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 experi-
mental evidence. Whereas other controls are better studied, the effect of fO2, beyond oxide stability, remains less con-
strained. 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 com-
positions 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, increas-
ing fO2 (1) stabilises Fe3+-rich spinel, leading to SiO2-richer melts and, therefore, less pronounced ASI (alumina satura-
tion 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