Figure 3. ⁴⁰Ar/³⁹Ar ages and (³⁷Ar_Ca/³⁹Ar_K) ratio spectra of plagioclase 83R77-82 and 85R18-23. The apparent age error bar for each temperature step is at the l level. The composition of the plagioclases measured by the microprobe is shown on the ³⁷Ar_Ca/³⁹Ar_K spectrum: the Ca/K ratio measured with the microprobe has been converted in ³⁷Ar_Ca/³⁹Ar_K ratio by the relationship (K/Ca) = 0.539 × (³⁹Ar_K/³⁷Ar_Ca).