Fe isotope systematics and their role in constraining front- and rear-arc processes in Iran Cenozoic magmatism: Beyond compositional and radiogenic isotopes evidence

The Cenozoic arc of Iran preserves archetypal exposures of the continental arc developed during the north-eastward subduction of the Neo-Tethys oceanic slab and records the history of flare-up magmatism within the Zagros Orogen. Here we present a new dataset of high-precision Fe–Sr–Nd isotopic compositions of Eocene–Oligocene frontal- and rear-arc (i.e., pre-collisional) magmatic products associated with subduction of the Neo-Tethys Ocean beneath Iran to provide new constraints on the fractionation of Fe isotopes and cycling of Fe within these tectonic settings. The δ56Fe values range from +0.07 ‰ to +0.15 ‰, falling within the range of arc-related rocks worldwide. The rear arc samples display higher δ56Fe (+0.135 ± 0.010 (2σ)) than samples from the coeval frontal arc (+0.090 ± 0.006 (2σ)). The low Fe3+/ƩFe values of all investigated samples in the range from 0.26 to 0.36 and the lack of negative correlation between δ56Fe and Fe3+/ƩFe argue against incorporation of any significant sulfate- and/or carbonate-bearing fluids. Based on geochemical and isotopic data, we postulate that the high-flux magmatic events (i.e., flare-ups) are related to partial melting of a garnet-free lherzolite mantle source and limited crustal assimilation. Our results also demonstrate that the Fe isotope fractionation in the Zagros orogen is primarily governed by fractional crystallization of Fe2+-rich phases (e.g., olivine, clinopyroxene), particularly in rear-arc domains where deeper crystallization promotes heavier δ56Fe signatures. Although changes in oxidation state can affect Fe isotope fractionation when iron is added or lost from the system, the relatively narrow range in Fe3+/ΣFe ratios and the absence of a systematic covariation with δ56Fe suggest that redox-driven effects were likely secondary with respect to crystal–melt partitioning in this setting. This stands in marked contrast to global arc systems, particularly oceanic arcs, where slab-derived oxidizing fluids typically dominate over magmatic differentiation, underscoring the Zagros as a case where source composition and fractional crystallization outweigh subduction-related redox processes.