40Ar/39Ar analyses on quaternary K-Ar standard BB-24: Evaluations

Quaternary K-Ar standard BB-24 is evaluated by 40Ar/39Ar analyses. Total-fusion results show that BB-24 is poorly reproducible either as 40Ar*(radiogenic 40Ar) or as 39Ar(k), at a sample size of 15-40 mg. Much lower ages produced by the purified aliquots of BB-24 show the excess 40Ar* sited in the olivine component within the standard. The step-heating experiment produces a different total gas age of 440 +/- 21.2 ka and a plateau age of 424.4 +/- 15.0 ka, indicating different components degassing at different temperatures. The first and last steps have elevated ages and distort the age spectrum. This is vital when total-fusion experiments are performed. Age spectra of purified aliquots produce concordant age spectra, implying mineral impurities or excess 40Ar within the sample. As a conclusion, BB-24 is not a suitable neutron monitor in 40Ar/39Ar dating. (c) 2007 Elsevier B.V. All rights reserved.