Metallicity distributions of all clusters sampled in our six zoom-in simulations of galaxies (top). The different simulations are indicated with the same colours as in Fig. 2. The minimal value of metallicity admitted in the FIRE-2 simulations is ∼10⁻⁴ Z_⊙, which causes a stacking at low-metallicity visible on the plot. Metallicity distributions in the simulations m11d (centre) and m12i (bottom) of all stars (black line), all clusters (coloured line), massive clusters (filled area), and clusters in which merging BBHs form (hatched area). All histograms are normalised to one.