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Since the first application of CL microscopy on rock samples (Sippel, 1965; Marfunin, 1979), this technique has become a standard tool in mineralogy. First CL-studies, however, have mainly focused on sedimentary (± magmatic) rocks and on zircon-zonations connected with ion probe dating.
Here, we would like demonstrate the great advantages of a "hot cathode" CL microscope applied to UHP metamorphic rocks, in particular in conjunction with electron microprobe (EMP)-studies. First results on Dora Maira and Kokchetav rocks were presented by Schertl et al. (2004); the compilation of data presented here refer to a number of "classical" UHP provinces like those from Dora Maira/Italy, Kokchetav/Kazakhstan, Sulu/China, Western Gneiss region/Norway, Pohorje/Slovenia, and Erzgebirge/Germany. CL allows to achieve colored images of important growth features and internal structures of minerals, like small-scale growth zoning, inhomogeneities, exsolution lamellae, dissolution and deformational effects etc. within seconds. Many of those specific features could easily have become overlooked without applying CL microscopy. Examples shown are oscillatory zoning patterns of garnet and kyanite, change of morphology during growth, regular and irregular zoning patterns of garnet, jadeite, omphacite, carbonates, clinopyroxene (partly with cracking/annealing structures), exsolution textures (dolomite/Mg-calcite; K-feldspar/pyroxene), different CL-colours of SiO2-phases (coesite: bluish-green, quartz: dark red to violet, chalcedony: yellow), and special features of accessory minerals like zircon, diamond, apatite, and bearthite.
Although being mainly descriptive, the conclusions drawn open new avenues for petrological applications. The knowledge of specific structural characteristics of rock-forming minerals has an invaluable impact on the more precise and detailed derivation of PT-paths and thus on the evolution of geodynamic processes with any metamorphic rocks involved. Since time-consuming X-ray intensity mapping of minerals using EMP-techniques take up to 50 hours, we propose the CL-technique to be used more intensively for routine investigations of thin sections as a pathfinder prior to chemical characterization by EMP.
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