Fig. 2: Distributions of ⁴⁰K–⁴⁰Ar system ages, measured by the K–Ar and ⁴⁰Ar–³⁹Ar methods.

Time is reported as both age (Ma, lower x axis) and time after Solar System formation (Myr_ss, upper x axis). Histograms reflect the summed distributions of ages from each dataset. K–Ar age density has a broad, shallow maximum at ~4,200 Ma, that gradually decays to low probability densities with minor local maxima extending to the present day. ⁴⁰Ar–³⁹Ar age density monotonically decreases from a sharp peak (>2× the height of the K–Ar peak) shortly after the age of the Solar System (dashed line), with minor local peaks between ~4,200 Ma and 3,600 Ma. By 3,500 Ma, the distribution converges on near-nil probability density until an approximately symmetric local maximum within the last 2,000 Ma. As these <2,000 Ma ages (shaded) are not associated with GPM (see text), we exclude them from our analyses. If we recalculate the ⁴⁰Ar–³⁹Ar age distribution with lower precision (σ = 6%) to mimic the K–Ar system, the early peak is more broad and shallow but the maximum remains >4,300 Ma.