Fig. 3.

(A) An unperturbed incremental heating spectra comprising 100% of the released ³⁹Ar. The dashed outlined plateau is a single-collector, multicrystal incremental heating experiment from Mark et al. (7) highlighting the nearly twofold greater resolution achievable per heating step by the Noblesse multicollector mass spectrometer. (B) Comparison of the integrated gas and incremental heating plateau dates. The two dates are not within 2σ analytical uncertainty for 17 of 49 crystals, illustrating the effect of compromised crystal domains excluded from the plateau age calculations. (C) Example of a concave-up age spectra reflecting contributions of excess Ar to the low and high temperature steps (gray boxes) that produce apparent ages older than the plateau (red boxes). The discordance of the plateau and integrated gas ages illustrates the potential bias to fusion analyses that can be excluded by IH-MCMS. (D) Isochron plot for the plateau steps plotted in C. The intercept within uncertainty of the atmospheric ⁴⁰Ar/³⁶Ar ratio and isochron date indistinguishable from the plateau date indicates that the excess Ar apparent in C does not affect the plateau steps.