AUTHOR=Tsunogai Urumu , Shingubara Ryo , Morishita Yuhei , Ito Masanori , Nakagawa Fumiko , Yoshikawa Shin , Utsugi Mitsuru , Yokoo Akihiko 

TITLE=Sampling Volcanic Plume Using a Drone-Borne SelPS for Remotely Determined Stable Isotopic Compositions of Fumarolic Carbon Dioxide

JOURNAL=Frontiers in Earth Science

VOLUME=10

YEAR=2022

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2022.833733

DOI=10.3389/feart.2022.833733

ISSN=2296-6463

ABSTRACT=<p>Both chemical and isotopic compositions of concentrated volcanic plumes are highly useful in evaluating the present status of active volcanoes. Monitoring their temporal changes is useful for forecasting volcanic eruptions as well. Recently, we developed a drone-borne automatic volcanic plume sampler, called SelPS, wherein an output signal from a sulfur dioxide (SO<sub>2</sub>) sensor triggered a pump to collect plume samples when the SO<sub>2</sub> concentration exceeded a predefined threshold. In this study, we added a radio transmission function to the sampler, which enabled our operator to monitor real-time SO<sub>2</sub> concentration during flights and thus obtain more concentrated volcanic plume samples through precise adjustment of the hovering position. We attached the improved SelPS to a drone at Nakadake crater, Aso volcano (Japan), and successfully obtained volcanic plume samples ejected from the crater more concentrated than those obtained by using previous version of SelPS in 2019. Additionally, we found a significant linear correlation between the reciprocal of the concentration and isotopic ratios for the <sup>2</sup>H/<sup>1</sup>H ratios of H<sub>2</sub>, <sup>18</sup>O/<sup>16</sup>O ratios of CO<sub>2</sub>, and <sup>13</sup>C/<sup>12</sup>C ratios of CO<sub>2</sub> within the plume samples. Based on the isotopic ratios of fumarolic H<sub>2</sub> (<italic>δ</italic><sup>2</sup>H = −239 ± 6‰) and fumarolic CO<sub>2</sub> (<italic>δ</italic><sup>13</sup>C = −3.58 ± 0.85‰ and <italic>δ</italic><sup>18</sup>O = +22.01 ± 0.68‰) determined from the linear correlations, we estimated the apparent equilibrium temperatures (AETs) with magmatic H<sub>2</sub>O simultaneously and precisely for the first time in erupting volcanoes, assuming hydrogen isotope exchange equilibrium between H<sub>2</sub> and H<sub>2</sub>O (AET<sub>D</sub> = 629 ± 32°C) and oxygen isotope exchange equilibrium between CO<sub>2</sub> and H<sub>2</sub>O (AET<sub>18O</sub> = 266 ± 65°C). We found that the AET<sub>18O</sub> was significantly lower than the AET<sub>D</sub> in the crater. While the temperature of the magmatic gases was originally 600°C or more, most of the gases cooled just beneath the crater to temperatures around the boiling point of water. The improved SelPS enable us to determine both AET<sub>D</sub> and AET<sub>18O</sub> in eruptive volcanoes, wherein fumaroles are inaccessible. Simultaneous and precise determination of both the AET<sub>18O</sub> and AET<sub>D</sub> can provide novel information on each volcano, such as the physicochemical conditions of magma degassing and the development of fluid circulation systems beneath each volcano.</p>