Some garnet microstructures: an illustration of the potential of orientation maps and misorientation analysis in microstructural studies.

Two samples with contrasting garnet microstructures are examined in this paper. The first is a typical deformed garnet porphyroclast in an upper-amphibolite facies shear zone that affects the eclogites of the Glenelg–Attadale inlier of NW Scotland. The second sample a typical garnet porphyroblast from the Schneeberg Complex of the N-E Italian Alps. Garnet is critical to many thermobarometers and major element, trace element and isotopic zoning patterns together with inclusion assemblages are used to infer metamorphic histories. Many mantle models suggest that majoritic garnet is a major component of the mantle transition zone and may control mantle rheology. Thus understanding processes within garnet is of particular importance and requires microstructural data. The microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction. In both cases there is a very strong crystallographic preferred orientation, with measurements clustered round a single dominant orientation. Each garnet grain is divided into domains with similar orientations, limited by boundaries with misorientations of 2° or more. In both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains. One sample is a deformed garnet that shows considerable distortion within the domains. Lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes. The domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery. The second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes. These boundaries probably formed by coalescence of originally separate garnets. We suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy. The example of the garnet microstructures shows two of the key elements of EBSD analysis: 1. The misorientation information generated by measuring the spatial variation in orientation can be used to test conceptual, numerical or analytical models for the processes that produce a microstructure. 2. Microstructural analysis can be carried out in nearly all crystalline materials.