New age constraints for the Volyn biota

The Volyn biota from the Precambrian Korosten pluton (Western Ukraine) represents an exceptional assemblage of (micro)fossils. Their significance lies in the extraordinary three- dimensional preservation as well as their unique mode of occurrence – found at depth of ≥100 m within miarolitic vacuoles in granitic pegmatites. Previously, we arrived at a minimum age of ~1.5 Ga for the Volyn biota, (Franz et al. doi.org/10.5194/bg-20-1901-2023), which was challenged by Popov (doi.org/10.3390/geosciences13100297). Head et al. (doi.org/10.5194/bg-21-1773-2024) doubted the fossil nature of the Volyn biota.

While confident in the classification as genuine fossils, we acknowledged that our minimum age estimate relies solely on one mineral (muscovite) and one isotope system (⁴⁰Ar/³⁹Ar; Franz et al. doi.org/10.5194/bg-21-4119-2024) and hence demands validation. Here, we present new geochronology data based on in situ Rb/Sr dating of igneous alkalifeldspar and Li-mica, post-igneous muscovite and hydrothermal buddingtonite together with in situ U/Pb dating of black opal (with organic matter). The latter acts as a cement within the pegmatites and therefore represents the latest mineral formation (see poster in session 01.21 Advances in Geochronology).

The Li-mica confirmed the previously known igneous event of ~1.7 Ga and records a hydrothermal event at ~1.5 Ga. The muscovite Rb-Sr data is in agreement with the previous ⁴⁰Ar/³⁹Ar age. Buddingtonite, which did not yield reliable ⁴⁰Ar/³⁹Ar ages, gave a reliable Rb/Sr isochron at ~520 Ma. Alkalifeldspar recorded the igneous event and several hydrothermal events at ~1.5 Ga, ~500 Ma, and ~200 Ma). Opal records the latter event and a Jurassic overprint at ~160 Ma.