Fig. 3. ⁴⁰Ar/³⁹Ar thermochronometry constraints on the formation age of breccia 15498.

(A) Multi-phase, multi-domain diffusion (MP-MDD) model predictions for diffusion of radiogenic ⁴⁰Ar* experienced by a 1-cm-diameter basalt clast within 15498 resulting from breccia formation between 650 and 3300 Ma (that is, from heating to temperatures ranging between 450° and 675°C), followed by daytime heating to effective mean temperatures ranging between 25° and 56°C after 600 Ma. Observed step heating ages ±1 SD (dark gray boxes) are plotted against the cumulative release fraction of ³⁹Ar released. The 3310 ± 24–Ma age inferred from the HT release steps represents the minimum crystallization age of the basalt clast. The colored steps are model release spectra calculated using MP-MDD model parameters corresponding to breccia formation at varying times (different formation ages are indicated with different colors). The inset displays the number of model degassing steps that are within error of the sample degassing path (individual steps connected by dashed red line) in the LT release fraction (heating steps 2 to 6) for different breccia formation ages (a value of n = 5 indicates all steps fit within error of the model). (B) Reduced χ² misfit values for model release spectra shown in (A). Misfits are shown both including (black circles) and excluding (gray circles) the first degassing step of the heating experiments. Red shaded box indicates formation ages precluded by the cosmogenic exposure age (≤600 Ma).