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Front. Earth Sci. | doi: 10.3389/feart.2019.00060

Geochemistry of CO2-rich gases venting from submarine volcanoes: the case of Kolumbo (Hellenic Volcanic Arc, Greece)

 Andrea Luca Rizzo1*,  Antonio Caracausi1,  Valérie Chavagnac2,  Paraskevi Nomikou3, Paraskevi N. Polymenakou4,  Manolis Mandalakis4,  Georgios Kotoulas4, Antonio Magoulas4, Alain Castillo2, Danai Lampridou3, Nicolas Marusczak2 and  Jeroen Sonke2
  • 1Istituto Nazionale di Geofisica e Vulcanologia, Italy
  • 2UMR5563 Géosciences Environnement Toulouse (GET), France
  • 3National and Kapodistrian University of Athens, Greece
  • 4Institute of Marine Biology, Biotechnology & Aquaculture, Hellenic Centre for Marine Research, Greece

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..

Keywords: Hellenic volcanic arc, Kolumbo submarine volcano, submarine gases, gas–water interaction, hydrothermal system, CO2, CH4

Received: 01 Aug 2018; Accepted: 12 Mar 2019.

Edited by:

Guido Giordano, Roma Tre University, Italy

Reviewed by:

Valerio Acocella, Roma Tre University, Italy
Yuri Taran, National Autonomous University of Mexico, Mexico  

Copyright: © 2019 Rizzo, Caracausi, Chavagnac, Nomikou, Polymenakou, Mandalakis, Kotoulas, Magoulas, Castillo, Lampridou, Marusczak and Sonke. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Andrea Luca Rizzo, Istituto Nazionale di Geofisica e Vulcanologia, Palermo, Italy, andrea.rizzo@ingv.it