The oxidizing capacity of volcanic gases relies on both the redox state of the mantle and the redox evolution of mantle-derived magmas. Arc magmas transfer abundant volatiles from the mantle and undergo extensive differentiation before degassing. But how the magmatic redox state evolves during differentiation remains enigmatic. The redox evolution of a magma depends on the mineral/melt partition coefficients of Fe3+ and Fe2+ (DFe3+ and DFe2+). However, no systematic DFe3+ and DFe2+ are reported due to the difficulties in analyzing Fe3+ of silicate minerals. Here, we conducted hydrous experiments at 900−1100 °C and 1−2 GPa and determined systematically DFe3+ and DFe2+ using a novel method. With the obtained partition coefficients, we investigated the redox evolution by performing fractional crystallization modelling. The almost invariable redox state during arc magma differentiation implies an oxidizing mantle predated the Great Oxidation Event if it was triggered by the increasing oxidizing capacity of volcanic gases.