Re-evaluation of the ages of 40Ar/39Ar sanidine standards and supereruptions in the western U.S. using a Noblesse multi-collector mass spectrometer

Abstract

The rewards from resolving the tempo of magmatic, paleobiologic, paleoclimatic, and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision, accuracy, and calibration of several chronometers, including the 40Ar/39Ar variant of the K-Ar clock. A new generation of multi-collector mass spectrometers for 40Ar/39Ar geochronology provides the means to address these issues, as well as to evaluate the ages of 40Ar/39Ar standards and key marker horizons deposited by large volume, caldera-forming supereruptions in the western U.S. at unprecedented temporal resolution. We highlight the utility of a 5-collector Noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36Ar+H35Cl from a 36Ar+H35Cl+12C3 sum peak at m/e 36, maintains a remarkably low and stable background, and thus yields 40Ar/39Ar dates of exceptionally high precision. We have calibrated a reference gas and developed a simple analytical routine involving one peak hop, both of which are used to evaluate detector-bias correction factors. Because the 40Ar/39Ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age, precise and accurate determination of the ages of these standards, and verification of isotopic composition and homogeneity, is essential. We present results from both total fusion and incremental heating experiments on single crystals of the widely used Alder Creek (ACs) and Fish Canyon (FCs) sanidine standards. Several recent, independent, measurements of ACs using multi-collector mass spectrometers, including those presented herein, favor an age of 1.1864±0.0003/0.0012Ma (95% confidence analytical/full external uncertainty), that is >1% younger than the widely used value obtained by Nomade et al. (2005) using a single collector mass spectrometer. Incremental-heating experiments on single crystals of FCs most commonly do not yield plateau ages, instead many crystals appear to be affected by Ar loss or excess Ar. Our findings, obtained using ion counters, independently confirm those recently obtained using a multiple-Faraday detectors or a combination of Faraday and ion counting detectors. Thus, careful consideration must be given to using FCs in future high-precision 40Ar/39Ar calibrations of the geologic timescale. Notwithstanding, 40Ar/39Ar dating of sanidine using the 5- collector Noblesse can now provide leverage critical to interpreting the large range of U–Pb zircon dates commonly found in rhyolitic lavas and tuffs.