An evidence-based approach to accurate interpretation of 40Ar/39Ar ages from basaltic rocks

Abstract

Since its inception in the mid-1960s, the 40Ar/39Ar dating technique has been the premier method for determining the eruption ages of basaltic rocks, providing valuable insights into a plethora of terrestrial and planetary processes. Advances in multi-collector mass spectrometry and improved sample preparation procedures are enabling ever-improving analytical precision and clearer evaluation of the isotopic disturbances that affect many basaltic samples and cause discordant 40Ar/39Ar age spectra.Here, we present 40Ar/39Ar step-heating data for multiple samples from two Quaternary basalt flows (0.8038 ± 0.0017 and 2.309 ± 0.009 Ma) of the intraplate Newer Volcanic Province, southeast Australia. A small proportion of these samples give concordant 40Ar/39Ar results, but most are variably discordant. The factors controlling these disturbances and implications for accurate age determination are examined and modelled in both step-heating spectra and inverse isochron space. We demonstrate that the proportion of radiogenic 40Ar (40Ar⁎) present in these samples strongly influences the nature of the discordance reflected in 40Ar/39Ar data. Mass-dependent fractionation appears to have a major influence on low-40Ar⁎ samples, whereas 39Ar recoil loss/redistribution effects are evident in samples with higher 40Ar⁎ proportions. The impact of mass fractionation is quantified via step-heating analyses of unirradiated basalt, whereby a ∼4% difference in 38Ar/36Ar ratios is observed between low- and high-temperature heating steps.On an inverse isochron plot (39Ar/40Ar vs 36Ar/40Ar), isotopic disturbance for groundmass samples primarily manifests as isochron rotation, leading to a negative correlation between initial 40Ar/36Ar ([40Ar/36Ar]i) values and associated 40Ar/39Ar ages. We propose a new framework for the interpretation of 40Ar/39Ar step-heating data for basaltic samples, through judicious evaluation of inverse isochron data, (40Ar/36Ar)i ratios and inverse isochron ages. Results from this study suggest that only samples exhibiting both flat 40Ar/39Ar age spectra and atmospheric (40Ar/36Ar)i ratios yield accurate eruption ages; in the case of more discordant age spectra, intermediate temperature steps with atmospheric (40Ar/36Ar)i ratios may provide the closest approximation of the eruption age.