40AR/39AR GEOCHRONOLOGY AND GEOCHEMICAL EVOLUTION OF PLANCHON-PETEROA VOLCANIC COMPLEX

Planchon-Peteroa is a ~60 km³ Pleistocene to recent volcanic complex with several active vents on the crest of the Andes (2500-4100 masl) straddling the Chile-Argentina border at 35 ^oS. Late Pleistocene collapse of the edifice generated a 95 km long, 10 km³ debris avalanche that travelled down the Rio Teno into now populated portions of the central valley (MacPhail, 1973). Recent episodes of seismic activity, along with minor phreatic eruptions, indicate hazards that have placed this system on yellow alert status by OVDAS since 2015. The eruptive history can be subdivided into four phases, including from oldest to youngest Azufre, Planchon I, Planchon II, and Peteroa. We have collected 64 new samples on foot and with helicopter support. Initial and published XRF data shows erupted compositions ranging from predominant basaltic andesite to more rare rhyodacite. A pilot set of three Planchon I lava flows have been dated using ⁴⁰Ar/³⁹Ar methods at SERNAGEOMIN yielding ages of 52 ± 12, 71 ± 14, and 72 ± 11 ka; two Azufre lavas are 219 ± 14 and 178 ± 5 ka, suggesting that it is far younger than previously thought based on K-Ar dates. Precise ⁴⁰Ar/³⁹Ar dates from 50 samples, geochemical data, and a thorough petrological investigation will be combined to revise the geologic map and further elucidate its evolution. The 1.4 ka mingled, basalt-dacite Los Banos fall deposit indicates that mixing of contrasting magmas may play a role in the genesis and triggering of volatile-rich explosive eruptions during the late Holocene. Diffusion chronology in plagioclase will be used to explore timescales of destabilization and eruption of the modern magma reservoir.

An additional goal is to intercalibrate the ⁴⁰Ar/³⁹Ar chronometer in the WiscAR Lab and the SERNAGEOMIN Laboratorio de Geología Isotópica. A common set of Planchon-Peteroa samples and several ⁴⁰Ar/³⁹Ar standard minerals are being measured in both labs to address challenges confronting dating young K₂O-poor samples with low radiogenic argon. Face-to-face visits between collaborators in each lab are facilitating dialog essential to rooting out sources of discrepancies and identifying similarities. SERNAGEOMIN monitors many active volcanoes in Chile, thus, accurate and precise dating of Pleistocene-Holocene eruptions is essential for providing context for future eruptions.