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Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert

Nature Geoscience volume 2, pages 640–643 (2009)Cite this article

A Corrigendum to this article was published on 01 December 2009

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Abstract

The 3,465-million year-old Apex chert of the Pilbara Craton in Western Australia has been controversially reported to contain some of the oldest fossil evidence for life: tiny carbonaceous filaments interpreted as cyanobacteria1. Yet, on the basis of the presence of barite and native metals, it has been suggested that the chert formed from hydrothermal fluids at temperatures greater than 250 ∘C (refs 2, 3), an unlikely environment for cyanobacteria. Here we use scanning electron microscopy to assess the mineralogy and depositional setting of the chert veins surrounding the nodes that contain the microfossils. In addition to rare native metals, we find an assemblage of iron oxides and clay minerals, which we interpret to have formed during repeated pulses of hydrothermal alteration at low to medium temperatures. We also find micrometre-sized silica structures that resemble microbial exopolymers4, and textures formed by the partial dissolution of tubular minerals that look similar to fossilized microbial mats5. Other branched microstructures suggest post-depositional colonization of microcracks and fissures by microbes5. These observations are not directly applicable to the carbonaceous filaments purported to be the earliest fossil evidence of life, as we did not recover any in our sample. However, because of the observed hydrothermal and groundwater alteration, we conclude that the Apex chert is unlikely to have preserved any early forms of life.

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Figure 1: Minerals in Apex chert.
Figure 2: Biomorphic microstructures.

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Change history

  • 22 November 2009

    It has been brought to our attention that the second sentence of the last paragraph on page 641 of this Letter and the sentence beginning "The microstructures are also present" in line 10 on page 642 of this Letter could be ambiguous. Both sentences have been amended in the HTML and PDF versions of the Letter.

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Acknowledgements

We wish to thank P. Philippot for donating the Apex chert sample PI-02-07. Discussions with V. Altermann, N. Grassineau, T. Kakegawa, S. Kesler, R. Maranger, M. Van Kranendonk, J. Valley and Y. Watanabe were greatly appreciated. The project was supported by NSERC Discovery Grant grant no. 314496 to D.L.P. This is GEOTOP contribution 2009-0015.

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Authors and Affiliations

  1. GEOTOP and Département des Sciences de la Terre et de l’Atmosphère, Université du Québec à Montréal, CP 8888, succ. Centre-Ville, Montréal, Quebec H3C 3P8, Canada

    Daniele L. Pinti & Raymond Mineau

  2. Institut du Physique du Globe de Paris, place Jussieu - Case 89 - 75252 Paris Cedex 05, France

    Valentin Clement

Authors
  1. Daniele L. Pinti
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  2. Raymond Mineau
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  3. Valentin Clement
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Contributions

D.L.P., R.M. and V.C. contributed equally to the analyses of Apex chert. R.M. treated SEM imagery. D.L.P. wrote the manuscript.

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Correspondence to Daniele L. Pinti.

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Pinti, D., Mineau, R. & Clement, V. Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert. Nature Geosci 2, 640–643 (2009). https://doi.org/10.1038/ngeo601

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