Temporal monitoring of fumarole composition at Santorini volcano (Greece) highlights a quiescent state after the 2011-2012 unrest Provisionally Accepted

Alexis Bernard^1* Anne Battani¹ Andrea L. Rizzo^{2, 3} Finlay Stuart^4* Uğur Balci⁴ Domokos Györe^{4, 5} Walter D'Alessandro⁶ Jean-Paul Callot¹ Konstantinos Kyriakopoulos⁷ Magali Pujol⁸

• ¹UMR5150 Laboratoire des Fluides Complexes et Leurs Reservoirs (LFCR), France
• ²Department of Environmental and Earth Sciences, School of Science, University of Milano-Bicocca, Italy
• ³Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Milano, Milano, Italy., Italy
• ⁴Scottish Universities Environmental Research Centre, University of Glasgow, United Kingdom
• ⁵Isomass Scientific Inc., Calgary, Alberta, Canada., Canada
• ⁶Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy., Italy
• ⁷Faculty of Geology and Geoenvironment, School of Science, National and Kapodistrian University of Athens, Greece
• ⁸CSTJF, Avenue Larribau, TotalEnergie, France

Santorini Island (Greece) is an active volcano which has alternated between dormant and active periods over the last 650,000 years with the latest volcanic disturbances occurring in 2011-2012. Here we report a geochemical survey of fumarolic gases collected at Nea Kameni islet located in the center of the caldera over the period 2015-2022 in order to study the activity of the volcano and changes in hydrothermal conditions. This period is marked by the absence of significant geochemical anomalies compared to the volcanic unrest of 2011-2012, implying that no new magma upwelling has occurred. This is evident from the low CO2/CH4 ratio and H2 concentration of fumaroles. An increase of the atmospheric contribution in gases after the 2011-2012 unrest suggests a decrease of the deep gas flow and the chemical and C-He-isotope compositions are compatible with a model of Rayleigh fractionation in which CO2 2 dissolves in water at decreasing temperatures over time. These results are consistent with temperature estimates obtained using the H2/N2 geothermometer, seismic and geodetic evidences. This implies a slowing of the degassing of the hydrothermal/volcanic system and a cooling of the magma injected at shallow depth in 2011-2012.