Mineral growth in metamorphic rocks: relationships between chemical patterns, mineral microstructure and reaction kinetics
Abstract
Mineral growth in metamorphic rocks is governed by kinetic processes, which are strongly related to geometrical and physical properties of the chemical pathways involved. Information on the nature of chemical pathways and potential effects of deformation on kinetic processes is stored in chemical patterns and mineral microstruture. By using high-resolution analytical techniques (i.e. EBSD, FE-EMPA and TEM/FIB) and numerical simulations we explored natural and experimental samples in order to reveal the kinetics and the microstrutural evolution of both transport controlled mineral reactions and exchange reactions. We found reaction rims between reactant minerals, which are composed of several grains. These grains are subdivided into subgrains perpendicular to the reaction fronts. The subgrains often show a systematic lateral orienation change, which is possibly caused by grain-scale plastic deformation in the form of creep polygonization. Such an arrangment of grain boundaries allows for short-circuit diffusion and amplifies the necessary mass transfer across reaction rims. Information about the contribution of grain boundary diffusion to bulk material flow in Grt is stored during the formation of asymmetric growth zonings and during micron- scale compositional variations along grain and phase boundaries. Our evaluation of these compositional patterns by diffusion modelling allows for the derivation of Digb/Divol ratios and rim growth rates. Information about phase boundary material properties is also stored in the zoning of Grt porphyroblasts, which formed during the retrograde Fe-Mg exchange between Grt and biotite. If temperature falls below a limiting value during cooling, non-equilibrated rim compositions start to develop along Grt-Qtz phase boundaries due to insufficient chemical transport rates. As the extent of non-equilibrated rim compositions reflects the transport capacity of the present interganular medium, our simulations by diffusion modelling of observed compositional patterns formed during cooling at Grt-Bt-Qtz triple junctions allows for the extraction of boundary material properties.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMMR14A..04K
- Keywords:
-
- 1012 Reactions and phase equilibria (3612;
- 8412);
- 1094 Instruments and techniques;
- 3600 MINERALOGY AND PETROLOGY;
- 3625 Petrography;
- microstructures;
- and textures;
- 3660 Metamorphic petrology