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CNRS, Géosciences Environnement Toulouse, 14 Avenue Edouard Belin, Toulouse, France
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- PublicationOpen AccessGeochemistry of CO2-Rich Gases Venting From Submarine Volcanism: The Case of Kolumbo (Hellenic Volcanic Arc, Greece)(2019-04-12)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;Studies of submarine hydrothermal systems in Mediterranean Sea are limited to the southern Italian volcanism, while are totally missing in the Aegean. Here we report on the geochemistry of high-temperature fluids (up to 220°C) venting at 500 m b.s.l. from the floor of Kolumbo submarine volcano (Hellenic Volcanic Arc, Greece), which is located 7 km northeast of Santorini Island. Despite the recent unrest at Santorini, Kolumbo submarine volcano is considered more active due to a higher seismicity. Rizzo et al. (2016) investigated the He-isotope composition of gases collected from seven chimneys and showed that are dominated by CO2 (>97%), with only a small air contamination. Here we provide more-complete chemical data and isotopic compositions of CO2 and CH4, and Hg(0) concentration. We show that the gases emitted from different vents are fractionated by the partial dissolution of CO2 in water. Fractionation is also evident in the C-isotope composition (13CCO2), which varies between –0.04‰ and 1.15‰. We modelled this process to reconstruct the chemistry and 13CCO2 of intact magmatic gases before fractionation. We argue that the CO2 prior to CO2 dissolution in water had 13C ~ –0.4‰ and CO2/3He ~ 1•1010. This model reveals that the gases emitted from Kolumbo originate from a homogeneous mantle contaminated with CO2, probably due to decarbonation of subducting limestone, which is similar to other Mediterranean arc volcanoes (e.g. Stromboli, Italy). The isotopic signature of CH4 (13C ~ –18‰ and D ~ –117‰) is within a range of values typically observed for hydrothermal gases (e.g. Panarea and Campi Flegrei, Italy), which is suggestive of mixing between thermogenic and abiotic CH4. We report that the concentrations of Hg(0) in Kolumbo fluids are particularly high (~61 to 1300 ng m–3) when compared to land-based fumaroles located on Santorini and worldwide aerial volcanic emissions. This finding may represent further evidence for the high level of magmatic activity at Kolumbo. Based on the geo-indicators of temperature and pressure, we calculate that the magmatic gases equilibrate within the Kolumbo hydrothermal system at about 270°C and at a depth of ~1 km b.s.l..666 150 - PublicationRestrictedKolumbo submarine volcano (Greece): An active window into the Aegean subduction system(2016-06-17)
; ; ; ; ; ; ; ; ; ; ;Rizzo, A. L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Caracausi, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Palermo, Palermo, Italia ;Chavagnac, V.; CNRS, Géosciences Environnement Toulouse, 14 Avenue Edouard Belin, Toulouse, France ;Nomikou, P.; Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Panepistimiopolis, Zographou, Greece ;Polymenakou, P. N.; Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture, Heraklion Crete, Greece ;Mandalakis, M.; Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture, Heraklion Crete, Greece ;Kotoulas, G.; Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture, Heraklion Crete, Greece ;Magoulas, A.; Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture, Heraklion Crete, Greece ;Castillo, A.; CNRS, Géosciences Environnement Toulouse, 14 Avenue Edouard Belin, Toulouse, France ;Lampridou, D.; Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Panepistimiopolis, Zographou, Greece; ; ; ; ; ; ; ; ; 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 mantlederived volatiles with potential hazard at the Kolumbo submarine volcano.427 184