Abstract

Closed-conduit volcanoes develop hydrothermal aquifers where infiltrating waters dissolve magmatic gases such as SO2, HCl, and HF. The extent of this "scrubbing effect" is traditionally tracked using CO2/SO2 or CO2/STotal, but these ratios can also be modified by secondary processes. Given the high solubility of gaseous HCl(g) into aqueous water, we assess here chlorine isotopic compositions (delta 37Cl) in fumarolic HCl(g) as a potential quantitative index of scrubbing. We present three years and a half (January 2018 to May 2021) of delta 37Cl and chlorine concentrations [Cl] data from fumarolic gases at La Soufrie`re de Guadeloupe, alongside traditional geochemical tracers in volcanology. Our data show strong negative correlation between delta 37Cl and [Cl] values, suggesting variable extents of HCl dissolution into the hydrothermal aquifer through time. High delta 37Cl values associated to low [Cl] indicate intense 35Cl loss into the aquifer, while low delta 37Cl values with high [Cl] likely reflect more pristine magmatic signatures with minimal extents of scrubbing. The observed delta 37Cl-[Cl] trends suggest Cl isotopic fractionation factors between gas and liquid Delta g-l varying between +2.5 and +5 parts per thousand. These values are higher than expected at equilibrium, suggesting kinetic fractionations likely resulting from rapid gas flow (preventing isotopic equilibration with liquid water). The fact that comparable delta 37Cl-[Cl] trends are found at Poas (Costa Rica) and Vulcano (Italy) suggests that such kinetic fractionations during HCl scrubbing may be widespread. Comparisons between delta 37Cl and CO2/CH4 ratios and the modeled groundwater level within the dome suggest that variations in scrubbing at La Soufrie`re are primarily driven by varying magmatic inputs. The progressive decrease in delta 37Cl, associated with increasing [Cl] from 2019 to 2021, likely reflects long-term aquifer drying, predating the observed rise in fumarolic temperatures above 150 degrees C in mid-2023.