This study presents an approach that combines equilibrium thermodynamic modelling with oxygen isotope fractionation modelling for investigating fluid–rock interaction in metamorphic systems. An application to subduction zones shows that chemical and isotopic zoning in minerals can be used to determine feasible fluid sources and the conditions of interaction. Slab-derived fluids can cause oxygen isotope variations in the mantle wedge that may result in anomalous isotopic signatures of arc lavas.