The role of oxygen fugacity in hydrous basaltic phase equilibria: experimental constraints at 0.2 and 0.8 GPa

Crystallisation-differentiation drives arc magma evolution, yet discrepancies remain among field, geochemical and experimental evidence. Whereas other controls are better studied, the effect of fO₂, beyond oxide stability, remains less constrained. We investigate fO₂-pressure effects on olivine-clinopyroxene-spinel phase relations with implications for arc magmas. We conducted phase equilibria experiments at 200 MPa between 1010 and 1100 °C. We used basaltic compositions with different xMg* [MgO/(MgO + FeO^tot)] (0.5 to 0.7) at multiple fO₂ conditions (NNO-0.5 to NNO + 2.3), deconvolving the effects of Fe³⁺/Fe²⁺ and xMg^eff [MgO/(MgO + FeO)] on phase equilibria. Additionally, we ran 800 MPa experiments between NNO-0.4 and NNO + 2.5 to explore the combined effects of fO₂ and pressure. At 200 MPa, increasing fO₂ (1) stabilises Fe³⁺-rich spinel, leading to SiO₂-richer melts and, therefore, less pronounced ASI (alumina saturation index, ASI = Al₂O₃/(CaO + Na₂O + K₂O) molar) increase relative to SiO₂, and (2) expands olivine stability relative to clinopyroxene in ol-cpx cotectic melts, resulting in lower ASI melts (for a given SiO₂ content) that better match arc rocks. This is only observed under spinel-absent conditions. The 800 MPa experiments reveal decreasing spinel stability with increasing pressure, while fO₂ has a negligible effect on the ol-cpx cotectic. This suggests that the previously documented pressure effect on the olivine-clinopyroxene equilibrium is stronger than the effect of fO₂. Our results demonstrate that fO₂ increasingly influences the olivine-clinopyroxene cotectic equilibrium as pressure decreases. This supports models where decompression-driven polybaric crystallisation under oxidising conditions shapes arc magmatic compositions. The reported pressure-fO₂ interplay helps reconcile natural and experimental arc records.