MICROBIAL BIOALTERATION TEXTURES IN GREENSTONE BELT PILLOW LAVAS

Microorganisms that inhabit sub-seafloor pillow lavas etch volcanic glass creating micron sized tunnels and pits. These textural traces of life can be regarded as trace fossils that may be identified back to the Archean as some of the earliest physical traces of life [1]. Such bioalteration textures in metavolcanic glasses can be used to explore the Precambrian sub-seafloor biosphere.

Investigations of microbial bioalteration textures from non-metamorphosed volcanic glass have identified 5 trace fossil types or ichnospecies: a granular form; a simple tubular form; an annulated tubular form; a helicoidal tubular form and a branched tubular form [2]. The traces may be hollow and/or partially infilled by clays and iron-oxyhydroxides. These textural traces are associated with elemental and isotopic signatures that record microbial activity down to ≤550m beneath the modern seafloor [3].

In greenschist facies pillow lavas mineralization of the trace fossils most commonly by titanite ensures their preservation. Examples from Archean Greenstone belts include: the Barberton of S Africa [1, 4]; the Pilbara Craton of W Australia [5] and the Wutai Group of N China [6]. In-situ U/Pb dating of the titanite infilling the tubes confirms their Archean age [5]. The oldest yet described are from the 3.46Ga Hooggenoeg Complex of the Barberton Greenstone Belt S Africa. These occur as clusters of tubes ~50μm long and ~4μm wide that are overgrown by early chlorite that causes segmentation of the tubes.

The Barberton bioalteration textures are concentrated in one horizon within the upper part of the ~2700m thick Hooggenoeg pillow lava sequence beneath chert HC10. This “biomarker” zone was targeted by the Barberton Scientific Drilling Project [7]. Investigation of the drill core will allow us to further investigate controls upon the distribution, abundance and preservation of these bioalteration textures in the Archean sub-seafloor.

[1] Furnes et al. (2004) Science 304, 578; [2] McLoughlin et al. (2009) J. Geol. Soc. London 166, 159; [3] Staudigel et al. (2008) Earth Sci. Rev. 89, 156; [4] Banerjee et al. (2006) EPSL 241, 707-722; [5] Banerjee et al. (2007) Geology 35, 487; [6] McLoughlin et al. (in press). Chinese Sci. Bull.; [7] Grosch et al. (in press). Eos & Scientific Drilling.