Redox conditions in magmatic environments control phase equilibria, metal mobility, and the behavior of volatiles like sulfur. Understanding the interplay between oxygen fugacity (fO2) and magma differentiation provides insights into the roles of magmatism, ore deposit formation, and volcanic degassing in Earth's redox evolution. However, there are only a few studies on oxybarometric methods calibrated at elevated pressure and water activity, conditions typical of magma differentiation at convergent plate boundaries.

We conducted experiments (200 MPa, 1030−870 °C, logfO2: -1 to +3.5 ∆FMQ) to develop oxybarometers based on redox sensitive trace element (i.e. vanadium, V) partitioning between mafic minerals and hydrous silicate melts. The results show that V partitioning is largely independent of pressure (P), temperature (T), and silicate melt composition (X). Especially in the case of olivine-silicate melt pairs, we can demonstrate a robust and precise update on V-based oxybarometers, reproducing fO2 within a 2σ median error of one log unit over a wide range of T-P-X. Together with other oxybarometric systems involving iron-bearing minerals (clinopyroxene, orthopyroxene, amphibole, and spinel), we provide a versatile tool to track fO2 along magma differentiation from mafic to intermediate magmatic systems under water-saturated conditions.

These oxybarometers are applicable to volcanic rocks with suitable pairs of silicate melt inclusion and host mineral for reconstructing magma reservoir redox histories. This study also highlights the effect of fO2 on stable phase assemblages in mafic to intermediate magmas at subduction zones, and how this influences the liquid lines of descent of medium-K calc-alkaline basalts and shoshonites in upper crustal magma reservoirs.