Gas buffering of magma chamber contraction during persistent explosive activity at Mt. Etna volcano

Decrease of the local gravity field and ground deflation were observed at Mt. Etna through continuous measurements, during a 2-month period when more than 20 short-lasting explosive eruptions took place. Results from the joint inversion of the gravity and ground deformation data are cross-checked against the output of a numerical code providing independent geochemical insight on how the density of the magmatic liquid/gas mixture in the source reservoir varies as a function of the pressure. This cross-analysis provides a framework to explain why (i) the bulk volume reduction sensed by the ground deformation data is much lower than the volume of the erupted products and (ii) the observed gravity changes point to a strong mass decrease, that is incompatible with a pure mechanism of magma withdrawal. Contraction of the source reservoir was mostly buffered by pressure-driven exsolution and expansion of H2O and CO2, which compensated the withdrawal of magma and led to the inferred mass decrease.