INTERCALIBRATION OF 40AR/39AR MINERAL STANDARDS WITH BODIE HILLS SANIDINE

The ⁴⁰Ar/³⁹Ar dating technique measures age by comparing the ratio of radiogenic ⁴⁰Ar to K-derived ³⁹Ar (⁴⁰Ar*/³⁹Ar_K) in a neutron irradiated sample to this same ratio measured in a co-irradiated mineral standard or “monitor” of known age. Best results are obtained using precisely known monitor minerals with ages similar to those of the unknown samples. Doing so permits optimization of irradiation durations and resulting isotopic ratios for both. In choosing a monitor for ⁴⁰Ar/³⁹Ar dating of young volcanic rocks, we sought the youngest well-behaved mineral separate yielding ≥99% ⁴⁰Ar*, ultimately selecting sanidine from the 9.8-Ma Rhyolite of Bodie Hills (BHs) in eastern California. This sanidine has been intercalibrated with the ANU biotite standard, GA1550, and with sanidines of the Fish Canyon tuff (FCs), Taylor Creek Rhyolite (TCs), and Rhyolite of Alder Creek (ACs), based on 1986 analyses of 201 different packets in 12 irradiations. Although FCs has been calibrated recently with other decay schemes and minerals tied to astronomically tuned cyclostratigraphy, we have chosen to calibrate all standards to the best characterized K-Ar standard measured by first-principles, GA1550 biotite, with an age of 98.79±0.96 Ma (McDougall and Wellman, 2011, Chem. Geol., 280; Renne et al., 1998, Chem. Geol., 145). However, use of intercalibration factors (R), as described by Renne et al. (1998), permits recalculation of ages to any value for one of the standards, including values derived from external calibration to time scales considered more accurate. We obtain an age for BHs of 9.80 ± 0.09 Ma with propagation of full external errors. Without uncertainties in decay constants and GA1550, the uncertainty decreases to 0.003 Ma. Results also show ages and analytical uncertainties of 28.35 ± 0.01 Ma for TCs, 28.10 ± 0.02 Ma for FCs, and 1.181 ± 0.002 Ma for ACs. Whereas ages reported here for GA1550 and TCs are essentially the same as cited by Renne et al. (1998), that for FCs is about 80 ka older and ACs is younger by 1.1%. Using the astronomically tuned value for FCs, 28.198 Ma, the age of BHs is 9.83 Ma and ACs is 1.185 Ma. These results and those of recent interlaboratory studies suggest that ⁴⁰Ar/³⁹Ar laboratories may produce individual differences in R and should consider reporting laboratory-specific intercalibration factors.