Penetration of groundwater into crystalline rocks

The extent to which crystalline rocks formed at high temperatures become infiltrated by groundwater during prolonged residence near the earth’s surface is evaluated. The study was carried out because rock matrix diffusion, defined as diffusion through pore water in crystalline rocks, has been proposed to facilitate the dispersal of radionuclides from buried waste. This assumes that rocks are water-saturated, but ubiquitous granite minerals such as biotite are not stable at low temperatures in the presence of water, irrespective of water composition. Nevertheless, the filling of almost all pores in a 3-mm-diameter core of fine-grained granite over 7 days was demonstrated by in situ X-ray computed tomography. Water infiltration could be inhibited by the formation of reaction products if granite reacted with the water, which was investigated using polished granite cores to facilitate observation of reaction products and etch pits. After 4 weeks at a temperature of 50 °C, etch pits in biotite and plagioclase and small secondary platelets on biotite were observed. At higher temperatures of 100 and 150 °C, secondary growths became more pronounced. To provide a natural analogue of possible water penetration, the history of the Mountsorrel Granite in the UK was examined. Despite > 300 Ma of residing within a few kilometers of the surface, the rock has fresh primary minerals, except for areas very close to spaced joints. Infiltration of groundwater into granite takes place along fractures but penetration of the matrix porosity is strongly inhibited, probably by secondary minerals resulting from hydration reactions.