HR: 17:05h
AN: V12I-05 INVITED     [PDF]
TI: Garnet Deformation Microstructures: TEM vs. EBSD
AU: * Brenker, F E
EM: brenker@min.uni-koeln.de
AF: Dept. of Mineralogy und Geochemistry, University of Cologne, Zuelpicher Str. 49 b, Koeln, 50674 Germany
AU: Prior, D J
EM: davep@liverpool.ac.uk
AF: Department of Earth and Ocean Sciences, Liverpool University, Liverpool, L69 3GP United Kingdom
AB: The development of electron backscatter diffraction (EBSD) and orientation contrast (OC) analysis enable us to study the microstructure of minerals and to measure the orientation of all crystallographic axes in thin sections with resolution down to sub micrometer scale. One of the great advantages of this methods is that it is now possible to study cubic crystals, such as garnet, which are isotropic for light. The application of this new technique to the study of garnets revealed an unexpected wide variety of microstructures, which are not all related to the deformation of garnet. The orientation distribution analyses help to distinguish between brittle or plastic deformation, grain or subgrain boundaries and growth defects. Garnet microstructures related to plastic deformation are characterized by the development of rotation axes if a transect across several garnet subgrains is measured with small crystallographic mismatches ($<3\deg$) across cell boundaries. Each specific rotation axis is directly related to the activated slip system. If more than one slip system is involved, the identification of individual is not always possible. In order to fully characterize the deformation microstructure additional transmission electron microscopical (TEM) work is needed. Here we present new data on garnet deformation microstructures characterized by EBSD and TEM techniques. In some cases a connection between cell boundary and chemical composition is observed by comparing OC, BSE and element mapping. The same area is studied by analytical TEM and shows subgrain boundaries as well as indications for brittle failure with subsequent refilling of a newly grown garnet. In the case of subgrain boundaries a higher diffusivity is expected along dislocation cores. In order to get a more general view about the net effect of microstructuring of garnet, bulk diffusion calculations were performed for homogeneously distributed defects and a network of subgrain boundaries. Assuming the same enhancement of diffusion by microstructuring as in metal, garnet remains an open system for homogeneously distributed defects below 600 to 650$\deg$C at strain rates in the order of 10-12 s$^{-1}$. A closely spaced network of subgrain boundaries will affect bulk diffusion at even higher temperatures. Our data suggest that deformation microstructures in garnet are more frequent than previously assumed and that microstructures may significantly enhance bulk diffusion coefficients. As a consequence the microstructure of garnet must be considered when applying geo-chronological or thermo-barometrical techniques.
DE: 1035 Geochronology
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
DE: 3694 Instruments and techniques
DE: 3904 Defects
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting