Periodic SiO$_2$ mineralization dated by LA–ICP–MS and ID–TIMS, an example from Golobradovo agate, Rhodopes, Bulgaria

Agates from Oligocene acidic volcanic rocks in the Eastern Rhodopes (Bulgaria) were investigated by combined mineralogical and radiometric methods. Petrographic–CL-spectroscopic observations reveal distinct SiO$_2$-rich zones made up by cristobalite, celadonite, zeolite, opal-CT, chalcedony, and quartzine with U contents up to 67 μg/g. Results of laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) of nine agate zones (Z1 to Z9), mostly <250 μm across, provide evidence for four, successively younger stages of SiO$_2$ mineralization at 19.3 ± 0.3 Ma (Z1-Z3), 13.2 ± 0.7 Ma (Z4-Z6), 10.3 ± 0.3 Ma (Z7), and 6.8 ± 0.2 Ma (Z8). The age of the biggest zone Z7, up to 5000 μm across, is independently confirmed by isotope dilution–thermal ionization mass spectrometry (ID–TIMS) of six fragments, which yielded within uncertainties an identical weighted mean $^{206}$Pb/$^{238}$U* age of 10.02 ± 0.05 Ma. The results show that agate formation can occur periodically by multiple fluid-rock interaction over ca. 12 million years, significantly later than formation of surrounding volcanic host rocks. In addition, they demonstrate that the Usingle bondPb system of respectively older agate zones, made up of different SiO$_2$ mineralizations remain stable during formation of younger agate generations, and that agate dating by LA–ICP–MS and ID–TIMS yield identical ages within uncertainties.