Qingshan sanidine (QSs): A new mesozoic 40Ar/39Ar dating standard tied to the M0r (Barremian–Aptian boundary)

Abstract

Accurate 40Ar/39Ar dating requires a standard (neutron fluence monitor) of an appropriate age to minimize the range of isotopic ratios to be measured between standard and unknown samples. Here we present a new Cretaceous standard, i.e., Qingshan sanidine (QSs), from the Jiaolai Basin in Shandong Province, China. In situ major element analysis revealed high and consistent K contents within QSs grains. Overall, we performed 39 single grain total fusion and 3 single grain step-heating experiments of QSs. The flat age spectra and average F values (40Ar*/39ArK, with a relative standard deviation of 0.24 %) demonstrated the reproducibility of QSs and its suitability as a reliable 40Ar/39Ar standard. We obtained a weighted mean age of 121.71 ± 0.29 Ma [0.24 %, mean squared weighted deviation (MSWD) = 1.5, n = 35 of 39, 2σ] for QSs by the recently revised decay constant values (5.531 ± 0.0135 × 10−10 yr−1) proposed by Renne et al. (2011). Therefore, QSs is recommended as a high quality Cretaceous 40Ar/39Ar standard. We also calculated RFCsQSs = 4.4631 ± 0.0286 (1σ), RGA1550QSs = 1.2448 ± 0.0081 (1σ), RACsQSs = 104.4647 ± 0.6138 (1σ), and RHB3grQSs = 0.0868 ± 0.0037 (1σ) that allowing direct comparison between QSs and any other standards. In addition, U–Pb analyses by the secondary ion mass spectrometry (SIMS) method of carefully selected single zircons from the same rhyolite produced a concordia age of 121.6 ± 1.4 Ma (MSWD = 1.9, n = 12), which is consistent to the 40Ar/39Ar age. Advantages of this QSs standard include: 1) Sanidine is an ideal standard for high precision dating; 2) Compared with the existing young sanidine standards, QSs filled the scarcity of sanidine standards before the Cenozoic and can be used to better calibrate the unknown sample from the Mesozoic to the Paleozoic; 4) A sufficient quantity of QSs is available for use and is freely available to the 40Ar/39Ar community. In addition, this standard provides time constraints of 121.71 ± 0.29 Ma for the onset of the magnetic polarity chron M0r and the associated Barremian–Aptian boundary (BAB).