Calibration of the interlaboratory ⁴⁰Ar³⁹Ar dating standard, MMhb-1

KAr data has been compiled from over 18 laboratories on the ⁴⁰Ar³⁹Ar dating standard, MMhb-1. The new recommended age for the monitor is 520.4 ± 1.7 Ma. The interlaboratory precision for K is 1.06% and for Ar is 1.89%. ⁴⁰Ar-³⁹Ar analyses of MMhb-1 have shown that the ⁴⁰Ar^{{black star} 40}K ratio is uniform at better than the 0.1% level, thus variations in the conventional determinations are a result of systematic variations and interlaboratory biases. We recommend a set of ⁴⁰Ar³⁹Ar standards be prepared and adopted by the KAr community to reduce uncertainty in intersample age comparison. Further, an organization like the U.S. National Bureau of Standards should be encouraged to "unconventionally" calibrate these monitors if the precision of which the ⁴⁰Ar³⁹Ar technique is capable is to be approached.