Gas loss from ascending magma controls the chemical and physical evolution of volcanic systems. Melt inclusions trapped in volcanic phenocrysts contain snapshots of volatile evolution, but are notoriously hard to interpret in terms of conventional degassing models because gas fluxing, diffusive re-equilibration and external sources can also contribute to the volatile budget. Here we supplement published melt inclusion volatile data from a wide range of volcanoes with new ion-microprobe data on H2O, CO2, Be, B, Li and Sc in melt inclusions from the 1980–1986 eruptions of Mount St. Helens. The new and published data demonstrate that suites of melt inclusions are typically displaced to higher CO2 contents than would be predicted from calculated
open or closed-system degassing trends. We reconcile all CO2–H2O data into a coherent framework based on the ratio of gas to melt during eruption. Interaction of shallow-stored magmas with vapours released from deeper in the magma system displaces melts to CO2-rich compositions. Our observations require that arc
magmas are significantly more CO2-rich than is estimated from melt inclusions alone because of the low probability of trapping high CO2 melts during magma ascent. We illustrate this process with calculations in the model system albite–H2O–CO2.