MULTI-COLLECTOR REVOLUTION IN 40AR/39AR GEOCHRONOLOGY

Many of the most transformative advances in geoscience resulted directly from technological innovations in mass spectrometry. Multi-collector mass spectrometers for ⁴⁰Ar/³⁹Ar geochronology, equipped with ion counting multipliers and Faraday detectors, are currently revolutionizing our ability to resolve the tempo of magmatic, tectonic, and paleobiologic processes from the Holocene to the age of the Earth. These instruments feature remarkable sensitivity as well as high mass resolving power, such that ³⁶Ar can be partially or fully distinguished from H³⁵Cl and ¹²C₃ at m/e 36. Experiments that would have never been considered feasible using traditional single collector instruments such as incremental heating of single Holocene sanidine phenocrysts and in-situ UV laser dating of Cenozoic fault rocks are now possible. In addition, the ages of key marker horizons, stage boundaries and paleomagnetic transitions spanning the entire geologic timescale are being re-assessed to improve their precision and accuracy. Because the ⁴⁰Ar/³⁹Ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age, multi-collector data has recently been obtained from several laboratories to precisely determine the ages and evaluate the homogeneity of commonly used standards. Single crystal incremental heating of the Alder Creek sanidine (ACs) standard yields plateaus with ages consistent with the weighted mean of the fusions, suggesting ACs is a viable standard. However, single crystal incremental heating of the Fish Canyon sanidine often do not yield plateaus. Single crystal total fusion dates span ~ 100 ka, which is an unacceptable level of heterogeneity. The ⁴⁰Ar/³⁹Ar community needs to develop a new set of standard materials to fully capitalize on these technological developments. The FCs results as well as those from several Quaternary to Cretaceous rhyolitic super-eruptions suggest that some crystals reflect pre-eruptive cooling to sub-solidus conditions, accumulation of radiogenic argon, and entrainment immediately prior to or during eruption. New ultra- precise ⁴⁰Ar/³⁹Ar dates of single sanidine crystals now provide leverage critical to determining an eruptive age, and thus to interpreting the range of U-Pb zircon dates commonly found in rhyolitic lavas and tuffs.