ZMT04 muscovite: a potential Paleoproterozoic reference material for 40Ar/39Ar dating

ZMT04 muscovite serves as a quality control reference material for K-Ar dating, with a recommended age of 1804 +/- 21 Ma (2 sigma). Here, we check the homogeneity for ZMT04 muscovite and compare its age against the reference material Fish Canyon sanidine (FCs). Five sub-samples of ZMT04 muscovite were irradiated in three different reactors with different irradiation doses, analysed at varying sample masses over different time periods, and subjected to different mass spectrometric measurement procedures in two independent laboratories, which yielded consistent plateau ages with a mean of 1772.2 +/- 2.7 Ma (2 sigma, MSWD = 1.30, and P = 0.18) and R-FCs=28.294 +/- 0.072Ma(ZMT04) = 105.53 +/- 0.25 (2sd, MSWD = 0.88, and P = 0.57). The single-grain results are consistent with the multi-grain stepwise heating results, indicating good reproducibility and homogeneity of Ar-40* and K at the single-grain level. A single grain is sufficient to yield a high precision result when the J-value >0.01. We recommend 1 mg as the minimum sample mass to yield enough neutron-induced Ar-39 signals for high-precision analysis (J-value <0.001). Accordingly, ZMT04 muscovite can represent a reliable Ar-40/Ar-39 dating and K-Ar dating reference material for Paleoproterozoic geological samples.