Fig. 4.

(A) Sensitivity of the isochron intercepts to ⁴⁰Ar_xs in BT sanidine. The squares are the maximum potential deviation from the atmospheric ⁴⁰Ar/³⁶Ar ratio [Δ ³⁶Ar/(⁴⁰Ar_atm + ⁴⁰Ar_xs)] of the trapped Ar component of each crystal, i.e., the upper bound of the 95% confidence interval of the isochron intercept, plotted against the ³⁹Ar-weighted average fraction of Ar_atm of the plateau steps; red squares indicate isochron intercepts insufficiently precise to resolve a ⁴⁰Ar_xs component that would produce a 3-ky difference in apparent age. Curves show the expected isochron intercepts of crystals containing ⁴⁰Ar_xs that would produce age differences of 3 ky to 12 ky. (B) Plateau date (with 2σ analytical uncertainties) vs. % Ar_atm for BT sanidines. Older plateau dates are not associated with imprecise isochron intercepts, indicating unresolved ⁴⁰Ar_xs is not their source. (C) Results of the Ar diffusion numerical model using the diffusion parameters of Wartho et al. (53). The y axis shows the apparent age that would be measured today for a crystal initially containing 10 ky to 85 ky of accumulated ⁴⁰Ar* following an interval of storage at 475 °C to 785 °C. Whereas crystals residing at less than 475 °C would accumulate ⁴⁰Ar*, the temperatures recorded by BT mineral thermometry would reset the apparent sanidine age in no more than a few centuries.