δ13C of CO2 vs. CO2/3He plot of the gas EM35, EM61 and EM52BIS (red dots, data in Table S1).
The mixing lines between a mantle composition and the end-members of limestone and organic sediment22 are reported for comparison. The boxes represent the fields of the fumaroles of Ischia Island, Campi Flegrei and Somma-Vesvius59,60,61. BdM samples are on the mixing trend of the Campanian volcanoes. The end-member gases of the mixing line are a mantle source and the gases produced by decarbonation reaction of carbonate minerals.