Fractionation of Amphibole as an Endogenic Oxidizer in Continental Arc Magmas

Abstract Arc magmas are major contributors to continental crust formation and are more oxidized compared to mid‐ocean ridge basalts, promoting several proposed explanations. Amphibole, which is stable across a broad range of arc conditions, remains central to ongoing debate about its role on controlling arc magma oxidation. Here we report Fe isotopic compositions of whole rocks and amphibole separates from a well‐characterized amphibole‐dominated non‐cumulate intrusive suite and genetically related hornblendite cumulates in the eastern Gangdese arc, Tibet. Amphibole separates are isotopically light compared to their parental whole rocks (δ 56 Fe = 0.05–0.08‰ vs. 0.08–0.09‰) as well as more evolved rocks (δ 56 Fe = 0.06–0.17‰). The δ 56 Fe values of non‐cumulate rocks increase with magma differentiation (e.g., Mg# and Fe 2 O 3 T ), while the hornblendite cumulates exhibit a distinct trend. This suggests that fractional crystallization of amphibole, which preferentially incorporates light Fe isotopes, drives the residual melt toward higher δ 56 Fe values. The positive correlation between δ 56 Fe values and Fe 3+ /ΣFe ratios in the non‐cumulate suite further supports melt oxidation driven by amphibole removal. Our study demonstrates that amphibole fractionation results in heavy Fe isotopic compositions and contributes to arc magma oxidation, emphasizing the critical role of amphibole in continental crust formation.