<sup>40</sup>Ar/<sup>39</sup>Ar dating of a hydrothermal pegmatitic buddingtonite–muscovite assemblage from Volyn, Ukraine

Abstract

Abstract. We determined 40Ar/39Ar ages of buddingtonite, occurring together with muscovite, with the laser-ablation method. This is the first attempt to date the NH4-feldspar buddingtonite, which is typical for sedimentary–diagenetic environments of sediments, rich in organic matter, or in hydrothermal environments, associated with volcanic geyser systems. The sample is a hydrothermal breccia, coming from the Paleoproterozoic pegmatite field of the Korosten Plutonic Complex, Volyn, Ukraine. A detailed characterization by optical methods, electron microprobe analyses, backscattered electron imaging, and IR analyses showed that the buddingtonite consists of euhedral-appearing platy crystals of tens of micrometers wide, 100 or more micrometers in length, which consist of fine-grained fibers of ≤ 1 µm thickness. The crystals are sector and growth zoned in terms of K–NH4–H3O content. The content of K allows for an age determination with the 40Ar/39Ar method, as well as in the accompanying muscovite, intimately intergrown with the buddingtonite. The determinations on muscovite yielded an age of 1491 ± 9 Ma, interpreted as the hydrothermal event forming the breccia. However, buddingtonite apparent ages yielded a range of 563 ± 14 Ma down to 383 ± 12 Ma, which are interpreted as reset ages due to Ar loss of the fibrous buddingtonite crystals during later heating. We conclude that buddingtonite is suited for 40Ar/39Ar age determinations as a supplementary method, together with other methods and minerals; however, it requires a detailed mineralogical characterization, and the ages will likely represent minimum ages.

The Volyn biota (Ukraine) – indications of 1.5 Gyr old eukaryotes in 3D preservation, a spotlight on the “boring billion”

“…In combination with textural arguments, the age determination of muscovite, formed in pseudomorphs after beryl, points to a minimum age of 1.5 Ga (Franz et al, 2022b); the maximum age is restricted by the intrusion of the igneous rocks at 1.760 Ga (Shumlyanksyy et al, 2021).…”

“…Their fossilization conditions included sudden influx of hot hydrothermal waters in the geyser system, where magmatic fluids rich in SiF 4 mixed with meteoric waters (Franz et al, 2022a), infiltration of Si-Al into the outermost layer of the fossils, and formation of dominantly clay mineral encrustations. The 1.76 Ga intrusion age of the pegmatites (U-Pb zircon; Shumlyanskyy et al, 2021) provides a maximum age of the fossils; the minimum age of 1.5 Ga is provided by the age of formation of a breccia, which contains degraded OM, brown opal with OM, buddingtonite whose NH 4 content was provided by the degraded OM, and muscovite ( 40 Ar-39 Ar laser ablation data; Franz et al, 2022b). An additional argument for this age comes from the brown opal, which cements the breccia and contains organic matter (Franz et al, 2017, and references therein).…”

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

“…Franz et al [32,48,49] researched how and when organic matter was brought to one of the Volyn rare-element pegmatites that are hosted by the Korosten granitic pluton in Ukraine. According to independent U-Pb dating of zircon and baddeleyite [50,51], the pluton was assembled between 1817 ± 15 and 1743 ± 5 Ma (the level of uncertainty was not indicated in the original sources).…”

“…The second study of Franz et al [32] was based on the same pseudomorph and sought to verify the claim about the Precambrian age of the organic matter within it by 40 Ar/ 39 Ar dating of muscovite and buddingtonite. In contrast to the earlier paper, they asserted there that all of the minerals in the pseudomorph formed simultaneously, which was supported by the image from Figure 4b, where muscovite sheets supposedly "partly enclose buddingtonite".…”

“…Relatively few previous studies of pegmatites present spatially resolved isotopic dates that are supplemented by detailed petrological characterisation of the analysed materials, and they often report significant scatter in the obtained apparent ages [24][25][26][27][28][29][30][31][32][33]. Some of these results were or can be taken as evidence for protracted, multi-stage and potentially polygenic origin of individual pegmatites, which challenges the very foundation of thinking of them as fast-forming products of the late-stage evolution of felsic magmas.…”