Temperature-driven micro-fracturing in Granite: The interplay between microstructure, mineralogy and tensile strength

High temperatures exert a significant influence on the mechanical and fluid flow properties of rocks and minerals. In crystalline rocks, differential thermal expansion of minerals is known to induce microfracture damage leading to changes in bulk volume and tensile strength. Here we report new data from thermally treated core samples of Devon Granite in order to constrain the interplay between tensile strength and thermally-induced damage with respect to the background mineralogy. A series of core samples was cyclically heated at temperatures ranging from 25 to 800 C, with P-wave velocity and porosity measured after each cycle. Tensile strength decreased significantly from 9 MPa to less than 3 MPa as thermal treatment increased from 25 to 800 C. The mechanical data were then compared to fracture density values obtained by optical maps of microfracture damage to assess the quantity and degree of linkage of intergranular and intragranular fractures using the FraqPaQ toolbox. The fracture density increased from 0.02 mm^(-2) to 2.0 mm^(-2) which is consistent with results obtained from direct physical parameters as calculated from elastic wave data. We conclude that the combined effects of thermal expansion and the α-β phase transition within quartz crystals has a pronounced effect on tensile strength.