Grande Comore and Mayotte gas-geochemistry and evidence of deep fluid migration during the 2018-2020 submarine eruption off Mayotte

<p>Located within the Mozambique Channel, the Comoros archipelago is situated within a complex geodynamic system of great interest owing to recent volcanic and seismic activity (2018-20), but where little gas geochemistry research has been conducted.</p><p>Focusing on Grande Comore and Petite Terre, a small islet off the northeast coast of Mayotte, our investigations set out to identify the gas-geochemistry characteristics of the islands, and explore any potential influence from the then ongoing unrest and/or volcanic activity.</p><p>Geochemical surveys included measurements of soil CO₂ flux on both islands, and gas sampling from fumarolic areas at Karthala volcano (Grande Comore) and two bubbling areas at Petite Terre, with the aim of determining the chemical and isotopic characteristics of the main gases (CO₂, CH₄, He, Ne, Ar) and equilibrium temperatures of the hydrothermal system at Petite Terre.</p><p>&#948;¹³C values of soil CO₂ emissions highlight evidence of a low magmatic contribution at Grande Comore, while a significantly higher contribution is evident at Petite Terre. ³He/⁴He data are consistent with average values of fluid inclusions for both Grande Comore and Petite Terre rocks, and are fixed at low value ranges (4.7&#8804;Rc/Ra&#8804;5.9 and 5.3&#8804;Rc/Ra&#8804;7.5 respectively). The gases detected at the two sites of Petite Terre primarily reflect the signature of deep gases in terms of geochemical tracers such as R/Ra and &#948;¹³C in CO_2.&#160;At one of the two emission sites at Petite Terre, namely the meromictic lake Dziani Dzaha, the gases are relatively more variable in relative proportion of CO₂, CH₄&#160; and C isotopes; at this specific site, a significant influence from microbial activity is evidenced.</p><p>Our results allow us to infer that the general degassing characteristics between the two islands are similar. They also shed light on their reciprocal differences, which may either be attributable to local specifics within Petite Terre, or to different states of volcanic activity between Grande Comore and Petite Terre at the time of the surveys, the latter being a consequence of fluid migration to the mainland of Mayotte during the offshore submarine activity (2018-20).</p><p>The outcomes of this work provide a necessary step towards filling gaps in the knowledge of gas-geochemistry in Comoros, and contribute potential support for volcanic and environmental monitoring programmes.</p>