omphacite from the Streaked Eclogite, Sesia-Lanzo Zone, NW Italian Alps, is a key tool to understanding how this mineral forms strong LPOs and what is the role of mechanisms such as dislocation creep, pressure solution and grain growth (McNamara work in progress) (Fig. 4). Comparison of the EBSD and EDS results on the omphacite from the Streaked Eclogite with results from omphacite fans growing in an undeformed eclogite from the same locality, suggest that the pronounced subgrain structure observed may originate during grain growth and not during deformation. Chemical analysis show subtle or no chemical variations in the omphacite (Fig 4).
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