Intercalibration of 40Ar–39Ar age standards NL-25, HB3gr hornblende, GA1550, SB-3, HD-B1 biotite and BMus/2 muscovite

40Ar–39Ar dating requires precise and accurate age data of standard fluence monitors. Besides primary calibration of K and 40Ar concentrations (i.e. the ages are calculated with precise K–Ar dating), intercalibration of the different monitors in use provides important cross-checks of the validity of standard ages and, implicitly, reliability and uncertainty of chronological studies that rely on these standards. Here we report intercalibration of 5 primary international 40Ar–39Ar standards and one internal laboratory monitor. A major goal of this study was to check the ages of the NL-25 hornblende standard and of the muscovite laboratory standard BMus/2 against the international standards HD-B1, GA1550, SB-3 and HB3gr. In replicate measurements of three irradiation series, 40Ar–39Ar ages for the fluence monitors were found which agree within 1σ-error limits with the reported age values of 2660 ± 9 Ma for NL-25, Hb3gr: 1073.6 ± 4.6 Ma, BMus/2: 328.5 ± 1.1 Ma, SB-3: 162.9 ± 0.8, GA1550: 98.79 ± 0.54 and 24.21 ± 0.32 for HD-B1. For the hornblende NL-25 fluence monitor a new – slightly different – 40Ar–39Ar age of 2657 ± 4 Ma is suggested. For the age of the muscovite BMus/2 laboratory standard we recommend 328.5 ± 1.1 Ma, and for biotite HD-B1 a more precise value of 24.18 ± 0.09 Ma.