Development of a fracture network in crystalline rocks during weathering: Study of Bishop Creek chronosequence using X-ray computed tomography and 14C-PMMA impregnation method

The progressive development of porosity during subsoil weathering of granodiorite clasts was studied at the Bishop Creek moraine chronosequence in east-central California. Fractures and other pores were examined using two complementary imaging techniques, X-ray computed tomography (XRCT) and a C-14-PMMA (C-14-polymethylmethacrylate) method. The well-known XRCT method allows the investigation of three-dimensional (3-D) pore space. C-14-PMMA is a less-known method based on the complete impregnation of pore space with C-14-doped PMMA, and subsequent autoradiograph of a rock section. These imaging methods allow us to decipher the evolution of pore space in the granodiorite during the 120 k.y. weathering period. The C-14-PMMA imaging technique was found to be more suitable for following the evolution of the whole sequence, from "intact" bedrock to saprock, in terms of crack density, porosity, and aperture. Working with hand specimens, this method was adapted to detect both the low-aperture fractures (microcracks) and macrocracks. Only a slight and progressive increase in total fracture density was observed during the whole weathering period. However, this trend does not hold if macrocracks and microcracks are separated: Microcrack density slightly decreases, whereas macro-crack density increases due to a progressive expansion of microcracks. The total porosity of the rock increases during weathering and is correlated to the progressive aperture increase of all types of cracks. This evolution is accompanied by a change of crack morphology and connectivity, and an overall increase in intragranular porosity of biotite and plagioclase aggregates.