Garnet-filled trails associated with carbonaceous matter mimicking microbial filaments in Archean basalt

Authors

  • K. LEPOT,

    1. Département de Géologie, UR Paléobotanique, Paléopalynologie et Micropaléontologie, Université de Liège, Liège, Belgium
    2. Laboratoire Géobiosphère Actuelle et Primitive, Institut de Physique du Globe de Paris – Institut de Minéralogie et de Physique des Milieux Condensés, CNRS, Universités Paris 6 and 7, Paris, France
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  • P. PHILIPPOT,

    1. Laboratoire Géobiosphère Actuelle et Primitive, Institut de Physique du Globe de Paris – Institut de Minéralogie et de Physique des Milieux Condensés, CNRS, Universités Paris 6 and 7, Paris, France
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  • K. BENZERARA,

    1. Laboratoire Géobiosphère Actuelle et Primitive, Institut de Physique du Globe de Paris – Institut de Minéralogie et de Physique des Milieux Condensés, CNRS, Universités Paris 6 and 7, Paris, France
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  • G.-Y. WANG

    1. Institut de Chimie et des Matériaux Paris-Est, CNRS, Université Paris 12, Thiais, France
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K. Lepot. Tel.: +32 4366 5349; fax: +32 4366 5338; e-mail: kevin.lepot@ulg.ac.be

Abstract

The study of the earliest traces of life on Earth can be complicated by abiotically formed biomorphs. We report here the finding of clustered micrometer-sized filaments of iron- and calcium-rich garnets associated with carbonaceous matter in an agate amygdale from a 2.7-billion-year-old basalt of the Maddina Formation, Western Australia. The distribution of carbonaceous matter and the mineral phases composing the filaments were analyzed using a combination of confocal laser scanning microscopy, laser-Raman micro-spectroscopy, focused ion beam sectioning and transmission electron microscopy. The results allow consideration of possible biogenic and abiotic processes that produced the filamentous structures. The filaments have a range of sizes, morphologies and distributions similar to those of certain modern iron-mineralized filamentous bacteria and some ancient filamentous structures interpreted as microfossils. They also share a high morphological similarity with tubular structures produced by microbial boring activity. However, the microstructures and the distribution of carbonaceous matter are more suggestive of an abiotic origin for the filaments. They are characteristic features of trails produced by the displacement of inclusions associated with local dissolution of their silica matrix. Organic compounds found in kerogen or bitumen inclusions may have contributed significantly to the dissolution of the quartz (or silica gel) matrix driving filamentous growth. Discriminating the products of such abiotic organic-mediated processes from filamentous microfossils or microbial borings is important to the interpretation of the scarce Precambrian fossil record and requires investigation down to the nanoscale.

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