Evaluation of the geothermal effects caused by the weathering of crystalline rocks

Abstract

Abstract The acquisition of a permeability high enough to constitute an aquifer in crystalline rocks is a result of physico-chemical weathering through the transformation of minerals by the chemical processes of oxidation and hydration. The volumetric changes resulting from hydration induce stresses that fracture unweathered rocks. These reactions are exothermic, suggesting that the heat produced may generate a geothermal signature and even some hydrothermal effects. This paper develops a simplified model of thermal disturbance related to exothermic hydration to determine the relevance of this potential thermal effect. The fundamental thermal parameter is the rate of heat production, which is the product of the heat generation per unit volume multiplied by the velocity of the propagation of the front. When the front velocity is c. ≤1 mm a −1 , the temperature disturbance is negligible. The thermal effect only becomes significant if propagation of the front is extremely rapid (several centimetres per year). This observation led to an investigation of the instantaneous value of the front propagation velocity. This parameter was evaluated using a physical model coupling diffusion and chemical reactions with rock fracturing. Such extreme front velocities were only reached in exceptional circumstances; in most common situations, the weathering of crystalline rocks does not cause geothermal effects.