The use of high resolution analytical methods for the characterization of reaction microstructures
Abstract
Crystallization of new minerals in metamorphic rocks is strongly controlled by chemical transport rates towards the crystallization sites. Grain and phase boundaries play an essential role in this transport, but information on geometrical and physical properties of boundaries and potential boundary materials are largely absent. In order to gain insights into chemical transport processes during the crystallization of new phases we explored reaction microstructures by using high-resolution analytical techniques. Reactions of type A + plagioclase (Pl) = garnet (Grt) + C form garnet rims between reactants. EBSD microstructure analysis reveals that the reaction rims are composed of grains and subgrains, which form a palisade microstructure perpendicular to the reaction fronts. Based on the known geometry of the grain boundary network, TEM foils can be cut by FIB perpendicular to garnet grain boundaries and TEM investigations reveal distinct nanometer scale compositional patterns across the garnet grain boundaries. In addition, FEG-EPMA and AEM allows to quantitatively measuring micron-scale asymmetric compositional zoning patterns across the reaction rims. These compositional patterns were formed during short-circuit diffusion, which amplifies the necessary mass transfer across the growing reaction rims. Information about the contribution of grain boundary diffusion to bulk material flow in Grt is stored during the formation of these growth zonings and allows for the derivation of Digb/Divol ratios and rim growth rates by diffusion modeling. Reaction progress needs the transfer of material through the interior of reacting Pl towards the reaction fronts. TEM investigations reveal about 100 nm wide pores in Pl and Grt-Pl phase boundaries, which both are filled with a non-crystalline material. This suggests that a coupled diffusion and dissolution/precipitation process, during which internal and surface dissolution/precipitation formed new nano-scale pathways and a non-crystalline transport medium, likely enhanced mass transfer in Pl and along Grt-Pl phase boundaries.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V33B2210K
- Keywords:
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- 1012 Reactions and phase equilibria (3612;
- 8412);
- 1094 Instruments and techniques;
- 3600 MINERALOGY AND PETROLOGY;
- 3625 Petrography;
- microstructures;
- and textures;
- 3660 Metamorphic petrology