Vapour pressures and thermodynamics of a silicate melt of BSE composition. Upper: Similar to Fig. 3, but for a melt of BSE composition (including Na₂O). The dominance of Na(g) at intermediate redox states (ΔIW-3 to ΔIW+2) is evident, which is consistent with the findings of other studies (Schaefer & Fegley 2004; Miguel et al. 2011; Wolf et al. 2023; van Buchem et al. 2023). Upper center: Concentration of oxides in the silicate melt. FeO concentration deplets under reducing conditions, as Fe is formed. This is evident in Lower center: here, the bulk system composition is shown, highlighting the conversion of FeO to Fe. Lower: Activity of Fe and Fe₂O₃ in the melt, compared to the far more dominant SiO₂ and MgO.