Correlated increase in CO2 fumarolic content and diffuse emission from la Fossa crater (Vulcano, Italy): Evidence of volcanic unrest or increasing gas release from a stationary deep magma body?

The peculiarity of the quiescent La Fossa volcano is the occurrence of "crises" characterized by strong increases of fumarole T and output and by chemical changes indicative of an increasing input of magmatic fluids. Several surveys carried out during a new "crisis" began in November 2004 indicate that the total diffuse CO2 emission for the crater area increases by one order of magnitude during crises (up to 1600 ton-d -1 in December 2005). Concern exists on the possibility that these crises be related to an unrest process leading to eruption. The repetition along decades of the same gas compositional variations during crises, their temporal coincidence with increases of the local shallow seismicity, and the lack of any significant ground motion, rather suggest that they correspond to moments of increasing volatile release from a stationary magma system.