Permeability data for impact breccias imply focussed hydrothermal fluid flow John Parnell a, ⁎, Colin W. Taylor a , Scott Thackrey a , Gordon R. Osinski b , Pascal Lee c a Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK b Department of Earth Sciences/Physics and Astronomy, University of Western Ontario, London, ON, Canada N6A 5B7 c NASA Ames Research Center, MS 245-3 Moffett Field, CA 94035-1000, USA abstract article info Article history: Received 8 May 2009 Accepted 10 December 2009 Available online 21 December 2009 Keywords: Astrobiology Hydrothermal systems Impact crater Permeability New measurements of permeability from 14 samples of impact breccia in meteorite impact craters all indicate values of 1 mD or lower. These values are low and suggest that fluid flow through impact craters, evidenced by hydrothermal systems in numerous craters, is predominantly through fracture systems. Mineral precipitation by circulating fluids would eventually seal the fractures. The focussing of hydrothermal fluid flow in fractures emphasizes their importance as potential sites for microbial colonization, and suggests that the sealing minerals are good targets to search for evidence of life. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Numerous meteorite impact craters exhibit evidence of hydro- thermal activity, which is assumed to be a consequence of the transient residual heat left after the impact event (e.g. Newsom et al., 1986; Ames et al., 1998; Osinski et al., 2001; Hagerty and Newsom, 2003; Osinski et al., 2005b). In addition to the heat created by the energy of impact, uplift of the central parts of many craters causes introduction of rocks to the surface which had been at greater depths and therefore bring heat to the surface. Theoretical modelling suggests that a thermal anomaly may last for a million years or more in the case of the largest craters in the terrestrial geological record (McCarville and Crossey, 1996; Abramov and Kring, 2004, 2007). Hydrothermal activity is of interest due to the potential for discovery of economic mineral deposits in some craters (Grieve and Masaitis, 1994; Grieve, 2003). However hydrothermal systems are also of interest for their astrobiological significance, and they have been proposed as environments that could support organic synthesis, the evolution of life or the maintenance of life (Farmer, 2000; Reysenbach and Cady, 2001). There is now confidence that hydro- thermal deposits occur at the Martian surface, based on a range of criteria that could point towards hydrothermal activity, including volcanic activity, magmatic-driven tectonism, impact cratering in icy terrains, hydrous alteration of minerals and typical hydrothermal mineralogies (Schulze-Makuch et al., 2007). Hydrothermal activity in Martian craters is therefore the subject of modelling efforts (Barnhart et al., 2009; Abramov, 2009). Many planetary scientists have specifically advocated search of hydrothermal deposits in Martian impact craters for evidence of past or present life (Farmer, 2000; Reysenbach and Cady, 2001; Rathbun and Squyres, 2002; Schulze- Makuch et al., 2007). Therefore, it is important to predict the likely distribution of hydrothermal deposits within craters. A critical aspect of this is whether fluids move by diffusion or localized channelling, which relates to whether heat loss from the cooling crater-fill rock is achieved by conduction or convection (Sanford, 2005). The move- ment of heat and mass (fluid) is in turn related to permeability. Permeability is a fundamental characteristic of rocks and sedi- ments that controls fluid flow and the rate at which it occurs. In a planetary context, such as on Mars, it is of interest for a range of reasons, including studies of fluid seepage at the surface, penetration of oxidants beneath the surface, diagenesis of sediments, and fluid movement driven by heating (magmatism, impacts). Several workers have incorporated permeability into models for the circulation of hydrothermal fluids within impact craters (Abramov and Kring, 2004; Sanford, 2005; Versh et al., 2006; Abramov and Kring, 2007). In most cases, the permeability values used have been necessarily estimates given the uncertainty over what real values might be, or ranges of values to test model sensitivity to this parameter. Some workers have made a tacit assumption that the permeability of impact breccias is likely to be high. However, a single data base of values for suevites in the Chicxulub crater is in the range 0.001 mD to 1 mD (10 -18 to 10 -15 m 2 )(Mayr et al., 2008). A useful frame of reference for interpreting these values is that permeabilities of 1 mD and 0.1 mD are the minimums required for production from oil and gas reservoirs respectively (Law et al., 2001). In fact most oil reservoirs that do not require stimulation to produce their oil have permeabilities above 100 mD. Values of less than 1 mD are, therefore, low and not likely to support the flow of water. On the contrary they Journal of Geochemical Exploration 106 (2010) 171–175 ⁎ Corresponding author. Tel.: +44 1224 273464; fax: +44 1224 272785. E-mail address: J.Parnell@abdn.ac.uk (J. Parnell). 0375-6742/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2009.12.002 Contents lists available at ScienceDirect Journal of Geochemical Exploration journal homepage: www.elsevier.com/locate/jgeoexp