The effect of microscale pore structure on matrix diffusion—a site-specific study on tonalite

The matrix diffusion of non-sorbing tracers was studied in rocks from the Syyry area, Central Finland (SY1). The effect of alteration and weathering on rock matrix porosity and on the available pore space, which affects diffusivity, are discussed. The main rock type in the crystalline bedrock of Syyry is a slightly foliated, gray tonalite with mica gneiss inclusions as well as minor, more mafic inclusions. The total porosity and the spatial porosity distribution and microstructure of the rocks were investigated using infiltration of carbon- 14-methylmethacrylate, electron microscopy and Hg-porosimetry. The laboratory-scale diffusion experiments were performed using (1) the out-leaching technique of methylmethacrylate (MMA), (2) the through-diffusion of tritiated water (HTO) and chloride in the liquid phase and (3) the through-diffusion of helium in the gas phase. The results of structure investigations indicate that alteration and weathering create micrometric-scale pore apertures, visible as fissures and cracks. These migration pathways dominate in laboratory-scale diffusion experiments and result in high effective porosities and effective diffusivities. Alteration also creates spherical pores, smaller than 1 μm, especially in heavily altered biotite and plagioclase minerals. The altered mineral phases act as quasi dead-end pores hindering the diffusion of non-sorbing tracers. The heterogeneity of the matrix is increased. The heterogeneous spatial pore structure of altered and weathered rock samples causes uncertainty in the diffusion coefficients owing to an assumed homogeneous matrix in Fickian diffusion models