Geobiology (2009), 7, 50–65 DOI: 10.1111/j.1472-4669.2008.00184.x © 2008 The Authors 50 Journal compilation © 2008 Blackwell Publishing Ltd Blackwell Publishing Ltd ORIGINAL ARTICLE Terrestrial volcanic glass alteration Alteration textures in terrestrial volcanic glass and the associated bacterial community C. S. COCKELL, 1 K. OLSSON-FRANCIS, 1 A. HERRERA 1 AND A. MEUNIER 2 1 Geomicrobiology Research Group, PSSRI, Open University, Milton Keynes, MK7 6AA, UK 2 HydrASA INSU-CNRS, University of Poitiers, 40 Avenue Recteur Pineau, 86022 Poitiers Cedex, France ABSTRACT Alteration textures were examined in subglacial (hyaloclastite) deposits at Valafell, Southern Iceland. Pitted and ‘elongate’ alteration features are observed in the glass similar to granular and tubular features reported previously in deep-ocean basaltic glasses, but elongate features generally did not have a length to width ratio greater than five. Elongate features were found in only 7% of surfaces. Crystalline basalt clasts, which are incorporated into the hyaloclastite, did not display elongate structures. Pitted alteration features were poorly defined in crystalline basalt, comprising only 4% of the surface compared to 47% in the case of basaltic glass. Examination of silica-rich glass (obsidian) and rhyolite similarly showed poorly defined pitted textures that comprised less than 15% of the surface and no elongate features were observed. These data highlight the differences in alteration textures between terrestrial basaltic glass and previously studied deep-ocean and sub- surface basaltic glass, and the important role of mineralogy in controlling the type and abundance of alteration features. The hyaloclastite contains a diverse and abundant bacterial population, as determined by 16S rDNA analysis, which could be involved in weathering the glass. Despite the presence of phototrophs, we show that they were not involved in the production of most alteration textures in the basaltic glass materials we examined. Received 11 October 2008; accepted 23 November 2008 Corresponding author: Charles S. Cockell, Tel.: +44 (0)1908 652588; fax: +44 (0)1908 858022; e-mail: c.s.cockell@open.ac.uk INTRODUCTION Microbially mediated alteration textures in minerals have now been described in a variety of deep-ocean and terrestrial settings (e.g. Thorseth et al., 1992, 1995, 2003; Fisk et al., 1998; Staudigel et al., 1998; Torsvik et al., 1998; Furnes & Staudigel, 1999; Etienne & Dupont, 2002; Banerjee & Muehlenbachs, 2003; Storrie-Lombardi & Fisk, 2004; Furnes et al., 2005, 2007; Staudigel et al., 2006, 2008; Walton, 2008). The best-studied alteration textures are in deep-ocean and subsurface basaltic glass, which contributes substantially to nutrient availability in the oceans (Staudigel & Hart, 1983; Staudigel et al., 1995) on account of the approximately one billion cubic kilometres of the material that is produced each year (Morgan & Spera, 2001). It has also been shown that microbial etching of glass occurs in non-volcanic settings (Drewello & Weissman, 1997). Some of the most conspicuous features attributed to microbial action are tubular or channel-like structures that are developed perpendicular to alteration surfaces in cracks and fractures within glass. These weathering textures have been used to assess the possible biogenicity of tunnel-like structures in ancient rocks on Earth (Furnes et al., 2004; Banerjee et al., 2006, 2007), and they might even be sought in Martian rocks (Fisk et al., 2006; McLoughlin et al., 2007). The organisms responsible for producing weathering fea- tures in volcanic materials are not known. Thorseth et al. (1992) postulated the role of photosynthetic microorganisms, specifically cyanobacteria, in the formation of elongate features in exposed terrestrial glasses. They proposed that high pH generated around photosynthesizing cells would facilitate rock leaching. The observation of elongate features only in the upper 6 –7 mm of the material they studied was explained as corresponding to the zone of light availability. The ubiquity of different types of weathering textures in the terrestrial environment has not been examined in materials other than basaltic glasses. Different volcanic mineralogical end-members exhibit different chemical weathering behaviours on account of their different crystallinities or silica compositions (Kirkman & McHardy, 1980; Wolff-Boenisch et al., 2004, 2006). Chemical weathering has previously been shown to be slower in crystalline rocks compared to glass, particularly when