Distorted crystals carry useful information on processes involved in their formation, deformation and growth. The distortions are accommodated by geometrically necessary dislocations, and therefore characterising those dislocations is an informative task, to assist in, for example, deducing the slip systems that produced the dislocations. Electron Backscatter Diffraction (EBSD) allows detailed quantification of distorted crystal orientations and we summarise here a method for extracting information on dislocations from such data. The Weighted Burgers Vector (WBV) method calculates a vector at each point on an EBSD map, or an average over a region. The vector is a weighted average of the Burgers vectors of dislocation lines intersecting the map surface. It is weighted towards dislocation lines at a high angle to the map but that can be accounted for in interpretation. The method is fast and does not involve specific assumptions about dislocation types. It can be used, with care, to analyse subgrain walls (sharp orientation changes) as well as gradational orientation changes within individual grains. We describe new and published examples of the use of the technique to illustrate its potential; case studies to date mainly use the WBV direction not the magnitude. EBSD orientation data have angular errors, and so does the WBV. We present an analysis of these angular errors, showing there is a trade-off between directional accuracy and area sampled. In summary the technique is fast, free from assumptions, and errors can be taken into account to allow testing of hypotheses about dislocation types.

Deformation is a near ubiquitous process that is observed within nearly all naturally forming rocks, terrestrial and extra-terrestrial. Large area electron backscatter diffraction (EBSD) is a technique that enables slip-systems (a form of plastic deformation) to be inferred at a comparable scale to representative texture analysis (≥100 crystals). Extensive laboratory and studies on naturally occurring samples have identified preferential extrinsic parameters for specific slip-system signatures within olivine and clinopyroxene for mantle conditions. Slip-systems in both olivine and augite (high Ca-clinopyroxene) for 21 large area EBSD datasets sourced from 16 different Martian nakhlite meteorites were analysed and assessed against these parameters. When investigating the high and low deformation regions within the samples 10 of the 21 sections exhibited a shift in the slip-system patterns between the low and high deformation regions. The secondary signatures identified within the low deformation regions are inferred to relate to emplacement deformation. Thus, these samples exhibit both shock and emplacement signatures. The observed variations in deformation patterns for the two main regimes of deformation indicate heterogeneous sampling of the nakhlite ejecta crater. Our findings indicate that shock deformation is prevalent throughout the nakhlites, and that great care needs to be taken when interpreting slip-deformation of crystals within apparent lower deformation regions.

The Martian nakhlite meteorites, ormed from a single magma source region, and emplaced during multiple events spanning at least 93 ± 12 Ma, represent a key opportunity to study the evolution of Martian volcanic petrogenesis. Here 16 of the 26 identified nakhlite specimens are studied using coupled large area electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) mapping to determine shape preferred orientation (SPO) textures of contained augite (high Ca-clinopyroxene) phenocrysts by considering crystallographic preferred orientation (CPO). Textural parameters derived from EBSD and EDS analyses were used to calculate maximum and minimum magma body crystallization thicknesses via three endmember emplacement scenarios: thermal diffusion, crystal settling, and crystal convection. Results from CPO textural analyses indicate weak to moderate fabric textures that are comparable to those in terrestrial clinopyroxenites. In all samples, a consistent foliation within the {001} axis of augite is observed. In all but two of the studied nakhlites this {001} foliation is typically coupled with a weaker lineation fabric in one or more of the {100}, {010}, {001} axes. Results from the calculated magma body thicknesses are consistent with an emplacement mechanism for the nakhlites driven by crystal settling. These crystal settling results infer magmatic body thicknesses ranging from <1 m to several 10’s m, forming two distinguishable groups that appear random when assessed against observed texture, geochemical, and age parameters. Coupled textural and modelling results therefore suggest that the nakhlite source volcano varied in thickness over time yet consistently solidified via the mechanism of crystal settling.