Comment on bg-2021-332

doi:10.5194/bg-2021-332-rc2

Abstract

Abstract. We report on Precambrian microfossils from igneous rocks of the Volyn pegmatite district, associated with the Paleoproterozoic Korosten pluton, northwestern Ukraine. The fossils were recovered from meter-sized miarolitic cavities and show a well-preserved 3D morphology, mostly filamentous but with a large variety of types and also in irregular, flaky shapes reminiscent of former biofilms, as well as rare spherical objects. Based on literature data, 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 1â2âÂµm 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 in which N and S content is low, indicating simultaneous N and S loss during anaerobic oxidation. Mineralization with AlÂ silicates starts at the rim of the microfossils, continuing in its outer parts into identifiable encrustations and intergrowths of clay minerals, feldspar, CaÂ sulfate, CaÂ phosphate, FeÂ sulfide, and fluorite. Breccias, formed during collapse of some the miarolitic cavities, contain decaying OM, which released high concentrations of dissolved <math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn></msub><mo>+</mo></msup></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="47abf32743cd28df9573e01430c76658"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-19-1795-2022-ie00001.svg" width="29pt" height="14pt" src="bg-19-1795-2022-ie00001.png"/></svg:svg>, responsible for the late-stage formation of tobelite-rich muscovite and buddingtonite. The age of the fossils can be restricted to the time between the pegmatite formation, at â¼1.760âGa, and the breccia formation at â¼1.49âGa. As the geological environment for the 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 organisms in the miarolitic cavities were derived from microorganisms at the surface. Fossilization was induced by magmatic SiF4-rich fluids. The Volyn occurrence is a distinct and uncommon example of Precambrian fossils, and the results underline the importance of cavities in granitic rocks as a possible habitat for microorganisms preserved in the deep biosphere.

Highlights

Precambrian soft-tissue fossils are generally rare and their morphology is generally not well preserved
Objects with spherical morphology are rare, we restrict to the fibers and flakes, which could be found in thick sections of organic matter (OM) embedded in epoxy, and available for more detailed analytical investigations
Common occurrences of (Precambrian) fossils are in sediments, especially in chert, but there is more and more evidence of OM in the pore space of igneous rocks (Ivarsson et al, 2020)

Summary

Introduction

Precambrian soft-tissue fossils are generally rare and their morphology is generally not well preserved They occur mostly in (meta)sediments, but in recent years it became evident that pores, fissures and other open spaces in igneous rocks can be a habitat for microorganisms. In miarolitic cavities in pegmatites from the Volyn pegmatite district, Ukraine, genetically associated with the Paleoproterozoic Korosten pluton of the western Ukrainian shield (Fig. 1), organic matter (OM) occurs in a conspicuous fibrous form. It is known as 'kerite' and was first described by Ginzburg et al (1987) as a result of abiogenic sublimation and polymerization of volatile hydrocarbons from the pegmatite.

Results

Discussion

Conclusion