Symplectites are fine grained mineral aggregates with vermicular or lamellar intergrowth, which replace a homogeneous precursor phase in a solid state reaction. As they reflect processes acting during rock formation and evolution, they play a key role in understanding the P-T evolution of rocks. Since the formation of symplectites is a diffusion-controlled process, there is a great potential in symplectite microstructures to apply them as “geospeedometers” to estimate the rates and duration of their formation. This opens new perspectives in the dating of metamorphic processes in high grade rocks where conventional geochronologic methods are not applicable because of the high temperature. In addition to this, fine-grained symplectites have high surface energy, which may have significant effect on their stability. Accordingly, the behaviour of rocks containing symplectites during further metamorphic evolution may deviate from what would be predicted from equilibrium thermodynamics.
Efficient and up-to-date study of symplectites require the knowledge of the 3Dmicrostructure and elemental distribution of phases involved in the reaction as well asphysical properties of the of the grain and interphase boundaries and phases within thereaction zone. Only recent developments in electron beam microanalytics made possible to address these problems. These include the development of the electron source to achieve brighter electron beam and higher spatial resolution as well as the introduction of site-specific specimen preparation with focused ion beam technique (FIB). The latter one allows for preparing site-specific electron transparent foils for transmission electron microscopy (TEM) and for the three dimensional (3D) analysis of microstructures by combining the FIB with a scanning electron microscope (SEM) in a Dual beam SEM/FIB instrument.
The goal of this project is to apply these new analytical techniques on natural symplectites to refine conceptual mdoels on symplecite formation and to constrain P-T histories. In particular the susceptiblity of symplecitet micorstructtures to post peak metamoprhic processes such as fluid or melt infiltration will be addressed.