Preprints
https://doi.org/10.5194/bg-2021-332
https://doi.org/10.5194/bg-2021-332

  21 Dec 2021

21 Dec 2021

Review status: this preprint is currently under review for the journal BG.

Fossilization of Precambrian microfossils in the Volyn pegmatite, Ukraine

Gerhard Franz1, Peter Lyckberg2, Vladimir Khomenko3, Vsevelod Chernousenko4, Hans-Martin Schulz5, Nicolaj Mahlstedt5, Richard Wirth5, Johannes Glodny5, Ulrich Gernert6, and Jörg Nissen6 Gerhard Franz et al.
  • 1Institut für Angewandte Geowissenschaften, Technische Universität Berlin, D-10587 Berlin, Germany
  • 2Luxembourg National Museum of Natural History, 25 Rue Münster, 2160 Luxembourg, Luxembourg
  • 3The National Academy of Sciences of Ukraine, M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation, 34, Palladina av., Kyiv, 03142, Ukraine
  • 4Volyn Quartz Samotsvety Company, Khoroshiv (Volodarsk-Volynski), Ukraine
  • 5GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany
  • 6Zentraleinrichtung Elektronenmikroskopie, Technische Universität Berlin, D-10623 Berlin, Germany

Abstract. We report on Precambrian soft-tissue microfossils from igneous rocks of the Volyn pegmatite district, associated with the Paleoproterozoic Korosten Pluton, north-western Ukraine. The fossils were recovered from m-sized miarolitic cavities and show a well-preserved 3D morphology, mostly fibrous, but with a large variety of fiber types, and also in irregular, flaky shapes reminiscent of former biofilms, and rare spherical objects. Based on literature data, own pyrolysis experiments and reflected light microscopy results, the organic matter (OM) is characterized as (oxy)kerite. Further investigations with microscopic techniques, including scanning and transmission electron microscopy, and electron microprobe analysis show that fossilization likely occurred during a hydrothermal, post-pegmatitic event, by silicification dominantly in the outermost 1–2 µm of the microfossils. The hydrothermal fluid, derived from the pegmatitic environment, was enriched in SiF4, Al, Ca, Na, K, Cl, and S. The OM shows O enrichment where N and S content is low, indicating simultaneous N and S loss during anaerobic oxidation. Mineralization with Al-silicates starts at the rim of the microfossils, continues in its outer parts into identifiable encrustations and intergrowths of clay minerals, feldspar, Ca-sulfate, Ca-phosphate, Fe-sulfide, and fluorite.

Breccias, formed during collapse of some the miarolitic cavities, contain also decaying OM, which released high concentrations of dissolved NH4+, responsible for the late-stage formation of buddingtonite and tobelite-rich muscovite. The age of the fossils can be restricted to the time between the pegmatite formation, at ~1.760 Ga, and the breccia formation at ~ 1.49 Ga. As geological environment for growth of the microorganisms and fossilization we assume a geyser system, in which the essential biological components C, N, S, and P for growth of the orgabisms n the miarolitic caves were derived from microorganisms at the surface. Fossilization was induced by magmatic SiF4-rich fluids. The Volyn occurrence is a prime example of Precambrian fossils and the results underline the importance of cavities in granitic rocks as a possible habitat for microorganisms of the deep biosphere.

Gerhard Franz et al.

Status: open (until 01 Feb 2022)

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Gerhard Franz et al.

Gerhard Franz et al.

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Short summary
In pegmatites from Ukraine Precambrian fossils between 1.5 Ga and 1.76 Ga old were preserved in cavities, connected to the surface in a geyser system. The fossilization process is silicification of the outermost rim of the fossils, stabilizing the remaining part of the organisms. The variety of organisms points to an ecosystem of several microorganisms, which was active in the continental environment, and igneous rocks such as the pegmatites seem to be an ideal habitat for the deep biosphere.
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