The aim of this project is to investigate depletion haloes around porphyroblasts with corona structures from both essentially fluid saturated and undersaturated systems. The strategy will be to combine electron beam microanalytical techniques with high spatial resolution (FEG-EMS, FEG-SEM, TEM) and FIB sample preparation technique in order to directly observe the traces of intergranular diffusion both, within the depletion halos and across the corona structures. From this the relative contributions of volume and grain boundary diffusion within the rock matrix should be extracted. Given that volume diffusion coefficients are known for most of the major elements in most of the rock forming minerals, this will allow to get a handle on the bulk component mobility in the rock matrix. A hierarchic structure of observations on a wide range of length scales, from the thin section scale (size of depletion halos) down to the nanometer scale (chemical zoning across grain- and phase boundaries) will give a new and profound insight into the interplay of the kinetic processes, which control the microstructure, mineralogy and chemical evolution during metamorphic crystallization.