Conference Agenda

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 SiO₂-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 SiO₂ mineralisation 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 3000 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 ²⁰⁶Pb/²³⁸U* age of 10.03±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 U–Pb system of respectively older agate zones, made up of different SiO₂ mineralizations remain stable during formation of younger agate generations, and that agate dating by LA–ICP–MS and ID–TIMS yield identical results within uncertainties.

Agates are among the most intriguing mineralogical materials and have attracted scientific and cultural interest for more than 2,000 years. However, both historical and contemporary studies often lack precise information regarding their age. In most cases, only the age of the host rock can be determined, which is commonly interpreted as the formation age of the agates. The present study aimed to determine the crystallization age of agates from the Coyamito Range in Chihuahua, northern Mexico. For this purpose, samples from multiple deposits along the Range were analyzed using high-precision U–Pb ID-TIMS dating as well as U–Pb LA-ICP-MS measurements. The incorporation of uranium into the structure of chalcedony, where it is fixed during the crystallization process in the form of a uranyl–silicate-complex (Pan et al., 2021), provides a reliable tool for directly determining the timing of crystallization. The results obtained for the agate and quartz mineralization of the Coyamito Range show consistent ages from both analytical methods (LA-ICP-MS and ID-TIMS), which are significantly younger than those of the host rocks. These results indicate that agate formation occurred long after the formation of the Rancho El Agate andesite host rock, which crystallized at 38 Ma. Furthermore, they demonstrate that the magmatic host rock primarily provided the vesicles which are necessary for agate formation and did not directly supply the mineralizing fluids, as suggested in many other studies.

The zircon record of ancient orogenic systems constitutes a key petrochronological asset for resolving crustal processes and timescales of orogenic evolution. In long-lived orogens affected by HT-UHT metamorphism and regional anatexis, zircon commonly records multi-stage histories of growth, recrystallization and dissolution, leading to dispersed age spectra and decoupling among isotopes and trace elements. These features reflect re-equilibration along the P-T-t-D path and hinder the interpretation of single events.

We investigate these processes in the Socorro Nappe – a lower to upper crustal section of a Neoproterozoic magmatic arc remnant formed during the convergence between the São Francisco and Paranapanema paleoplates. Zircon from garnet-free, lower-crustal granulites and garnet-bearing mid- to upper-crustal migmatites exhibits variable coupling behaviour between Lu-Hf isotopes, Th/U ratios and U-Pb ages from intragrain to regional scales. In granulites, high-U, dark-CL zircon displays mottled textures, porous domains, and diffuse boundaries, consistent with dissolution-reprecipitation processes and producing age dispersions of up to ~100 m.y. Despite Pb mobility during fluid-melt interaction, Lu-Hf isotopes are only partially affected.

The absence of garnet in the granulites is consistent with high zircon-scale ¹⁷⁶Lu/¹⁷⁷Hf, while more radiogenic ¹⁷⁶Hf/¹⁷⁷Hf (t) reflect variable isotopic evolution and heterogeneous protolith sources. Integrated data indicate limited melt-related zircon growth at ca. 620 Ma in the granulites, in contrast to extensive growth at ca. 590-610 Ma in the migmatites, locally associated with fluid/melt-related high-Th domains. Zircon populations include pre-anatectic metamorphic and autocrystic domains (640-700 Ma) to inherited xenocrysts (800-950 Ma; 1.2-3.0 Ga). The latter are more prevalent at upper-crustal levels.

Sulfur isotope signatures (δ³⁴S) in sulfide minerals (e.g., pyrite, chalcopyrite, pyrrhotite) are crucial geochemical tracers and can provide information about the source of sulfur, temperature, and redox conditions during ore formation. The latest achievements in laser ablation inductively coupled plasma tandem mass spectrometry (LA-ICP-MS/MS) have made rapid, high-spatial-resolution in-situ sulfur isotope analysis possible. Recent studies have introduced an analytical approach based on LA-ICP-MS/MS that enables in-situ δ³⁴S measurements in sulfides by removing interferences on masses ³²S and ³⁴S through mass-shifting techniques (e.g., Börner et al., 2022; LaFlamme et al., 2025). In this approach, a reaction gas (e.g., O₂ or N₂O) is introduced into the reaction cell between two quadrupoles, promoting ion–molecule reactions that shift the mass of the target isotope, thereby enabling separation from isobaric interferences in the second quadrupole. Because analytical precision and accuracy are strongly influenced by differences in the ablation behaviour of minerals, this method requires the use of matrix-matched reference materials for the different sulfide phase, in order to minimize matrix-induced bias (LaFlamme et al., 2025). Given that the LA-ICP-MS/MS configuration can significantly influence analytical results, we review recent advances in in-situ sulfur isotope analysis, focusing on a setup with an Iridia 193 nm excimer laser (Teledyne Photon Machines) coupled to an iCAP TQ ICP-MS/MS instrument (Thermo Fisher Scientific).

Börner et al., 2022. https://doi.org/10.3389/feart.2022.916107

LaFlamme et al., 2025. https://doi.org/10.1016/j.chemgeo.2025.122946