Black opal is a rare variety of opal-CT, which is pigmented by organic matter (OM) and can therefore be considered as an example of geo-bio interaction (Gouzy et al., 2025). The locality of Volyn, Ukraine with its famous chamber pegmatites is well-known for interaction between OM and igneous rocks (Franz et al., 2017). The locality was recently renamed Khoroshiv, but because in the geological-mineralogical literature the name Volyn was introduced, we use this name here. The intrusion of the pegmatites is closely connected to the intrusion of the host rocks, granites of the southwestern part of the Korosten Pluton, and the intrusion age was determined as 1.76 Ga (Shumlyanskyy et al., 2021). OM was identified as kerite (fossilized remains of organisms; Franz et al., 2023, and references therein), and in fluid inclusions in beryl and topaz (Vozniak et al., 2012; Vozniak and Pavlyshin, 2008). Furthermore, formation of NH4-bearing feldspar (buddingtonite) and muscovite (tobelite) in breccia (identified together with the pegmatites) point to the interaction between decayed OM and the igneous minerals (Franz et al., 2017). The timing of the interaction between OM and igneous (and other) fluids is an important question (Franz et al., 2024), and therefore we also give age constraints on the formation of the black opal, which from textural arguments seems to be one of the latest mineral formations.
We give detailed information about the sample sites, the macroscopic features of the samples of different types of opal, and the analytical procedures. The description of the black opal samples is presented in images from secondary electron microscopy (SEM), back-scattered electron images (BSE) obtained with by electron microprobe (EMPA), element distribution maps obtained by µXRF (X-ray fluorescence), Fourier-transformed infrared spectroscopy (IR), and X-ray powder (XRD) characterization.
Chemical analyses were obtained by wave-length dispersive (WDS) analyses with the EMPA as well as by energy-dispersive (EDX) analyses with both the SEM and the EMPA instruments, to identify and characterize inclusions in the black opal. Trioctahedral Li-mica (polylithionite) is included by opal in one sample, and because this type of mica has not been described in detail from the Volyn peg-matites, we present the EMPA analytical data here in detail.
The presence of OM, which is known to absorb U in sufficient amounts, allows dating by the U-Pb decay system. The results of the isotopic dating with the laser-ablation sector-field inductively-coupled mass spectroscopy system (LA-SF-ICP-MS) is presented for the selected individual do-mains in three samples. The operating conditions are summarized in a separated pdf document.
Silicon is a beneficial element for many plants, and is deposited in plant tissue as amorphous bio-opal (phytoliths). The biochemical processes of uptake and precipitation induce isotope fractionation: the mass-dependent shift in the relative abundances of the stable isotopes of silicon. At the bulk scale, the silicon isotope composition reported as δ30Si span from -2 to +6 ‰. To further constrain these variations, at the scale of individual phytolith fragments we applied in situ femtosecond laser ablation multicollector inductively coupled plasma mass spectrometry (fsLA-MC-ICP-MS) to a set of 7 natural phytolith samples.
Two phytoliths samples (Norway spruce Picea abies and European beech Fagus sylvatica L.) were extracted from the organic-rich topsoil horizon (O) of two studies sites in Germany (Beerenbusch, close to village Rheinsberg and Wildmooswald, in the southern Black Forest). The other five phytolith samples (bushgrass Calamagrostis epigejos, common reed Phragmites australis, common horsetail Equisetum arvense, annual and perennial rough horsetail Equisetum hyemale) were separated from plant materials.
The individual phytolith fragments were analysed by fsLA-MC-ICP-MS and Si isotope results are reported in the δ-notation (delta) as permil deviation relative to NIST SRM610, which is isotopically indistinguishable from the reference material NBS28 (quartz NIST SRM8546 alias NBS28, δ29Si ≡ 0 and δ30Si ≡ 0). Raw data processing and background corrections were made according to the protocol described in Schuessler and von Blanckenburg (2014) that also involves application of several data rejection/acceptance criteria. Of these, the most important ones are that A) only 30/28Si and 29/28Si ratios are used for the calculation which deviate less than 3 standard deviation from the mean and B) only results which follow the mass-depended terrestrial fractionation line in a three-isotope-plot of δ29Si vs. δ30Si within analytical uncertainties and C) have a mass bias drift between the two bracketing standards of less than 0.30 ‰ in 30/28Si are accepted and reported in this study.
Detailed description of the sample origin, preparation steps, and the measurement protocol can be found in Frick, D. A.; Schuessler, J. A.; Sommer, M.; von Blanckenburg, F. (2018): Laser ablation in situ silicon stable isotope analysis of phytoliths. Geostandards and Geoanalytical Research. https://doi.org/10.1111/ggr.12243. With this supplement we aim to provide a comprehensive dataset for in situ stable silicon isotope composition of individual phytolith fragments.