WA1ms: A ∼2.61 Ga muscovite standard for 40Ar/39Ar dating

Benefits of this standard include: (1) an increased relative precision per crystal analyzed relative to other standards for Proterozoic and Archaean samples, (2) no post-irradiation waiting time due to the absence of Ca (no need to wait for 37ArCa decay), and (3) loading of standard grains that are optically different from the unknown (e.g., pyroxene or hornblende) to avoid standard-unknown mixing. WA1ms is freely available to the 40Ar/39Ar community.