The temporal evolution of the redox conditions undergone by degassing magmas has petrologic implications for magmatic phase equilibria. We present new experimental data that demonstrate that a Cl-bearing magmatic volatile phase may affect the concentration and activity of ferrous iron in silicate liquid via the preferential mass transfer of Fe2+ into the volatile phase. The data are consistent with a model wherein the exsolution of a Cl-bearing volatile phase from the melt modifies the MgO/FeO ratio of the residual degassed melt, increasing the Fe3+/ΣFe ratio of the melt. Degassing-induced alteration of the melt may cryptically affect the phases that constitute the hypersolidus crystallizing assemblage. The new results indicate that a low- to moderate-salinity magmatic volatile phase may contain several weight percent iron, present as FeCl2. As such, passive degassing over geologically plausible, protracted periods has the potential to extract significant quantities of Fe2+, resulting in a residual melt depleted in FeO relative to Fe2O3. Notably, preferential mass transfer of Fe2+ from the melt to the fluid results in self-oxidation of the melt, driving the oxygen fugacity of the melt across the phase boundary that demarcates the stability fields of sulfide and sulfate phases, stabilizing sulfate at the expense of sulfide. Such a process has significant implications for the remobilization of chalcophile metals during the evolution of magmatic-hydrothermal systems.

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