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 + FeOtot)] (0.5 to 0.7) at multiple fO2 conditions (NNO-0.5 to NNO + 2.3), deconvolving the effects of Fe3+/Fe2+ and xMgeff [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.

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Abrajano T, Sturchio N, Bohlke J, Lyon G, Poreda R, Stevens C (1988). Methane-hydrogen gas seeps, Zambales Ophiolite, Philippines: Deep or shallow origin? Chemical Geology 71(1): 211–222. https://doi.org/10.1016/0009-2541(88)90116-7.

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