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Discussion papers | Copyright
https://doi.org/10.5194/se-2018-47
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Jun 2018

Research article | 12 Jun 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Solid Earth (SE).

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

Ismay Vénice Akker1, Josef Kaufmann2, Guillaume Desbois3, Jop Klaver3,4, 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, Aachen, 52056, Germany
  • 4Map Microstructures and Pores GmbH, Aachen, 52056, Germany

Abstract. Estimating porosity of slates is of great interest for the recently rising industries dealing with the underground 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 understanding estimates of porosity of slates from the Infrahelvetic Flysch Units (IFU) in the Glarus Alps (eastern Switzerland) and their changes as function of varying metamorphic grade. Surface and sub-surface samples are collected along a temperature gradient from 200 to 320°C and give therefore the opportunity to link pore types along the deformation path and to surface processes or indicate what artificially induced porosity is. A developed workflow consists of a combination of bulk rock measurements such as Helium pycnometry (He-pycnometry) and Mercury Intrusion Porosimetry (MIP) with image analysis. Image analysis is performed on high scale resolution with Scanning Electron Microscopy (SEM) on Broad Ion Beam (BIB) prepared cross sections (BIB-SEM). Different vein generations give evidence for porosity formation at depth. Towards peak metamorphic conditions (prograde path) porosity reduces to <1vol%, indicated by matrix porosity detected by BIB-SEM, as this technique has shown the least artificially induced porosity by polishing. 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 macrofractures (up to 1mm), which are interpreted to be the effect of weathering processes such as freeze and thaw cycles or these are artificially induced by sample preparation. Additionally, porosity and pore morphology are strongly dependent on mineralogy, sample homogeneity and strain.

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