Major compositions and mode of Kohistan lower crustal cumulates

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 capabili...