Arc magmas, a major contributor to continental crust growth, are thought to be more oxidized than mid-ocean ridge basalts, but how arc magmas become oxidized is debated. In particular, fractionation of Fe-rich phases may change the Fe valence and thus the oxidation state of the derivative melt. Here we evaluate the redox effect of Fe-rich amphibole fractionation during arc magma differentiation. We present high precision Fe valence data determined by Mössbauer spectroscopy, and whole-rock Fe³⁺/∑Fe ratios by wet chemistry. Our results show that bulk Fe³⁺/∑Fe ratio of cumulates in mature island arc is mainly controlled by amphibole, and decreases (from 0.4 to 0.2) with decreasing Mg^# due to decreasing Fe³⁺/∑Fe ratios of amphibole (from 0.35 to 0.2). Our modeling suggests that amphibole fractionation from arc magmas with Fe³⁺/∑Fe significantly above 0.2 can lead to increased Fe³⁺/∑Fe ratios of residual melt (Fe³⁺/∑Fe from 0.5 to 0.7 with Mg^# from 0.4 to 0.25), and the redox filter capability of amphibole increases with the evolution of arc magmas. However, the formation of the Kohistan cumulates with Fe³⁺/∑Fe of 0.3-0.4 from initial parental magma with Fe³⁺/∑Fe of ~0.2 may require additional mechanism. In addition, although garnet has much lower Fe³⁺/∑Fe ratios, its ability to control the whole-rock Fe³⁺/∑Fe in evolved arc magma in mature island arc is negligible compared with that of amphibole because of the low proportions. Importantly, our findings provide a new perspective for understanding the formation of porphyry Cu deposit and the crustal evolution from the insight of internal petrological processes.

The FeO contents are determined by wet chemistry, and the mode are determined by observations.