evidence for mixing between fluids exsolved at different depths in the magmatic system of Mt Etna (Italy). Geochimica et Cosmochimica Acta, Elsevier, 2012Elsevier, , 84, pp.380-394. <10.1016Elsevier, /j.gca.2012.01.028>. 3 1 GEOCHEMICAL EVIDENCE FOR MIXING BETWEEN FLUIDS EXSOLVED AT DIFFERENT DEPTHS IN THE MAGMATIC SYSTEM OF MT ETNA (ITALY)A

Five gas discharges in the area of Mount Etna volcano (Italy) and in the near Hyblean plateau have been monitored since 1996. All the emissions displayed low contributions from crustal fluids, whereas magmatic gases were the main component. Selective dissolution of these gases into hydrothermal aquifers has been recognized and modeled, allowing us to calculate the original composition of the magma‐released gases. The inferred composition of the magmatic gases exhibits synchronous variations of He/Ne and He/CO2 ratios, which are coherent with the magma degassing process. On the basis of numerical simulations of volatile degassing from Etnean basalts we have computed the initial and final pressures of the magma batches feeding the emissions. We thus can define the levels of the Etna plumbing system where magmas are stored. Pressure values were around 360 and 160 MPa for initial and final stages, respectively, meaning related depths of about 10 and 3 km below sea level, matching those obtained by geophysical investigations for the deep and shallow magma reservoirs. In addition, we have been able to recognize episodes of magma migration from the deeper reservoir toward the shallow one. An important magma injection into the shallow storage volume was detected during the onset of the 2001 eruption (17 July). No further injection had taken place during this period until September 2001, providing a possible reason for the quick exhaustion of the eruption. In view of this we suggest that the sampled emissions are a powerful geochemical tool to investigate the Etna's plumbing system and its magma dynamics, as well as the development of eruptive events.

We present unprecedented data of real-time measurements of the concentration and isotope composition of CO 2 in air and in fumarole-plume gases collected in 2013 during two campaigns at Mount Etna volcano, which were made using a laser-based isotope ratio infrared spectrometer. We performed approximately 360 measurements/h, which allowed calculation of the δ 13 C values of volcanic CO 2 . The fumarole gases of Torre del Filosofo (2900 m above sea level) range from À3.24 ± 0.06‰ to À3.71 ± 0.09‰, comparable to isotope ratio mass spectrometry (IRMS) measurements of discrete samples collected on the same dates. Plume gases sampled more than 1 km from the craters show a δ 13 C = À2.2 ± 0.4‰, in agreement with the crater fumarole gases analyzed by IRMS. Measurements performed along~17 km driving track from Catania to Mount Etna show more negative δ 13 C values when passing through populated centers due to anthropogenic-derived CO 2 inputs (e.g., car exhaust). The reported results demonstrate that this technique may represent an important advancement for volcanic and environmental monitoring.

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Five years of gas monitoring from selected sites suggest that Mt Etna's plumbing system is much more extensive than previously reported. It extends at least 40 km SW from the volcano's boundary along the NE‐SW regional fault, where it discharges about 200 tons/day of gas, containing helium with mantle‐type isotopic composition. Synchronous variations of 3He/4He isotopic ratios in gas sampled at sites located 60 kilometers apart have allowed us to detect pulses of ascending magma in the plumbing system, thus providing a powerful tool for eruption forecasting. Following summer 2001 eruption, the still increasing trend of the 3He/4He ratios indicates that magma storage is even now occurring at a shallow depth. Hence, the volcano maintains a high capacity to re‐erupt within the next few months.

In 2011-2012, Santorini was characterized by seismic-geodetic-geochemical unrest, which was unprecedented since the most-recent eruption occurred in 1950 and led to fear an eruption was imminent. This unrest offered a chance for investigating the processes leading to volcanic reactivation and the compositional characteristics of involved magma. We have thus analyzed the He-Ne-Ar-isotope composition of fluid inclusions in olivines and clinopyroxenes from cumulate mafic enclaves hosted in cogenetic dacitic lavas of the 1570-1573 and 1925-1928 eruptions of Nea Kameni. These unique data on Aegean volcanism were compared with those of gases collected in quiescent periods and during the unrest. The 3 He/ 4 He ratios (3.1-4.0 Ra) are significantly lower than the typical arc-volcano values (R/Ra $ 7-8), suggesting the occurrence of magma contamination in Santorini plumbing system, which would further modify the 3 He/ 4 He ratio of parental magmas generated in the local metasomatized mantle. The 3 He/ 4 He values of enclaves (3.1-3.6 Ra) are comparable to those measured in gases during quiescent periods, confirming that enclaves reflect the He-isotope signature of magma residing at shallow depths and feeding passive degassing. A significant increase in soil CO 2 flux from Nea Kameni and anomalous compositional variations in the fumaroles were identified during the unrest, accordingly with previous studies. Simultaneously, 3 He/ 4 He ratios up to 4.0 Ra were also measured, demonstrating that the unrest was due to the intrusion into the shallow plumbing system of a more-primitive 3 He-rich magma, which is even volatile richer and less contaminated than mafic magma erupted as enclaves. This new intrusion did not however trigger an eruption.

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Submarine volcanism represents ~80% of the volcanic activity on Earth and is an important source of mantle-derived gases. These gases are of basic importance for the comprehension of mantle characteristics in areas where subaerial volcanism is missing or strongly modified by the presence of crustal/atmospheric components. Though, the study of submarine volcanism remains a challenge due to their hazardousness and sea-depth. Here, we report 3He/4He measurements in CO2–dominated gases discharged at 500 m below sea level from the high-temperature (~220 °C) hydrothermal system of the Kolumbo submarine volcano (Greece), located 7 km northeast off Santorini Island in the central part of the Hellenic Volcanic Arc (HVA). We highlight that the mantle below Kolumbo and Santorini has a 3He/4He signature of at least 7.0 Ra (being Ra the 3He/4He ratio of atmospheric He equal to 1.39×10−6), 3 Ra units higher than actually known for gases-rocks from Santorini. This ratio is also the highest measured across the HVA and is indicative of the direct degassing of a Mid-Ocean-Ridge-Basalts (MORB)-like mantle through lithospheric faults. We finally highlight that the degassing of high-temperature fluids with a MORB-like 3He/4He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano.