INTERCALIBRATION OF THE SERVICIO NACIONAL DE GEOLOGÍA Y MINERÍA (SERNAGEOMIN), CHILE AND WISCAR 40AR/39AR LABORATORIES FOR QUATERNARY DATING

Accurate and precise dating of Quaternary lavas and pyroclastic flow or fall deposits is essential for understanding the evolution of active volcanoes and providing context for future eruptions and hazard assessment. The ⁴⁰Ar^/39Ar method is often employed to date many types of volcanic materials, however, dating young (<100 ka) K₂O poor samples can be challenging owing to low radiogenic ⁴⁰Ar* contents, which can be difficult to distinguish from trapped atmospheric argon. To address this challenge, a collaborative intercalibration between the UW-Madison WiscAr and SERNAGEOMIN laboratories is underway on a common set of samples with the aim of optimizing precision and accuracy. Groundmass and plagioclase samples were analyzed on a 5-Collector Noblesse ion counting mass spectrometer, whereas measurements in the SERNAGEOMIN lab were performed using an ARGUS VI spectrometer equipped with faraday detectors and one compact discrete dynode electron multiplier. Samples for the intercalibration were collected jointly from three Andean Southern Volcanic Zone volcanoes to test the capability of each lab in dating different materials. Lava flows were collected from Planchon-Peteroa volcanic complex (1.0 – 3.2 wt. % K₂O) and Calbuco Volcano (< 1.0 wt. % K₂O). Single crystals of plagioclase will be measured from the voluminous Diamante (Pudahuel) ignimbrite sourced from the Diamante Caldera. Multiple rounds of experiments include co-irradiation of samples at Oregon State University, as well as irradiations using the reactor in Chile to investigate differences in neutron fluence procedure and parameters. Although less precise than plateau ages, all isochron ages generated in the two laboratories agree at 2σ. Five of 6 co-irradiated samples from Planchon-Peteroa yield plateau ages that also show inter-lab agreement at 2σ. The low K₂O lavas from Calbuco are more challenging with only 1 out of 5 plateau ages in agreement between labs. Differences in the variability of the ³⁶Ar blank measurements between the two laboratories may explain the discrepancy. The consistency of the results between labs is promising. Additional new precise dates will significantly improve our understanding of the temporal evolution of these active volcanoes.