Radiometric age dating of black opal (Volyn, Ukraine)

Opal (SiO₂ • nH₂O) is one of the few non-crystalline minerals. Due to its amorphous nature, high porosity, arrangement in spheres of silica nanograins, and elevated H₂O content, it poses significant challenges for radiometric dating. Consequently, opal has rarely been used as material for dating purposes, although it occurs relative widespread in both sedimentary and hydrothermal environments. Here we present results of U-Pb dating of black opal (pigmented by organic matter), which occurs in miarolitic pockets of pegmatitic rocks within the Precambrian Korosten Pluton (Western Ukraine). The opal formation age is of particular interest, as these pockets also host the Volyn biota (Franz et al. 2023, doi.org/10.5194/bg-20-1901-2023, see talk in session 01.10).

U-Pb dating was carried out by LA-ICP-SF-MS at KIT (Karlsruhe) on three different samples, using an ELEMENT XR (Thermo-Scientific) coupled to a 193 nm Excimer laser. Opal from a Li-mica-rich domain, with very low common Pb content, yielded a Triassic age of 201+8/−14 Ma, reflecting the minimum time of opal formation. In contrast, opal from alkali-feldspar-rich domains, which is always enriched in common Pb, yield Jurassic lower intercept ages of 168±6 Ma and 156±5 Ma, perhaps dating fluid infiltration. An altered opal crust gave a (sub)recent age, suggesting Pb-loss during a very recent hydrothermal overprint. The results show that LA-ICP-MS techniques provide a potential tool to place new constraints on the timing opal formation, fluid infiltration and alteration, and to gain new information about the late-stage hydrothermal evolution of the Eastern European platform.