Abstract
Abstract. Estimating the porosity of slates is of great interest for the industries dealing with sub-surface areas such as CO2 sequestration, nuclear waste disposal and shale gas but also for engineering purposes in terms of mechanical stability for underground or surface constructions. In this study, we aim at understanding estimates of the porosity of slates from the Infrahelvetic flysch units (IFUs) in the Glarus Alps (eastern Switzerland). Surface and sub-surface samples were collected along a temperature gradient from 200 to 320 °C and therefore give the opportunity to link pore types along this temperature and deformation path. In addition, we indicate which porosity is the effect of surface processes and indicate the contribution of artificially induced porosity. The developed workflow consists of a combination of bulk rock measurements including helium pycnometry (He pycnometry) and mercury intrusion porosimetry (MIP) with image analysis. Image analysis was performed with high-resolution scanning electron microscopy (SEM) on broad ion beam (BIB) prepared cross sections (BIB-SEM). Different vein generations provide evidence of porosity formation at depth, as they present paleo-porosity. Towards peak metamorphic conditions (prograde path), porosity reduces to < 1 vol%, indicated by matrix porosity detected by BIB-SEM. During exhumation (retrograde path) porosity increases due to the formation of microfractures interpreted as the effect of unloading (open fractures). At the surface, porosity is further increased due to the formation of macro-fractures (fracture apertures up to 1 mm), which are interpreted as being either due to the effect of weathering processes such as freeze and thaw cycles or artificially induced by sample preparation. Additionally, porosity and pore morphology are strongly dependent on mineralogy, sample homogeneity and strain, which change dynamically in time and space.
Highlights
Slates as representatives of sheet-silicate-rich rocks delineate a fine-grained anisotropic microstructure and are common low-grade metamorphic rocks
We use a combination of He pycnometry and mercury intrusion porosimetry (MIP) as bulk rock measurements and image analysis to obtain estimates of the porosity of slates
Bulk rock measurements obtain the total connected porosity, which can be subdivided into fracture and matrix porosity determined by image analysis
Summary
Slates as representatives of sheet-silicate-rich rocks delineate a fine-grained anisotropic microstructure and are common low-grade metamorphic rocks. These rock types play an important role in the industry of underground storage, such as nuclear waste disposal and geological carbon sequestration Loon, 2008; Thury and Bossart, 1999) but are of importance in the surface and sub-surface building industry because of the low mechanical strength and associated geotechnical problems (e.g. Blümling et al, 2007). Slates are crucial in mountain building processes, since they can act as a mechanically weak phase and localize large amounts of strain in low- to very low-grade metamorphic domains The pore network defines the major fluid pathways, and a detailed investigation of microstructural porosity is a key to better understanding the fluid flux and circulation of fluids in collisional orogens and orogenic wedges
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