We have conducted a series of experiments to evaluate the intrinsic effects of dissolved chlorine on Fe3+/∑Fe and magnetite solubility in hydrous chloride-rich rhyodaciticliquids. The addition of Cl to the melt appears to have two prominent effects on iron in the melt: (1) dissolved Cl appears to perturb the magnetite-melt equilibrium, such that greater FeOtotal contents are required to support magnetite saturation in Cl-bearing melts than in Cl-free melts of equivalent bulk compositions; and (2) a systematic and progressive decrease of the measured Fe3+/∑Fe as fO2 is increased. These two intimately related effects each have important implications for redox processes occurring in Cl-enriched arc magmas.

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