| Summary: | The Fangaia mud pool provides a “window” into the hydrothermal system underlying the degassing Solfatara crater, which is the most active volcanic centre inside the restless Campi Flegrei caldera, Southern Italy. The present study aimed at unravelling the degassing dynamics of CO<sub>2</sub> and H<sub>2</sub>S flushing through the pH 1.2 steam-heated Fangaia mud pool, an ideal field laboratory as a proxy of an active crater lake. Our results from MultiGAS measurements above Fangaia’s surface show that H<sub>2</sub>S scrubbing, demonstrated by high CO<sub>2</sub>/H<sub>2</sub>S ratios, was most efficient in the portions of the basin affected by diffusive degassing. Convective bubbling degassing instead was the most effective mechanism to release gas in quantitative terms, with lower CO<sub>2</sub>/H<sub>2</sub>S ratios, similar to the Solfatara crater fumaroles, the high-T end member of the hydrothermal system. Unsurprisingly, total estimated CO<sub>2</sub> and H<sub>2</sub>S fluxes from the small Fangaia pool (~184 m<sup>2</sup> in June 2017) were at least two orders of magnitude lower (CO<sub>2</sub> flux < 64 t/d, H<sub>2</sub>S flux < 0.5 t/d) than the total CO<sub>2</sub> flux of the Campi Flegrei caldera (up to 3000 t/d for CO<sub>2</sub>), too low to affect the gas budget for the caldera, and hence volcano monitoring routines. Given the role of the rising gas as “sediment stirrer”, the physical and chemical processes behind gas migration through a mud pool are arguably the creating processes giving origin to Fangaia. Follow-up studies of this so far unique campaign will help to better understand the fast dynamics of this peculiar degassing feature.
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