Discriminating carbon dioxide sources during volcanic unrest: The case of Campi Flegrei caldera (Italy)

Abstract

Large calderas are among the main emitters of volcanic CO2, which is mainly supplied by the deep degassing of magmatic fluids. However, other sources of non-magmatic CO2 can also occur due to the intense interaction among magmatic fluids, wide hydrothermal systems, and their host rocks. In particular, massive amounts of CO2 are released by calderas during unrest phases and have been often detected before eruptions. An accurate assessment of CO2 sources is thus fundamental to properly understand gas monitoring signals during volcanic crises. We focused on the restless Campi Flegrei caldera, in southern Italy, where CO2 fluxes at the Solfatara-Pisciarelli hydrothermal site have been progressively increasing up to 4000–5000 t/d during the ongoing unrest that started in 2005. Theoretical models of magma degassing have been able to reproduce the CO2-N2-He variations at the Solfatara fumaroles. However, a time-dependent deviation between measured and modeled N2/CO2 and He/CO2, well correlated with the temporal evolution of ground uplift and temperature of the hydrothermal system, has been observed since 2005. We show that these variations are controlled by intense physical-chemical perturbation of the hydrothermal system, which is driving the decarbonation of hydrothermal calcite stored in reservoir rocks. This process is providing large volumes of non-magmatic CO2 during the current unrest, contributing up to 20%–40% of the total fumarolic CO2.