“…We note that our 4‐layer slab structure is most representative of fast‐spreading ''pacific‐type'' subduction settings. Atlantic‐type environments that lack a continuous mafic layer are relevant as well (Agard, 2021) and they would include highly contrasted sediment/serpentinite interfaces (e.g., Galvez et al., 2013; Malvoisin et al., 2012; Scambelluri et al., 2022) which are not represented here. Interestingly, the first fluids produced in the altered ultramafic and mafic layers may be rich in both K and Mg, ie.…”