FIGURE 4. 40Ar/39Ar age spectra and inverse isochron plots for samples CH-31 (above) and CH-32 (opposite page). Age spectra illustrate apparent age versus cumulative fraction of 39Ar released. Shaded boxes indicate steps used to calculate weighted mean plateau ages. Plotting these steps suggests that most of them fall on simple isochrons with a 40Ar/36Ar ratio of the trapped component somewhat higher than atmospheric gas, giving a more reliable estimate of the ages at ~19.5 Ma, or ~19.8 Ma when corrected for a revised age of the Fish Canyon Tuff standard.