Articles | Volume 9, issue 5
Solid Earth, 9, 1141–1156, 2018
Solid Earth, 9, 1141–1156, 2018

Research article 27 Sep 2018

Research article | 27 Sep 2018

Multiscale porosity changes along the pro- and retrograde deformation path: an example from Alpine slates

Ismay Vénice Akker1, Josef Kaufmann2, Guillaume Desbois3, Jop Klaver4,5, Janos L. Urai3, Alfons Berger1, and Marco Herwegh1 Ismay Vénice Akker et al.
  • 1Institute of Geological Sciences, University of Bern, Bern, 3012, Switzerland
  • 2Empa, Swiss Federal Laboratories for Materials Testing and Research, Dübendorf, 8600, Switzerland
  • 3Structural Geology, Tectonics and Geomechanics, Energy and Mineral Resources Group, RWTH Aachen University, 52056 Aachen, Germany
  • 4Structural Geology, Tectonics and Geomechanics, Energy and Mineral Resources Group, RWTH Aachen University, 52056 Aachen, Germany
  • 5Map Microstructures and Pores GmbH, 52056 Aachen, Germany

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.

Short summary
We studied porosity changes of slates from eastern Switzerland, which were deposited in an ocean in front of the emerging Alps during the Cenozoic. The Alpine collision between the European and African plates brought the rocks from this basin to today’s position in the Alps. From the basin to the surface, the porosity first decreased down to a small number of round cavities (<1 vol%) to microfractures, and once at the surface, the porosity increased again due to the formation of macro-fractures.