Intercalibration of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile and WiscAr 40Ar/39Ar laboratories for Quaternary dating

Abstract

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 40 Ar/ 39 Ar method is commonly employed to date these volcanic materials, however, dating young (<150 ka) K 2 O-poor materials can be challenging owing to low radiogenic 40 Ar* contents that can be difficult to distinguish from trapped atmospheric argon. To address this challenge, a collaborative intercalibration exercise involving the University of Wisconsin-Madison WiscAr Laboratory and the 40 Ar/ 39 Ar Laboratory of the Servicio Nacional de Geología y Minería (SERNAGEOMIN), Chile was conducted on a common set of samples with the aim of refining our methods and optimizing precision and accuracy of age determinations. Groundmass and plagioclase samples were analyzed on a 5-collector Noblesse ion counting mass spectrometer in the WiscAr lab, 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 evaluate the capability of each laboratory to date different materials. Samples from lava flows with 1.0–3.2 wt % K 2 O from Planchon-Peteroa volcanic complex and with <1.0 wt % K 2 O from Calbuco Volcano that are the focus of ongoing geological studies were measured in both laboratories. Single crystals of plagioclase (0.6–1.0 wt% K 2 O) were measured from the voluminous Diamante (Pudahuel) ignimbrite sourced from the Diamante Caldera. Multiple rounds of experiments were conducted including co-irradiation of samples at Oregon State University, as well as irradiations using the CCHEN reactor in Chile to investigate differences in neutron fluence parameters. As a result, SERNAGEOMIN has modified long-used protocols for the CCHEN reactor so that Quaternary samples may be irradiated for periods of time most appropriate for their age. Although less precise than plateau ages, the isochron ages generated in the two laboratories agree at 2σ for each sample. Six of six co-irradiated samples from Planchon-Peteroa yield plateau ages that also show inter-lab agreement at 2σ. The low K 2 O lavas from Calbuco proved more challenging with only three out of five plateau ages in agreement between labs. SERNAGEOMIN blanks were higher and more variable in Calbuco experiments, thus, differences in the variability of the measured 36 Ar blanks between the two laboratories may explain the discrepancy in plateau ages. Analysis of single plagioclase crystals from the Diamante Ignimbrite show excellent agreement between labs for both weighted mean apparent ages and isochron ages. We favor an isochron age for the ignimbrite of 132.4 ± 2.2 ka, however, discrepancies in results between samples from three different outcrops present an interesting geochronologic problem that warrants further study. Overall, the consistency of the results between labs is promising. These new precise age determinations significantly improve our understanding of the temporal evolution of these active volcanoes. • Broad agreement of ages from tephras and lavas measured in two labs. • 40 Ar/ 39 Ar dating of low K 2 O lavas from Calbuco is challenging due to limited 40 Ar*. • We favor an isochron age of ∼132 ± 2 ka ka for the 270 km 3 Diamante Ignimbrite. • Care in sample preparation and irradiation, and control of blanks are essential.