Fig. 2

A Polished section of sample IS_Strýtan_01. The sample is stabilized with an epoxy resin. An enormous amount of pore space is obvious (red arrows). An internal stratification reveals depositional events (dashed lines). B Porosity analysis calculated from the XCT investigation. The color bar scales the pore volume. Apparently, few small pores have developed. Generally, large pores up to 6 mm³ prevail, showing a relatively homogenous spatial distribution. C XCT scan of the sample IS_Strýtan_01. The enormous amount of pore space in the sample is well visible (white—smectite, black—air/pore space). The red arrows mark the major water pathways. The red square shows one main duct as shown in E. D 2D projection of the porosity analysis. A homogenous distribution of the pores is well visible. The indicated great abundance of large pores hints on the large available pore space being involved in fluid migration. The material is highly permeable. E Sample investigated by Marteinsson et al. (2001). This sketch outlines clearly the layered construction of the chimney and shows the small tubes emitting the hydrothermal fluid (red arrows; modified after Marteinsson et al. 2001). F Sample IS_Strýtan_01 macroscopically. Red arrows mark the major dewatering tubes. Sample is in native orientation