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 homblende 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 la-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 homblende 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-B I a more precise value of 24.18 +/- 0.09 Ma. (C) 2007 Elsevier B.V. All rights reserved.