Geochemical evidence for mixing between fluids exsolved at different depths in the magmatic system of Mt Etna (Italy)

A 4-year geochemical survey of some fumaroles at the Voragine summit crater of Mt Etna was performed in combination
with synchronous monitoring of peripheral gas emissions at the base of the volcano. This was the first geochemical study at
Mt Etna to have included the abundances of Ar, He, and C isotopes. Once the effects of postmagmatic shallow processes were
identified and quantitatively removed, the He–Ar–CO2 systematics of the Voragine crater fumaroles and peripheral gas emissions
described the same degassing path. Combining the carbon-isotope composition with information about noble gases provided
evidence that the crater fumaroles are fed from a two-endmember mixture composed of a deep member coming from
pressures between 200 and 400 MPa (depending on time), and a shallower one exsolved at 130 MPa. Similar mixing processes
probably also occur in gases from peripheral vents. The simultaneous assessment of d13CCO2 and He/Ar values of crater
fumaroles over time has identified simple changes in the mixing proportion between the two endmembers and, moreover, periods
during which the exsolution pressure of the deep fluid increased. These periods seem to be linked to pre-eruptive phases of
the volcano. The identified open-system degassing processes are indicative of efficient bubble–melt decoupling at depth,
whereas the mixing process requires a convective transfer of the deeply exsolved fluids toward shallower levels of magma
where further vapor is exsolved. In agreement with the most recent geophysical and petrological data from Mt Etna, these
observations allow inferences about a deep portion of the plumbing system (5 to 12 km b.s.l.), comprising sill-like reservoirs
connected by small vertical structures, and a main reservoir at 2–3 km b.s.l. that is probably fluxed by magmatic volatiles.
2012 Elsevier Ltd. All rights reserved.