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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system</title><author><name sortKey="Rizzo, Andrea Luca" sort="Rizzo, Andrea Luca" uniqKey="Rizzo A" first="Andrea Luca" last="Rizzo">Andrea Luca Rizzo</name><affiliation><nlm:aff id="a1"><institution>Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo</institution>,<country>Italy</country></nlm:aff></affiliation></author><author><name sortKey="Caracausi, Antonio" sort="Caracausi, Antonio" uniqKey="Caracausi A" first="Antonio" last="Caracausi">Antonio Caracausi</name><affiliation><nlm:aff id="a1"><institution>Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo</institution>,<country>Italy</country></nlm:aff></affiliation></author><author><name sortKey="Chavagnac, Valerie" sort="Chavagnac, Valerie" uniqKey="Chavagnac V" first="Valèrie" last="Chavagnac">Valèrie Chavagnac</name><affiliation><nlm:aff id="a2"><institution>CNRS, Géosciences Environnement Toulouse</institution>, 14 Avenue Edouard Belin, Toulouse,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Nomikou, Paraskevi" sort="Nomikou, Paraskevi" uniqKey="Nomikou P" first="Paraskevi" last="Nomikou">Paraskevi Nomikou</name><affiliation><nlm:aff id="a3"><institution>Department of Geology and Geoenvironment, National and Kapodistrian University of Athens</institution>, Panepistimiopolis, Zographou,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Polymenakou, Paraskevi N" sort="Polymenakou, Paraskevi N" uniqKey="Polymenakou P" first="Paraskevi N." last="Polymenakou">Paraskevi N. Polymenakou</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Mandalakis, Manolis" sort="Mandalakis, Manolis" uniqKey="Mandalakis M" first="Manolis" last="Mandalakis">Manolis Mandalakis</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Kotoulas, Georgios" sort="Kotoulas, Georgios" uniqKey="Kotoulas G" first="Georgios" last="Kotoulas">Georgios Kotoulas</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Magoulas, Antonios" sort="Magoulas, Antonios" uniqKey="Magoulas A" first="Antonios" last="Magoulas">Antonios Magoulas</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Castillo, Alain" sort="Castillo, Alain" uniqKey="Castillo A" first="Alain" last="Castillo">Alain Castillo</name><affiliation><nlm:aff id="a2"><institution>CNRS, Géosciences Environnement Toulouse</institution>, 14 Avenue Edouard Belin, Toulouse,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Lampridou, Danai" sort="Lampridou, Danai" uniqKey="Lampridou D" first="Danai" last="Lampridou">Danai Lampridou</name><affiliation><nlm:aff id="a3"><institution>Department of Geology and Geoenvironment, National and Kapodistrian University of Athens</institution>, Panepistimiopolis, Zographou,<country>Greece</country></nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">27311383</idno><idno type="pmc">4911562</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4911562</idno><idno type="RBID">PMC:4911562</idno><idno type="doi">10.1038/srep28013</idno><date when="2016">2016</date><idno type="wicri:Area/Pmc/Corpus">000003</idno><idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">000003</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system</title><author><name sortKey="Rizzo, Andrea Luca" sort="Rizzo, Andrea Luca" uniqKey="Rizzo A" first="Andrea Luca" last="Rizzo">Andrea Luca Rizzo</name><affiliation><nlm:aff id="a1"><institution>Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo</institution>,<country>Italy</country></nlm:aff></affiliation></author><author><name sortKey="Caracausi, Antonio" sort="Caracausi, Antonio" uniqKey="Caracausi A" first="Antonio" last="Caracausi">Antonio Caracausi</name><affiliation><nlm:aff id="a1"><institution>Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo</institution>,<country>Italy</country></nlm:aff></affiliation></author><author><name sortKey="Chavagnac, Valerie" sort="Chavagnac, Valerie" uniqKey="Chavagnac V" first="Valèrie" last="Chavagnac">Valèrie Chavagnac</name><affiliation><nlm:aff id="a2"><institution>CNRS, Géosciences Environnement Toulouse</institution>, 14 Avenue Edouard Belin, Toulouse,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Nomikou, Paraskevi" sort="Nomikou, Paraskevi" uniqKey="Nomikou P" first="Paraskevi" last="Nomikou">Paraskevi Nomikou</name><affiliation><nlm:aff id="a3"><institution>Department of Geology and Geoenvironment, National and Kapodistrian University of Athens</institution>, Panepistimiopolis, Zographou,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Polymenakou, Paraskevi N" sort="Polymenakou, Paraskevi N" uniqKey="Polymenakou P" first="Paraskevi N." last="Polymenakou">Paraskevi N. Polymenakou</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Mandalakis, Manolis" sort="Mandalakis, Manolis" uniqKey="Mandalakis M" first="Manolis" last="Mandalakis">Manolis Mandalakis</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Kotoulas, Georgios" sort="Kotoulas, Georgios" uniqKey="Kotoulas G" first="Georgios" last="Kotoulas">Georgios Kotoulas</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Magoulas, Antonios" sort="Magoulas, Antonios" uniqKey="Magoulas A" first="Antonios" last="Magoulas">Antonios Magoulas</name><affiliation><nlm:aff id="a4"><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></nlm:aff></affiliation></author><author><name sortKey="Castillo, Alain" sort="Castillo, Alain" uniqKey="Castillo A" first="Alain" last="Castillo">Alain Castillo</name><affiliation><nlm:aff id="a2"><institution>CNRS, Géosciences Environnement Toulouse</institution>, 14 Avenue Edouard Belin, Toulouse,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Lampridou, Danai" sort="Lampridou, Danai" uniqKey="Lampridou D" first="Danai" last="Lampridou">Danai Lampridou</name><affiliation><nlm:aff id="a3"><institution>Department of Geology and Geoenvironment, National and Kapodistrian University of Athens</institution>, Panepistimiopolis, Zographou,<country>Greece</country></nlm:aff></affiliation></author></analytic><series><title level="j">Scientific Reports</title><idno type="eISSN">2045-2322</idno><imprint><date when="2016">2016</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><p>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 <sup>3</sup>He/<sup>4</sup>He measurements in CO<sub>2</sub>–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 <sup>3</sup>He/<sup>4</sup>He signature of at least 7.0 Ra (being Ra the <sup>3</sup>He/<sup>4</sup>He ratio of atmospheric He equal to 1.39×10<sup>−6</sup>), 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 <sup>3</sup>He/<sup>4</sup>He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Crisp, J A" uniqKey="Crisp J">J. A. Crisp</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Chavagnac, V" uniqKey="Chavagnac V">V. Chavagnac</name></author><author><name sortKey="German, C R" uniqKey="German C">C. R. German</name></author><author><name sortKey="Taylor, R N" uniqKey="Taylor R">R. N. Taylor</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Brewer, P G" uniqKey="Brewer P">P. G. 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<front><journal-meta><journal-id journal-id-type="nlm-ta">Sci Rep</journal-id><journal-id journal-id-type="iso-abbrev">Sci Rep</journal-id><journal-title-group><journal-title>Scientific Reports</journal-title></journal-title-group><issn pub-type="epub">2045-2322</issn><publisher><publisher-name>Nature Publishing Group</publisher-name></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">27311383</article-id><article-id pub-id-type="pmc">4911562</article-id><article-id pub-id-type="pii">srep28013</article-id><article-id pub-id-type="doi">10.1038/srep28013</article-id><article-categories><subj-group subj-group-type="heading"><subject>Article</subject></subj-group></article-categories><title-group><article-title>Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system</article-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>Rizzo</surname><given-names>Andrea Luca</given-names></name><xref ref-type="corresp" rid="c1">a</xref><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Caracausi</surname><given-names>Antonio</given-names></name><xref ref-type="aff" rid="a1">1</xref></contrib><contrib contrib-type="author"><name><surname>Chavagnac</surname><given-names>Valèrie</given-names></name><xref ref-type="aff" rid="a2">2</xref></contrib><contrib contrib-type="author"><name><surname>Nomikou</surname><given-names>Paraskevi</given-names></name><xref ref-type="aff" rid="a3">3</xref></contrib><contrib contrib-type="author"><name><surname>Polymenakou</surname><given-names>Paraskevi N.</given-names></name><xref ref-type="aff" rid="a4">4</xref></contrib><contrib contrib-type="author"><name><surname>Mandalakis</surname><given-names>Manolis</given-names></name><xref ref-type="aff" rid="a4">4</xref></contrib><contrib contrib-type="author"><name><surname>Kotoulas</surname><given-names>Georgios</given-names></name><xref ref-type="aff" rid="a4">4</xref></contrib><contrib contrib-type="author"><name><surname>Magoulas</surname><given-names>Antonios</given-names></name><xref ref-type="aff" rid="a4">4</xref></contrib><contrib contrib-type="author"><name><surname>Castillo</surname><given-names>Alain</given-names></name><xref ref-type="aff" rid="a2">2</xref></contrib><contrib contrib-type="author"><name><surname>Lampridou</surname><given-names>Danai</given-names></name><xref ref-type="aff" rid="a3">3</xref></contrib><aff id="a1"><label>1</label><institution>Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo</institution>,<country>Italy</country></aff><aff id="a2"><label>2</label><institution>CNRS, Géosciences Environnement Toulouse</institution>, 14 Avenue Edouard Belin, Toulouse,<country>France</country></aff><aff id="a3"><label>3</label><institution>Department of Geology and Geoenvironment, National and Kapodistrian University of Athens</institution>, Panepistimiopolis, Zographou,<country>Greece</country></aff><aff id="a4"><label>4</label><institution>Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture</institution>, Heraklion Crete,<country>Greece</country></aff></contrib-group><author-notes><corresp id="c1"><label>a</label><email>andrea.rizzo@ingv.it</email></corresp></author-notes><pub-date pub-type="epub"><day>17</day><month>06</month><year>2016</year></pub-date><pub-date pub-type="collection"><year>2016</year></pub-date><volume>6</volume><elocation-id>28013</elocation-id><history><date date-type="received"><day>09</day><month>02</month><year>2016</year></date><date date-type="accepted"><day>27</day><month>05</month><year>2016</year></date></history><permissions><copyright-statement>Copyright © 2016, Macmillan Publishers Limited</copyright-statement><copyright-year>2016</copyright-year><copyright-holder>Macmillan Publishers Limited</copyright-holder><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by/4.0/"><pmc-comment>author-paid</pmc-comment>
<license-p>This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit <ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/4.0/">http://creativecommons.org/licenses/by/4.0/</ext-link></license-p></license></permissions><abstract><p>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 <sup>3</sup>He/<sup>4</sup>He measurements in CO<sub>2</sub>–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 <sup>3</sup>He/<sup>4</sup>He signature of at least 7.0 Ra (being Ra the <sup>3</sup>He/<sup>4</sup>He ratio of atmospheric He equal to 1.39×10<sup>−6</sup>), 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 <sup>3</sup>He/<sup>4</sup>He ratio corroborates a vigorous outgassing of mantle-derived volatiles with potential hazard at the Kolumbo submarine volcano.</p></abstract></article-meta></front><body><p>Most of the volcanic activity worldwide occurs in the oceans<xref ref-type="bibr" rid="b1">1</xref>, with newly formed volcanoes at mid-ocean ridges, hot spots and volcanic island arcs. Any submarine volcanic eruption leads to a major regional disruption of the environment by modifying the chemical composition of adjacent seawaters and releasing a large amount of gases with consequences on the associated deep-sea ecosystems and atmospheric chemistry<xref ref-type="bibr" rid="b2">2</xref><xref ref-type="bibr" rid="b3">3</xref><xref ref-type="bibr" rid="b4">4</xref>. This degassing can be even more catastrophic when high volumes of volcanic gases reach the atmosphere and become lethal for humans, as occurred in 1650 A.D. at Kolumbo volcano (South Aegean Sea; <xref ref-type="fig" rid="f1">Fig. 1a,b</xref>). During this submarine eruption ~70 people were killed at the nearby island of Santorini<xref ref-type="bibr" rid="b5">5</xref>. Thus, the study of submarine volcanic degassing is of great importance to better constrain the characteristics and evolution of geodynamic settings showing elevated volcanic hazard with considerable impact on the daily life of local populations.</p><p>The geochemical studies carried out on submarine fluids have been mainly focused on mid-ocean ridge hydrothermal systems<xref ref-type="bibr" rid="b6">6</xref><xref ref-type="bibr" rid="b7">7</xref><xref ref-type="bibr" rid="b8">8</xref>, whereas only a few of them investigated subduction zone environments<xref ref-type="bibr" rid="b9">9</xref><xref ref-type="bibr" rid="b10">10</xref><xref ref-type="bibr" rid="b11">11</xref><xref ref-type="bibr" rid="b12">12</xref>. The HVA in the South Aegean Sea (East Mediterranean) results from the subduction of the African plate below the European one<xref ref-type="bibr" rid="b13">13</xref><xref ref-type="bibr" rid="b14">14</xref> (<xref ref-type="fig" rid="f1">Fig. 1a</xref>). In the last decades, several studies were dedicated to better constrain the mantle characteristics below the HVA and understand the dynamics of this complex geodynamic setting but a general understanding has not been reached thus far<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b16">16</xref><xref ref-type="bibr" rid="b17">17</xref><xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b19">19</xref><xref ref-type="bibr" rid="b20">20</xref><xref ref-type="bibr" rid="b21">21</xref><xref ref-type="bibr" rid="b22">22</xref>. The recent discovery of the high-temperature hydrothermal field at the Kolumbo submarine volcano<xref ref-type="bibr" rid="b23">23</xref> (<xref ref-type="fig" rid="f2">Fig. 2a–c</xref>), located 7 km northeast off Santorini Island (<xref ref-type="fig" rid="f1">Fig. 1b</xref>), has opened up a new opportunity to deepen our knowledge of Aegean mantle features. Indeed, Kolumbo is considered to be the most active volcanic system of the region at present time<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b23">23</xref><xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b25">25</xref><xref ref-type="bibr" rid="b26">26</xref>. Its magmatic activity is evidenced by numerous hydrothermal sites venting vigorously and continuously up to ~220 °C warm fluids<xref ref-type="bibr" rid="b27">27</xref> and CO<sub>2</sub>-rich gases at the crater seafloor (<xref ref-type="fig" rid="f2">Fig. 2a–c</xref>)<xref ref-type="bibr" rid="b4">4</xref>. This ongoing intense degassing contrasts radically with the low-temperature phenomena typically observed in the Santorini caldera, next to the Kameni islands<xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b23">23</xref><xref ref-type="bibr" rid="b28">28</xref> (<xref ref-type="fig" rid="f1">Fig. 1b</xref>).</p><p>The Santorini-Amorgos Basin surrounding Kolumbo system, divides the HVA into a seismic-active eastern zone and a relatively quiet western area<xref ref-type="bibr" rid="b29">29</xref>. The main seismic hypocentres beneath Kolumbo are located at depths of 6–9 km<xref ref-type="bibr" rid="b24">24</xref>, indicating a high level of magmatic activity below this volcano. Accordingly, this range of depth corresponds to the location of the main magma chamber<xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b25">25</xref> (<xref ref-type="fig" rid="f3">Fig. 3</xref>).</p><p>The Kolumbo hydrothermal system is poorly studied<xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b30">30</xref>, and little is known about the origin of fluids escaping from its seafloor (<xref ref-type="fig" rid="f2">Fig. 2c</xref>) and the potential link with those emitted from Santorini subaerial vents, as well as other active volcanoes across the HVA. Here, we report on the <sup>3</sup>He/<sup>4</sup>He ratios of gases discharging from seven different hydrothermal chimneys of the Kolumbo submarine volcano in order to constrain the He isotopic characteristics of the mantle beneath this key sector of the HVA. We focus on the <sup>3</sup>He/<sup>4</sup>He ratio because this is the most powerful geochemical tracers to define the origin of volatiles released from the solid earth and the magmatic/mantle features<xref ref-type="bibr" rid="b31">31</xref><xref ref-type="bibr" rid="b32">32</xref>. Indeed, <sup>3</sup>He has a primordial origin and is preferentially degassed from the Earth’s interior. Because He is highly mobile, chemically inert, physically stable and nonbiogenic, the interaction of this noble gas on its movement toward the surface is minimized, and its isotopic composition is not affected by subsequent chemical reactions<xref ref-type="bibr" rid="b8">8</xref>. It also gives unambiguous signals of magma rising up in volcanic plumbing systems<xref ref-type="bibr" rid="b33">33</xref><xref ref-type="bibr" rid="b34">34</xref><xref ref-type="bibr" rid="b35">35</xref><xref ref-type="bibr" rid="b36">36</xref>.</p><sec disp-level="1"><title>Geodynamic setting</title><p>The HVA is a 500 km-long curving chain of volcanoes, Pliocene to modern in age, extending from the Methana peninsular zone on the Greek mainland, through the islands of Milos-Antimilos, Santorini, Kos-Nisyros, towards the western peninsular zone in Turkey. The HVA results from the extended and elongated subduction of the African plate beneath the Eurasian continent (<xref ref-type="fig" rid="f1">Fig. 1a</xref>). The plate horizontal velocities measured by GPS in the Eastern Mediterranean highlight the presence of a clockwise rotation of a broad region relative to Eurasia at rates in the range of 20–30 mm/yr<xref ref-type="bibr" rid="b19">19</xref><xref ref-type="bibr" rid="b22">22</xref>. This rotation is combined with the continuing southward rollback of the Hellenic subduction around a pole located in Albania<xref ref-type="bibr" rid="b19">19</xref><xref ref-type="bibr" rid="b22">22</xref>. The complex regime of movements causes a NW–SE extension in the Western Anatolia–Aegean system, which in turn leads to the thinning of the Aegean crust from roughly 50 to 25 km<xref ref-type="bibr" rid="b19">19</xref>. Based on seismic tomography models, the Aegean subduction is characterized by a single slab, more than 1,500 km-long, which extends down to the lower mantle<xref ref-type="bibr" rid="b22">22</xref>. These models also suggest that the slab does not extend far eastward and that possible ruptures are present below Nisyros volcanic system and western Turkey, where modern alkaline magmatism is present<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b22">22</xref>. In addition, P-wave velocities along the HVA indicate the presence of a low velocity zone just above the downgoing slab, which can be interpreted as the occurrence of asthenospheric mantle<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b37">37</xref>. As a result, the interface between the slab and mantle wedge is influenced by primary magmas generated in the underlying asthenosphere<xref ref-type="bibr" rid="b18">18</xref>.</p><p>Within the HVA, Santorini volcano is one of the most active and it is very famous worldwide because of its large explosive eruption that occurred in Bronze Age, which caused significant impacts to human populations in the eastern Mediterranean<xref ref-type="bibr" rid="b38">38</xref>. In the Santorini area, volcanism extends northeastward into the submarine environment evidenced by a series of small craters and cones aligned along Christianna-Santorini-Kolumbo (CSK) tectonic line<xref ref-type="bibr" rid="b28">28</xref>, which likely controls the pathways of hydrothermal circulation within the region (<xref ref-type="fig" rid="f1">Fig. 1b</xref>). This is a vertical northeast trending zone, up to 30–40 km large and 45 km deep<xref ref-type="bibr" rid="b29">29</xref>. The CSK line created an area of structural weakness, characterized by increased seismic activity<xref ref-type="bibr" rid="b25">25</xref>, enabling the upward migration of fluids at Kolumbo. The focus of this study is dedicated to a better understanding of the composition of these fluids in order to further constrain the mantle features within the HVA.</p></sec><sec disp-level="1"><title>Results</title><p>The chemical composition of gases collected from each of the seven chimneys (<xref ref-type="fig" rid="f2">Fig. 2b,c</xref>) consists of almost pure CO<sub>2</sub> (<xref ref-type="table" rid="t1">Table 1</xref>), in line with the findings of previous work at the same area<xref ref-type="bibr" rid="b4">4</xref>. This feature is typical of submarine volcanic emissions and it has also been observed either in hot spot systems as Loihi<xref ref-type="bibr" rid="b39">39</xref> (Hawaii), or in subduction-related environments like Panarea (Aeolian islands, Italy<xref ref-type="bibr" rid="b9">9</xref>), offshore NE Taiwan<xref ref-type="bibr" rid="b40">40</xref>, along the Mariana and Tonga-Kermadec arcs<xref ref-type="bibr" rid="b11">11</xref>. The O<sub>2</sub> (<2.1%) and N<sub>2</sub> (<8.5%) concentration measured in Kolumbo gases suggests a contamination by atmospheric air, which, nevertheless, has a minor influence on helium isotopic composition, as evidenced by the <sup>4</sup>He/<sup>20</sup>Ne ratio (9–270; <xref ref-type="table" rid="t1">Table 1</xref>) being 30 to 250 times higher than that of ambient air (<sup>4</sup>He/<sup>20</sup>Ne = 0.318). At the bottom of Kolumbo crater, seawater exhibits acidic conditions (pH = 5), as a consequence of CO<sub>2</sub> release and dissolution<xref ref-type="bibr" rid="b3">3</xref><xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b30">30</xref>. This dissolution, the extent of which varies depending on the actual flux of gas coming out from each vent, is responsible for the variable helium concentrations (15–41 part per million) measured in our samples (<xref ref-type="table" rid="t1">Table 1</xref>).</p><p>The <sup>3</sup>He/<sup>4</sup>He ratios of seven gas samples are virtually identical to one another, irrespective of the vent being sampled and the extent of CO<sub>2</sub> dissolution, and vary in the narrow range of 7.0–7.1 Ra (<xref ref-type="fig" rid="f4">Fig. 4a</xref> and <xref ref-type="table" rid="t1">Table 1</xref>). These values are slightly higher than the single previous measurement of 6.8 Ra reported for Kolumbo<xref ref-type="bibr" rid="b4">4</xref>. Because the <sup>3</sup>He/<sup>4</sup>He ratio does not suffer any significant fractionation during gas-water interaction, it can be used to assess the origin of emitted fluids.</p></sec><sec disp-level="1"><title>Discussion</title><sec disp-level="2"><title>Kolumbo-Santorini volcanic system</title><p>The <sup>3</sup>He/<sup>4</sup>He ratios measured at Kolumbo (7.0–7.1 Ra) are within the typical range of values found in arc volcanoes worldwide and correspond to a MORB-like <sup>3</sup>He/<sup>4</sup>He signature of 8 ± 1 Ra<xref ref-type="bibr" rid="b8">8</xref><xref ref-type="bibr" rid="b41">41</xref> (<xref ref-type="fig" rid="f4">Fig. 4a</xref>). This finding indicates that magma degassing beneath Kolumbo and feeding its hydrothermal system has a <sup>3</sup>He/<sup>4</sup>He representative of the primary source. This signature is surprisingly higher (more than 3 Ra units) than that measured in gases and rocks (fluid inclusions) of the adjacent (<7 km) Santorini island<xref ref-type="bibr" rid="b34">34</xref> (<xref ref-type="fig" rid="f4">Fig. 4a</xref>). Indeed, gases collected between 1988 and 2001 from both Nea and Palea Kameni islets were in the range of 3.2–3.8 Ra<xref ref-type="bibr" rid="b17">17</xref><xref ref-type="bibr" rid="b42">42</xref><xref ref-type="bibr" rid="b43">43</xref><xref ref-type="bibr" rid="b44">44</xref>, while during the 2011–2012 seismic, geodetic and geochemical unrest these ratios increased from 3.6 to 4.0 Ra (ΔRa = 0.4)<xref ref-type="bibr" rid="b34">34</xref>. Also, the <sup>3</sup>He/<sup>4</sup>He ratios measured in some mafic enclaves, hosted by dacitic lavas recently erupted at Nea Kameni, yielded a range of 3.1–3.6 Ra<xref ref-type="bibr" rid="b34">34</xref> (<xref ref-type="fig" rid="f4">Fig. 4a</xref>). All these data imply that the magma residing at shallow depths into the Santorini plumbing system has a distinct and fairly consistent He-isotope signature ranging between 3 and 4 Ra.</p><p>Substantial spatial variations of the <sup>3</sup>He/<sup>4</sup>He ratios over a 10-km distance have been recently observed at the mid oceanic ridge (7.5–10.2 Ra<xref ref-type="bibr" rid="b45">45</xref>). However, to the best of our knowledge, it is rare that two distinct and active volcanic systems located only a few km apart from one another display at the same time such a wide variation in <sup>3</sup>He/<sup>4</sup>He ratio. This finding emphasizes the importance of investigating fluids being discharged by submarine volcanic clusters with sufficient spatial coverage and resolution, if precious insights about the mantle are to be gained around the planet. In light of this, while the high <sup>3</sup>He/<sup>4</sup>He ratios at Kolumbo can be considered representative of the mantle source beneath the submarine volcano, the lower <sup>3</sup>He/<sup>4</sup>He values measured at Santorini lead to two fundamental questions:</p><p>(1) Do the <sup>3</sup>He/<sup>4</sup>He ratios measured at Santorini reflect a local mantle heterogeneity, despite the very short distance among the two volcanic systems?</p><p>(2) Do these ratios reflect an addition by radiogenic <sup>4</sup>He within the Santorini plumbing system of magma and/or fluids originated from a homogeneous mantle?</p><p>To decipher between the two hypotheses, we compare the lowest <sup>87</sup>Sr/<sup>86</sup>Sr ratios measured in whole rocks from Santorini (0.7035<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref>) with <sup>3</sup>He/<sup>4</sup>He in gases and fluid inclusions of the two volcanic systems (<xref ref-type="fig" rid="f4">Fig. 4b</xref>). For completeness, in <xref ref-type="fig" rid="f4">Fig. 4b</xref> we report the range of <sup>87</sup>Sr/<sup>86</sup>Sr ratios measured in bulk rocks erupted in each volcanic system. In magmatic environments, these geochemical tracers are inversely correlated, in the sense that the highest He isotopic ratios correspond to the lowest Sr isotopic ratios, which are measured in the most mafic and primitive products, and vice versa. This behaviour has been observed in other magmatic systems worldwide<xref ref-type="bibr" rid="b48">48</xref><xref ref-type="bibr" rid="b49">49</xref>, and is simplified in the insert diagram of <xref ref-type="fig" rid="f4">Fig. 4b</xref> in which we plot a binary mixing line between a MORB-like mantle (<sup>3</sup>He/<sup>4</sup>He = 8.0 Ra; <sup>87</sup>Sr/<sup>86</sup>Sr = 0.7020) and a hypothetical Aegean continental crust (<sup>3</sup>He/<sup>4</sup>He = 0.03 Ra; <sup>87</sup>Sr/<sup>86</sup>Sr = 0.7150)<xref ref-type="bibr" rid="b50">50</xref><xref ref-type="bibr" rid="b51">51</xref>. We obtain a straight line because for simplicity we assumed <italic>k</italic> = 1 (being <italic>k</italic> = (He/Sr)<sub>crust</sub>/(He/Sr)<sub>mantle</sub>), however slight variations of this ratio do not modify our inferences. Based on this mixing, the right y axis of plot 4b reports the expected <sup>3</sup>He/<sup>4</sup>He ratios corresponding to the <sup>87</sup>Sr/<sup>86</sup>Sr ratios reported in the left y axis.</p><p>Petrological studies on the HVA indicated that volcanic rocks from Santorini display compositional features typical of subduction-related tectonic settings, with a high variability of trace elements and Sr-Nd-Pb isotopes mainly ascribed to crustal assimilation processes<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref>. Considering that the most primitive products erupted at Santorini showed <sup>87</sup>Sr/<sup>86</sup>Sr values as low as 0.7035<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref> (<xref ref-type="fig" rid="f4">Fig. 4b</xref>), the genesis of parental magmas can be ascribed to the partial melting of a depleted, MORB-like, mantle wedge<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b47">47</xref><xref ref-type="bibr" rid="b52">52</xref>. As shown in the insert plot of <xref ref-type="fig" rid="f4">Fig. 4b</xref>, these <sup>87</sup>Sr/<sup>86</sup>Sr values are highly consistent with the <sup>3</sup>He/<sup>4</sup>He ratios measured in gases at Kolumbo (7.0–7.1 Ra; see also secondary y axis in <xref ref-type="fig" rid="f4">Fig. 4b</xref>). Conversely, the <sup>3</sup>He/<sup>4</sup>He ratios measured in gases and rocks of Santorini (3.1–4.0 Ra) cannot account neither for the very low <sup>87</sup>Sr/<sup>86</sup>Sr measured in basalts, nor for the ratios measured in mafic enclaves hosted in dacitic lavas of Nea Kameni (<sup>87</sup>Sr/<sup>86</sup>Sr~0.7048<xref ref-type="bibr" rid="b34">34</xref>). This implies that the wide variability of Sr isotope composition observed among the erupted volcanic at Santorini (<sup>87</sup>Sr/<sup>86</sup>Sr from 0.7035 to 0.7062; <xref ref-type="fig" rid="f4">Fig. 4b</xref>) is related to magmatic differentiation which predominantly takes place in a shallow magma chamber plus assimilation of crustal rocks during magma ascent plays an important role in the evolution of Santorini magmas<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref> (<xref ref-type="fig" rid="f3">Fig. 3</xref>). Accordingly, several models of assimilation plus fractional crystallization involving Santorini basalts as mafic end-members and crustal basement rocks as contaminants have been proposed<xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref>.</p><p>The composition of rocks erupted from Kolumbo is much less investigated than in Santorini. Available data indicated that rocks erupted from Kolumbo volcano range from andesites to rhyolites and belong to calcalkaline series, similarly to most of the Santorini rocks<xref ref-type="bibr" rid="b15">15</xref>. Pumices are the main products of the last Kolumbo eruption at 1650 A.D. and lie along a magmatic evolution line similar to the suite of silicic pyroclastics from the nearby Santorinivolcano<xref ref-type="bibr" rid="b53">53</xref>. Unfortunately, no Sr isotopic data have been published yet for Kolumbo rocks.</p><p>Recent volcanological and geochemical investigations suggested the presence of two distinct plumbing systems beneath Santorini and Kolumbo volcanic systems<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b24">24</xref> (<xref ref-type="fig" rid="f3">Fig. 3</xref>). These two reservoirs are also characterized by distinct geochemical and mineralogical characteristics, suggesting that primary magmas beneath Santorini are modified by successive processes of crustal assimilation plus fractional crystallization and magma mixing<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b18">18</xref><xref ref-type="bibr" rid="b24">24</xref>. As already highlighted<xref ref-type="bibr" rid="b34">34</xref>, the occurrence of magma contamination by crustal rocks in Santorini plumbing system would eventually lead to the reduction of <sup>3</sup>He/<sup>4</sup>He ratio with regard to parental magmas. On the other hand, magmas feeding Kolumbo would derive directly from the mantle<xref ref-type="bibr" rid="b24">24</xref>.</p><p>Santorini and Kolumbo volcanic systems lie along the same tectonic alignment that starts from Christianna north-eastwards (CSK<xref ref-type="bibr" rid="b28">28</xref>, <xref ref-type="fig" rid="f1">Fig. 1b</xref>) and share a common region of lithospheric weakness. This unequivocally indicates that these volcanic systems originated from the same geological structure, but distinct magmatic reservoirs are active in the crust beneath each one of them<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b24">24</xref> (<xref ref-type="fig" rid="f3">Fig. 3</xref>). Furthermore, our data argue that the mantle below Kolumbo and Santorini can be considered homogeneous in terms of He isotope ratios (<sup>3</sup>He/<sup>4</sup>He ≥ 7.0 Ra). The observed He signature is MORB-like, as in most arc volcanoes worldwide in which the subduction of oceanic crust does not affect He isotopic composition of the mantle wedge<xref ref-type="bibr" rid="b8">8</xref><xref ref-type="bibr" rid="b41">41</xref>.</p><p>If petrological features argue for primary magma undergoing differentiation plus crustal assimilation process in the magmatic plumbing systems below Santorini and Kolumbo volcanoes, then why do we observe <sup>3</sup>He/<sup>4</sup>He ratios as high as 7.1 Ra only at Kolumbo? Two possible hypotheses can be proposed:</p><p>(1) Evolution and crustal assimilation by magma beneath Kolumbo are very low and do not imply a contribution of <sup>4</sup>He-rich rocks and/or fluids;</p><p>(2) Predominant degassing at Kolumbo occurs directly from the mantle and fluxes the plumbing system, overprinting the He isotopic signature of evolved/contaminated magmatic products.</p><p>The geological features following a cross section through Santorini and Kolumbo volcanoes indicate a similar crustal thickness (~25 km<xref ref-type="bibr" rid="b54">54</xref>; <xref ref-type="fig" rid="f3">Fig. 3</xref>) and common basement rocks<xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b30">30</xref>, discarding the first hypothesis. Furthermore, local seismological studies carried out in the area revealed an active tectonic regime beneath Kolumbo volcano characterized by the presence of faults<xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b55">55</xref>, whereas there is no evidence of continuous seismic activity beneath the caldera of Santorini volcano with the exception of unrest periods<xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b25">25</xref><xref ref-type="bibr" rid="b29">29</xref><xref ref-type="bibr" rid="b56">56</xref>. These geological and geophysical observations are in very good agreement with recent volcanological and marine studies that showed an intense high-temperature hydrothermal activity at Kolumbo, in comparison to the low-level activity of the Santorini caldera (submarine and subaerial fluid temperatures <100°)<xref ref-type="bibr" rid="b23">23</xref><xref ref-type="bibr" rid="b28">28</xref><xref ref-type="bibr" rid="b34">34</xref>. As a result, we conclude that magma residing in the shallow plumbing system beneath Santorini could be volumetrically small, strongly degassed in terms of helium, and prone to the crustal-derived <sup>4</sup>He contamination by surrounding rocks. On the other hand, the <sup>3</sup>He/<sup>4</sup>He ratios at Kolumbo could be the result of mantle degassing (fluxing) in a regional extensive regime characterized by the presence of active lithospheric faults beneath the central part of the HVA (<xref ref-type="fig" rid="f1">Fig. 1b</xref>), which would overprint the signature of <sup>3</sup>He/<sup>4</sup>He of evolved/contaminated magma residing at shallow depths, thus allowing to measure mantle ratios (<xref ref-type="fig" rid="f3">Figs. 3</xref> and <xref ref-type="fig" rid="f4">4a</xref>).</p><p>Under this prism, the Kolumbo submarine volcano is conceived to represent a sort of “window” into the mantle of the central HVA.</p></sec><sec disp-level="2"><title>Geodynamic implications on the Hellenic Volcanic Arc</title><p>The <sup>3</sup>He/<sup>4</sup>He ratios of Kolumbo (up to 7.1 Ra) are also the highest ever measured in gases and rocks belonging to the HVA (<xref ref-type="fig" rid="f4">Fig. 4a</xref>). Until now, the highest values were measured in gases emitted at Nisyros (6.2 Ra), located at the eastern side of the arc<xref ref-type="bibr" rid="b17">17</xref>. In this case, the presence of crustal assimilation at shallow depths below the island or the addition of radiogenic <sup>4</sup>He to a MORB-like mantle wedge were considered unlikely, while the <sup>3</sup>He/<sup>4</sup>He ratio of Nisyros was deemed to be representative of South Aegean mantle and consistent with the European Sub-Continental Lithospheric Mantle (SCLM; 6.1 ± 0.9 Ra). The same study identified a regional trend toward a westward decrease of <sup>3</sup>He/<sup>4</sup>He ratios from Nisyros to Sousaki volcanoes (<xref ref-type="fig" rid="f4">Fig. 4a</xref>) and it was attributed to a variable degree of crustal assimilation and/or to different magmatic activity in each volcanic system<xref ref-type="bibr" rid="b17">17</xref>.</p><p>Our current <sup>3</sup>He/<sup>4</sup>He measurements from Kolumbo provide new constrains on these interpretations, because the mantle below the HVA (at least in its central sector, i.e., Kolumbo-Santorini) has an He isotopic signature compatible with a MORB-like source, and the high <sup>3</sup>He/<sup>4</sup>He ratios at Kolumbo rule out the presence of a regional trend. In order to evaluate if the mantle signature beneath Kolumbo may extend up to Nisyros, we used the same approach described in the previous section based on He and Sr isotope signatures in gases and rocks. Petrological studies of the HVA indicate that the most primitive products erupted at Nisyros are characterized by <sup>87</sup>Sr/<sup>86</sup>Sr values as low as 0.7034<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b46">46</xref> (<xref ref-type="fig" rid="f4">Fig. 4b</xref>). These Sr isotope ratios are perfectly compatible with the high <sup>3</sup>He/<sup>4</sup>He ratios measured at Kolumbo (7.0–7.1 Ra; <xref ref-type="fig" rid="f4">Fig. 4b</xref>), indicating that this He isotopic signature can be considered also representative of the mantle below Nisyros. On the other hand, <sup>3</sup>He/<sup>4</sup>He ratios of 6.2 Ra would be compatible with <sup>87</sup>Sr/<sup>86</sup>Sr of ~0.7049 (<xref ref-type="fig" rid="f4">Fig. 4b</xref>). The <sup>87</sup>Sr/<sup>86</sup>Sr variability of Nisyros (0.7034–0.7064<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b46">46</xref>), which is in line with measured values at Santorini, shows that magmatic differentiation plus assimilation of crustal rocks during magma ascent could also occur in the evolution of Nisyros magmas. The occurrence of these processes in the plumbing system of the volcano is reasonably responsible of the slight lowering of <sup>3</sup>He/<sup>4</sup>He ratios at Nisyros (6.2 Ra) compared to the pristine values obtained at Kolumbo volcano (7.0–7.1 Ra; <xref ref-type="fig" rid="f4">Fig. 4a</xref>).</p><p>These new considerations can be extended westward up to Milos volcanic system, whose lowest <sup>87</sup>Sr/<sup>86</sup>Sr (i.e., 0.7037<sup>15</sup>) are comparable to those measured at Santorini and Nisyros (<xref ref-type="fig" rid="f4">Fig. 4b</xref>). On this ground, the <sup>3</sup>He/<sup>4</sup>He ratios <4 Ra<xref ref-type="bibr" rid="b17">17</xref><xref ref-type="bibr" rid="b44">44</xref> (<xref ref-type="fig" rid="f4">Fig. 4a</xref>) in our opinion indicate strong crustal contamination process that would mask the pristine isotope signature. On the other hand, the westernmost part of the HVA (from Methana to Sousaki) showed the lowest <sup>3</sup>He/<sup>4</sup>He (0.17–2.6 Ra<xref ref-type="bibr" rid="b17">17</xref><xref ref-type="bibr" rid="b44">44</xref>) and <sup>87</sup>Sr/<sup>86</sup>Sr values >0.7055 even in the most mafic volcanics (basaltic-andesite to andesite of Poros and Methana<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b46">46</xref>) (<xref ref-type="fig" rid="f4">Fig. 4a,b</xref>). A recent investigation based on Sr and Nd isotopes highlighted an extensive crustal assimilation at Methana, higher than in Santorini and Nisyros<xref ref-type="bibr" rid="b46">46</xref>. These features strongly support the recognized variability of the He isotopic signature in these volcanic systems, even if magma mixing and mingling could simultaneously have occurred to generate the recognized variability in the petrological features of erupted volcanics<xref ref-type="bibr" rid="b46">46</xref>.</p><p>Based on these arguments, we propose that the mantle below the central and eastern part of the HVA (from Milos through Santorini-Kolumbo up to Nisyros-Kos) is rather homogeneous in terms of <sup>3</sup>He/<sup>4</sup>He and preserves its MORB-like magmatic signature<xref ref-type="bibr" rid="b8">8</xref><xref ref-type="bibr" rid="b41">41</xref>.</p></sec><sec disp-level="2"><title>Volcanic hazard at Kolumbo</title><p>Among the subaerial volcanoes belonging to the HVA, Nisyros and Santorini volcanoes are considered as the most active ones. In 1995–1998, the former was characterized by a seismic activity, after which the geochemical monitoring of gases collected from the fumaroles highlighted an increase of the <sup>3</sup>He/<sup>4</sup>He (∆Ra = 0.7)<xref ref-type="bibr" rid="b17">17</xref>. This variation was related to the upward movement of subsurface magma, which triggered an enhanced contribution of mantle helium<xref ref-type="bibr" rid="b17">17</xref>. In 2011–2012, a seismic, geodetic and geochemical unrest took place in Santorini<xref ref-type="bibr" rid="b56">56</xref>, during which an increase of <sup>3</sup>He/<sup>4</sup>He ratios (∆Ra = 0.4) has been recorded in the fluids emitted at Nea and Palea Kameni<xref ref-type="bibr" rid="b34">34</xref>. This increase was interpreted as the result of the intrusion of a more-primitive <sup>3</sup>He-rich magma into the shallow plumbing system. These unrests observed at Nisyros and Santorini led to a general fear that an eruption could be imminent.</p><p>Based on our present results, we infer that Kolumbo submarine volcano is very active at present time and the associated volcanic hazard may be potentially high. With this in mind, the development of a regular geochemical monitoring program for this potentially dangerous submarine volcano is strongly suggested.</p><p>In summary, hydrothermal fluids from Kolumbo submarine volcano exhibited the highest <sup>3</sup>He/<sup>4</sup>He ratios (up to 7.1 Ra) across the HVA, which are surprisingly higher (>3 Ra units) than those previously reported for the adjacent island of Santorini. These fluids reflect a direct mantle degassing that can only be produced in an extensive tectonic regime characterized by the presence of active lithospheric faults. The range of <sup>3</sup>He/<sup>4</sup>He ratio measured at Kolumbo is the highest of all the HVA and is representative of the mantle below the central and eastern part of the HVA. The degassing of high-temperature fluids from the bottom of Kolumbo crater with a mantle-like <sup>3</sup>He/<sup>4</sup>He ratio suggests that this submarine volcano is characterized by a vigorous activity with potential volcanic hazard.</p></sec></sec><sec disp-level="1"><title>Methods</title><sec disp-level="2"><title>Sampling</title><p>The Kolumbo submarine volcano is characterized by a well-defined 1500-meter-wide crater (<xref ref-type="fig" rid="f2">Fig. 2a</xref>), with a rim as shallow as 17 m and a floor ~500 m below the sea level<xref ref-type="bibr" rid="b26">26</xref><xref ref-type="bibr" rid="b28">28</xref> (<xref ref-type="fig" rid="f2">Fig. 2a,b</xref>). On the active hydrothermal vent field at the crater floor of Kolumbo (<xref ref-type="fig" rid="f2">Fig. 2b</xref>), first discovered in 2006<xref ref-type="bibr" rid="b23">23</xref>, relatively high temperatures (up to ~220 °C<xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b27">27</xref>) have been measured. During the 4-SeaBiotech cruise on board of the <italic>R/V Aegaeo</italic> (Hellenic Centre for Marine Research) in May 2014, seafloor exploration of hydrothermal activity has been carried out with the Greek ROV Max Rover. Due to the pressure-temperature (P-T) conditions of the vents (P~50 bar, T > 200 °C), hydrothermal fluid/gas discharges are in form of clear waters together with continuous gas bubbling at different chimneys (<xref ref-type="fig" rid="f2">Fig. 2c</xref> and <xref ref-type="supplementary-material" rid="S1">supplementary movies</xref>).</p><p>High temperature hydrothermal gases were collected in titanium gas-tight bottles of 200 ml capacity each equipped with funnels. These bottles have been built specially to avoid out-gassing and gas leakage during recovery and were initially designed by IFREMER for the manned submersible Nautile. Additional samplers has been projected and developed by Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Palermo (Italy), and experimentally used at Kolumbo. Both types of bottles have been pre-evacuated on board of the oceanographic vessel, just before each submersion (internal pressure <10<sup>−3</sup> bar). We point out that we did not observe any difference in using the two types of gas-tight samplers. For sake of clarity, sample A2 was collected with INGV-type bottle (<xref ref-type="table" rid="t1">Table 1</xref>). The gas-tight bottles were held by the ROV Max Rover of HCMR arm over the bubble streams discharged from the chimneys and then triggered to collect a sample (see <xref ref-type="supplementary-material" rid="S1">supplementary movies</xref>).</p><p>We collected gas samples from seven different chimneys located at the bottom of Kolumbo crater (<xref ref-type="fig" rid="f2">Fig. 2b,c</xref>). After each sampling, once the gas-tight syringes were onboard of the research vessel, gases at 50 bar were immediately extracted in stainless steel or titanium bottles for safe transportation (at lower pressures) towards the laboratory.</p></sec><sec disp-level="2"><title>Analytical techniques</title><p>The analysis of chemical composition and He-Ne isotopes of the collected gases has been performed in the laboratories of INGV-Palermo. The concentrations of CO<sub>2</sub>, O<sub>2</sub> and N<sub>2</sub> were analyzed by a Perkin Elmer Clarus 500 gas chromatograph equipped with a 3.5-m Carboxen 1000 column and double detector (hot-wire detector and flame ionization detector), with analytical errors of <3%. <sup>3</sup>He, <sup>4</sup>He and <sup>20</sup>Ne and the <sup>4</sup>He/<sup>20</sup>Ne ratios were determined by separately admitting He and Ne into a split flight tube mass spectrometer (GVI-Helix SFT, for He analysis) and into a multicollector mass spectrometer (Thermo-Helix MC plus, for Ne analysis), after standard purification procedures<xref ref-type="bibr" rid="b32">32</xref><xref ref-type="bibr" rid="b34">34</xref>. The <sup>3</sup>He/<sup>4</sup>He ratio is expressed as R/Ra (being Ra the He isotope ratio of air and equal to 1.39·10<sup>−6</sup>)<sup>58</sup>, and the analytical error is generally below 0.3%. The <sup>3</sup>He/<sup>4</sup>He values were corrected for the atmospheric contamination based on the measured <sup>4</sup>He/<sup>20</sup>Ne ratio<xref ref-type="bibr" rid="b34">34</xref> as follows:</p><p><disp-formula id="eq1"><inline-graphic id="d33e1119" xlink:href="srep28013-m1.jpg"></inline-graphic></disp-formula></p><p>where subscripts M and A refer to measured and atmosphere theoretical values, respectively [(He/Ne)<sub>A</sub> = 0.318]<sup>58</sup>. The corrected <sup>3</sup>He/<sup>4</sup>He ratios reported in the text and in <xref ref-type="table" rid="t1">Table 1</xref> are expressed as Rc/Ra values. The correction is small or negligible for most of the samples, with the maximum bias of ~0.2 Ra appearing in the sample showing the lowest <sup>4</sup>He/<sup>20</sup>Ne.</p></sec></sec><sec disp-level="1"><title>Additional Information</title><p><bold>How to cite this article</bold>: Rizzo, A. L. <italic>et al.</italic> Kolumbo submarine volcano (Greece): An active window into the Aegean subduction system. <italic>Sci. Rep.</italic><bold>6</bold>, 28013; doi: 10.1038/srep28013 (2016).</p></sec><sec sec-type="supplementary-material" id="S1"><title>Supplementary Material</title><supplementary-material id="d33e24" content-type="local-data"><caption><title>Supplementary Information</title></caption><media xlink:href="srep28013-s1.pdf"></media></supplementary-material><supplementary-material id="d33e27" content-type="local-data"><caption><title>Supplementary Movie 1</title></caption><media xlink:href="srep28013-s2.avi"></media></supplementary-material><supplementary-material id="d33e30" content-type="local-data"><caption><title>Supplementary Movie 2</title></caption><media xlink:href="srep28013-s3.avi"></media></supplementary-material></sec></body><back><ack><p>We thank Giuseppe Riccobono and Paolo Cosenza for the technical support in the projecting and construction of the INGV-Palermo gas tight sampler used to collect part of the submarine fluids. The former also contributed to the gas collection and extraction, and in the activities on board the <italic>R/V Aegaeo</italic> of the Hellenic Centre for Marine Research. The officers and the crew of the R/V <italic>Aegaeo</italic> are especially acknowledged for their valuable help during sampling. Sampling campaign 4-Seabiotech was supported by Seabiotech project (spider.science.strath.ac.uk/seabiotech/) funded by the European Commission within its FP7 Programme with Grant Number 311932. We are grateful to Mariano Tantillo for the isotopic analyses of noble gases and Mauro Martelli and Francesco Salerno for the analyses in gas chromatography. Comments made by three anonymous Reviewers strongly improved the manuscript.</p></ack><ref-list><ref id="b1"><mixed-citation publication-type="journal"><name><surname>Crisp</surname><given-names>J. A.</given-names></name><article-title>Rates of magma emplacement and volcanic output</article-title>. <source>J. Volcanol. Geotherm. 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Res</source>. <volume>291</volume>, <fpage>101</fpage>–<lpage>111</lpage> (<year>2015</year>).</mixed-citation></ref><ref id="b56"><mixed-citation publication-type="journal"><name><surname>Parks</surname><given-names>M. M.</given-names></name><italic>et al.</italic><article-title>Evolution of Santorini volcano dominated by episodic and rapid fluxes of melt from depth</article-title>. <source>Nat. Geosci</source>. <volume>5</volume>, <fpage>749</fpage>–<lpage>754</lpage> (<year>2012</year>).</mixed-citation></ref><ref id="b57"><mixed-citation publication-type="journal"><name><surname>Gertisser</surname><given-names>R.</given-names></name>, <name><surname>Preece</surname><given-names>K.</given-names></name> & <name><surname>Keller</surname><given-names>J.</given-names></name><article-title>The Plinian Lower Pumice 2 eruption, Santorini, Greece: Magma evolution and volatile behaviour</article-title>. <source>J. Volcanol. Geotherm. Res</source>. <volume>186</volume>, <fpage>387</fpage>–<lpage>406</lpage> (<year>2009</year>).</mixed-citation></ref></ref-list><fn-group><fn><p><bold>Author Contributions</bold> A.L.R. and P.N. conceived the study. A.L.R., V.C., P.N., P.N.P., M.M. G.K., A.M. and A.C. participated to the collection of the gas samples. A.L.R. and A.C. analyzed the gases and elaborated the data. All authors contributed to the preparation of the manuscript.</p></fn></fn-group></back><floats-group><fig id="f1"><label>Figure 1</label><caption><p>(<bold>a</bold>) Simplified map of the present day geodynamic structure of the HVA, showing the modern volcanic arc developed behind the Hellenic trench, the Peloponnese–Crete island arc and the Cretan back-arc basin. The study area is located in the center of the HVA (modified from ref. <xref ref-type="bibr" rid="b28">28</xref>). (<bold>b</bold>) Swath bathymetric map of Christianna-Santorini-Kolumbo (CSK) volcanic fields (modified from ref. <xref ref-type="bibr" rid="b26">26</xref>). The location of the NE-SW profile of <xref ref-type="fig" rid="f3">Fig. 3</xref> is also reported (purple dotted line), as well as CSK tectonic alignment.</p></caption><graphic xlink:href="srep28013-f1"></graphic></fig><fig id="f2"><label>Figure 2</label><caption><p>(<bold>a</bold>) 3D Bathymetric map of Kolumbo submarine volcano, showing the shape of the crater in whose bottom are located the hydrothermal chimneys; (<bold>b</bold>) High resolution swath bathymetric map of Kolumbo crater bottom (<xref ref-type="fig" rid="f1">Fig. 1c</xref>) with the location of the sampled hydrothermal chimneys as labeled in <xref ref-type="table" rid="t1">Table 1.</xref> (<bold>c</bold>) Active hydrothermal vent discharging both gases (>99% CO<sub>2</sub>) and high-temperature fluids collected in this work.</p></caption><graphic xlink:href="srep28013-f2"></graphic></fig><fig id="f3"><label>Figure 3</label><caption><title>Cross section along ~40 km NE–SW profile (reported in <xref ref-type="fig" rid="f1">Fig. 1b</xref>) moving from the Kolumbo area towards Santorini Island showing a sketch of the distinct plumbing systems beneath the two volcanoes.</title><p>The depth of the magmatic chambers (dimensions not to scale) and the main volcanological, seismic and petrologic features characterizing Santorini and Kolumbo volcanic systems are also reported<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b23">23</xref><xref ref-type="bibr" rid="b24">24</xref><xref ref-type="bibr" rid="b25">25</xref><xref ref-type="bibr" rid="b28">28</xref><xref ref-type="bibr" rid="b29">29</xref><xref ref-type="bibr" rid="b55">55</xref><xref ref-type="bibr" rid="b57">57</xref>.</p></caption><graphic xlink:href="srep28013-f3"></graphic></fig><fig id="f4"><label>Figure 4</label><caption><p>Along-arc variations of (<bold>a</bold>) <sup>3</sup>He/<sup>4</sup>He<xref ref-type="bibr" rid="b4">4</xref><xref ref-type="bibr" rid="b17">17</xref><xref ref-type="bibr" rid="b34">34</xref><xref ref-type="bibr" rid="b42">42</xref><xref ref-type="bibr" rid="b43">43</xref><xref ref-type="bibr" rid="b44">44</xref> and (<bold>b</bold>) <sup>87</sup>Sr/<sup>86</sup>Sr ratios in the HVA<xref ref-type="bibr" rid="b15">15</xref><xref ref-type="bibr" rid="b16">16</xref><xref ref-type="bibr" rid="b46">46</xref><xref ref-type="bibr" rid="b47">47</xref>. Solid black rectangle in plot 4a reproduces the range of He isotope ratios of Kolumbo gases. In the insert diagram of plot 4b we report a binary mixing between a MORB-like mantle (<sup>3</sup>He/<sup>4</sup>He = 8.0 Ra; <sup>87</sup>Sr/<sup>86</sup>Sr = 0.7020) and a hypothetical Aegean continental crust (<sup>3</sup>He/<sup>4</sup>He = 0.03 Ra; <sup>87</sup>Sr/<sup>86</sup>Sr = 0.7150)<xref ref-type="bibr" rid="b50">50</xref><xref ref-type="bibr" rid="b51">51</xref>. See text for further details. Based on this mixing, the secondary y axis of plot 4b reports the expected <sup>3</sup>He/<sup>4</sup>He ratios (red coloured) corresponding to the <sup>87</sup>Sr/<sup>86</sup>Sr ratios reported in the left y axis.</p></caption><graphic xlink:href="srep28013-f4"></graphic></fig><table-wrap position="float" id="t1"><label>Table 1</label><caption><title>Analyses of major gaseous components, helium and neon in gases from Kolumbo hydrothermal vents.</title></caption><table frame="hsides" rules="groups" border="1"><colgroup><col align="left"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col><col align="center"></col></colgroup><thead valign="bottom"><tr><th align="left" valign="top" charoff="50">Sample</th><th align="center" valign="top" charoff="50">Depth (m)</th><th align="center" valign="top" charoff="50">Latitude</th><th align="center" valign="top" charoff="50">Longitude</th><th align="center" valign="top" charoff="50"><sup>4</sup>He (ppm)</th><th align="center" valign="top" charoff="50"><sup>20</sup>Ne (ppm)</th><th align="center" valign="top" charoff="50">O<sub>2</sub> (%)</th><th align="center" valign="top" charoff="50">N<sub>2</sub> (%)</th><th align="center" valign="top" charoff="50">CO<sub>2</sub> (%)</th><th align="center" valign="top" charoff="50">R/Ra</th><th align="center" valign="top" charoff="50"><sup>4</sup>He/<sup>20</sup>Ne</th><th align="center" valign="top" charoff="50">Rc/Ra</th><th align="center" valign="top" charoff="50">Error (+/−)</th></tr></thead><tbody valign="top"><tr><td align="left" valign="top" charoff="50">A2</td><td align="center" valign="top" charoff="50">497</td><td align="center" valign="top" charoff="50">36°31.5700′N</td><td align="center" valign="top" charoff="50">25°29.2110′E</td><td align="center" valign="top" charoff="50">23</td><td align="center" valign="top" charoff="50">0.09</td><td align="center" valign="top" charoff="50">0.17</td><td align="center" valign="top" charoff="50">1.1</td><td align="center" valign="top" charoff="50">98.0</td><td align="center" valign="top" charoff="50">7.01</td><td align="center" valign="top" charoff="50">270.3</td><td align="center" valign="top" charoff="50">7.02</td><td align="center" valign="top" charoff="50">0.13</td></tr><tr><td align="left" valign="top" charoff="50">V2</td><td align="center" valign="top" charoff="50">498</td><td align="center" valign="top" charoff="50">36°31.5700′N</td><td align="center" valign="top" charoff="50">25°29.2050′E</td><td align="center" valign="top" charoff="50">15</td><td align="center" valign="top" charoff="50">1.69</td><td align="center" valign="top" charoff="50">2.09</td><td align="center" valign="top" charoff="50">8.5</td><td align="center" valign="top" charoff="50">88.3</td><td align="center" valign="top" charoff="50">6.84</td><td align="center" valign="top" charoff="50">9.1</td><td align="center" valign="top" charoff="50">7.05</td><td align="center" valign="top" charoff="50">0.017</td></tr><tr><td align="left" valign="top" charoff="50">V3</td><td align="center" valign="top" charoff="50">498</td><td align="center" valign="top" charoff="50">36°31.5843′N</td><td align="center" valign="top" charoff="50">25°29.2046′E</td><td align="center" valign="top" charoff="50">26</td><td align="center" valign="top" charoff="50">0.10</td><td align="center" valign="top" charoff="50">0.10</td><td align="center" valign="top" charoff="50">0.9</td><td align="center" valign="top" charoff="50">98.4</td><td align="center" valign="top" charoff="50">7.04</td><td align="center" valign="top" charoff="50">248.3</td><td align="center" valign="top" charoff="50">7.05</td><td align="center" valign="top" charoff="50">0.011</td></tr><tr><td align="left" valign="top" charoff="50">V4</td><td align="center" valign="top" charoff="50">498</td><td align="center" valign="top" charoff="50">36°31.5846′N</td><td align="center" valign="top" charoff="50">25°29.2378′E</td><td align="center" valign="top" charoff="50">22</td><td align="center" valign="top" charoff="50">0.84</td><td align="center" valign="top" charoff="50">1.08</td><td align="center" valign="top" charoff="50">5.0</td><td align="center" valign="top" charoff="50">91.5</td><td align="center" valign="top" charoff="50">6.95</td><td align="center" valign="top" charoff="50">29.1</td><td align="center" valign="top" charoff="50">7.05</td><td align="center" valign="top" charoff="50">0.012</td></tr><tr><td align="left" valign="top" charoff="50">V5</td><td align="center" valign="top" charoff="50">500</td><td align="center" valign="top" charoff="50">36°31.5790′N</td><td align="center" valign="top" charoff="50">25°29.2060′E</td><td align="center" valign="top" charoff="50">25</td><td align="center" valign="top" charoff="50">0.51</td><td align="center" valign="top" charoff="50">0.67</td><td align="center" valign="top" charoff="50">3.1</td><td align="center" valign="top" charoff="50">95.4</td><td align="center" valign="top" charoff="50">7.05</td><td align="center" valign="top" charoff="50">48.0</td><td align="center" valign="top" charoff="50">7.10</td><td align="center" valign="top" charoff="50">0.016</td></tr><tr><td align="left" valign="top" charoff="50">V6</td><td align="center" valign="top" charoff="50">498</td><td align="center" valign="top" charoff="50">36°31.5824′N</td><td align="center" valign="top" charoff="50">25°29.2012′E</td><td align="center" valign="top" charoff="50">19</td><td align="center" valign="top" charoff="50">0.08</td><td align="center" valign="top" charoff="50">0.00</td><td align="center" valign="top" charoff="50">0.6</td><td align="center" valign="top" charoff="50">97.4</td><td align="center" valign="top" charoff="50">7.00</td><td align="center" valign="top" charoff="50">234.4</td><td align="center" valign="top" charoff="50">7.01</td><td align="center" valign="top" charoff="50">0.018</td></tr><tr><td align="left" valign="top" charoff="50">V7</td><td align="center" valign="top" charoff="50">498</td><td align="center" valign="top" charoff="50">36°31.5580′N</td><td align="center" valign="top" charoff="50">25°29.2160′E</td><td align="center" valign="top" charoff="50">41</td><td align="center" valign="top" charoff="50">0.29</td><td align="center" valign="top" charoff="50">0.17</td><td align="center" valign="top" charoff="50">1.3</td><td align="center" valign="top" charoff="50">96.7</td><td align="center" valign="top" charoff="50">6.98</td><td align="center" valign="top" charoff="50">139.4</td><td align="center" valign="top" charoff="50">7.00</td><td align="center" valign="top" charoff="50">0.015</td></tr><tr><td align="left" valign="top" charoff="50">NA007-081<xref ref-type="fn" rid="t1-fn1">*</xref></td><td align="center" valign="top" charoff="50">502</td><td align="center" valign="top" charoff="50">36°31.5735′N</td><td align="center" valign="top" charoff="50">25°29.2034′E</td><td align="center" valign="top" charoff="50">24</td><td align="center" valign="top" charoff="50">0.03</td><td align="center" valign="top" charoff="50">0.01</td><td align="center" valign="top" charoff="50">0.3</td><td align="center" valign="top" charoff="50">99.4</td><td align="center" valign="top" charoff="50">6.84</td><td align="center" valign="top" charoff="50">990.0</td><td align="center" valign="top" charoff="50">6.84</td><td align="center" valign="top" charoff="50">–</td></tr></tbody></table><table-wrap-foot><fn id="t1-fn1"><p><sup>*</sup>Sample from ref. <xref ref-type="bibr" rid="b4">4</xref>.</p></fn></table-wrap-foot></table-wrap></floats-group></pmc></record>Pour manipuler ce document sous Unix (Dilib)
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