jats:titleAbstract</jats:title>jats:pMt. Etna is among the largest global volcanic outgassers with respect to carbon and sulfur, yet questions remain regarding the source of these volatiles and their systematics in the crust and mantle. The importance of heterogeneous mantle sources, mixing, crustal assimilation, and disequilibrium degassing is investigated using melt inclusions erupted during the CE 1669 eruption of Mt. Etna, Italy. We find that the melt inclusion compositions define a mixing array between two geochemically distinct melts. One end‐member melt is depleted in light rare Earth elements (LREEs) and enriched in strontium (Sr), carbon, and sulfur; the other is enriched in LREE and depleted in Sr, carbon, and sulfur. We infer, through modeling, that the melts may either have been generated by melting a mantle source that includes a recycled oceanic crustal component or they may have assimilated carbonate material in the crust. The resulting LREE‐depleted, Sr‐enriched melts were also alkali‐rich, which enhanced the solubility of carbon and sulfur. The LREE‐depleted, Sr‐ and volatile‐rich melt ascended through the crust and likely became supersaturated with respect to COjats:sub2</jats:sub> and sulfur. The melt intruded into a LREE‐enriched, relatively degassed magma body in the shallow crust, cooled rapidly, and vesiculated, likely triggering eruption. The melt inclusion array trapped by growing olivines during this intrusion process records a snapshot of incomplete mixing between the two melts. Mt. Etna is renowned for the large increases in COjats:sub2</jats:sub> gas fluxes shortly before and during eruption. The intrusion of supersaturated, COjats:sub2</jats:sub>‐enhanced magmas into shallow reservoirs may be a common process at Mt. Etna.</jats:p>