San Cristóbal magma initiates outgassing between 7.4 and 16 km depths. Volatile solubility is controlled by polybaric crystal fractioning and CO2 fluxing. Raman corrections for postentrapment CO2 loss constrain deeper source. Homogeneous 3He/4He ratios indicate a common MORB-like mantle for erupted magmas. C-rich signature of San Cristóbal volcanic plume traces subducting sediment.