The impact of biological processes on the alteration of volcanic rocks has been studied over the last decade. Recent studies of volcanic glass in massive and pillow basalts from the oceanic crust and ophiolites have suggested the importance of microbes in the alteration process. Alteration textures of putative biological origin include granular and tubular morphologies that form during glass dissolution by microbes and subsequent precipitation of amorphous material (e.g. Furnes et al. 2007).
Here, we present observations suggesting widespread past microbial activity in vesicles of Devonian pillow basalts from Germany. Mineralized filaments within the vesicles of pillow basalts from Thuringia and Bavaria exactly mimic filaments recognized in pillow basalt from the Rheinisches Schiefergebirge in Hesse (cf. Peckmann et al. 2008), which have been interpreted to represent fossilized microorganisms. The filaments mineralized by clay minerals are found in carbonate amygdules (vesicles filled by carbonate cement) in the volcanic rock, where they started to form on the internal surface of the once water-filled vesicles. Biogenicity of the filaments is inferred from their size and shape resembling modern microorganisms including a constant diameter along the length of curved filaments. The filaments are independent of crystal faces or cleavage planes, show branching patterns reminiscent of modern microorganisms, and preferentially occur close to the margin of pillows. The most common type of filament in the Devonian basalts is up to 100 μm in length and between 3 to 8 μm in width. The authigenesis of clay minerals surrounding and replacing the filaments is similar to the encrustation of prokaryotes in modern iron-rich environments. Based on their occurrence in formerly water-filled vesicles, the basalt-hosted filaments represent cryptoendoliths. Our new findings suggest that cryptoendolithic activity was once widespread in basalt. Future work will show if this niche of life still persists today.
References Furnes H, Banerjee NR, Staudigel H, Muehlenbachs K, McLoughlin N, de Witt M, Van Kranendonk M (2007) Comparing petrographic signatures of bioalteration in recent to Mesoarchean pillow lavas. Precambrian Research 158, 156 - 176 Peckmann J, Bach W, Behrens K, Reitner J (2008) Putative crytoendolithic life in Devonian pillow basalt, Rheinisches Schiefergebirge, Germany. Geobiology 6, 125-135