High precision multi-collector 40Ar/39Ar dating of young basalts: Mount Rouse volcano (SE Australia) revisited

Abstract

The acquisition of high precision geochronological data for young basalts, particularly low potassium examples <500 ka old, is a non-trivial task. This is due to low 40Ar radiogenic argon yields, high atmospheric argon levels and, sometimes, extraneous argon contamination. In this study we present new, high precision argon isotopic data for a basalt flow from the Mount Rouse volcano of the Newer Volcanic Province, southeast Australia. These data were generated using a new generation, multi-collector, ARGUSVI mass spectrometer. We show that the multi-collector system achieves an order of magnitude improvement in analytical precision and a concomitant improvement in our ability to detect minor isotopic disturbances, compared to ‘conventional’ single-collector mass spectrometry. We report a revised eruption age of 284.4 ± 1.8 ka (0.6%, 95% CI) for Mount Rouse, based on laser step-heating experiments of several groundmass aliquants (50–100 mg) from a single sample (180–250 μm size fraction). This result is considerably more precise than our previous estimate of 303 ± 13 ka (4.3%, 2σ), obtained using an older generation VG3600 mass spectrometer and larger (∼200 mg) sample aliquants.