<p><strong>Abstract.</strong> 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

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... function of varying metamorphic grade. Surface and sub-surface samples are collected along a temperature gradient from 200 to 320<span class="thinspace"></span>°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 <<span class="thinspace"></span>1<span class="thinspace"></span>vol<span class="thinspace"></span>%, 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 1<span class="thinspace"></span>mm), 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.</p>