Schiavo, Benedetto

Volcanic lakes are complex natural systems and their chemical composition is related to a myriad of processes. The chemical composition of major, minor, Rare Earth Elements (REE) and physico-chemical parameters at the hyperacid crater lake of Rincón de la Vieja volcano (Costa Rica) are here investigated during February 2013–August 2014. The study of the lake chemical composition allows to identify the main geochemical processes occurring in the lake and to track the changes in the volcanic activity, both important for active volcanoes monitoring. The total REE concentration ( REE) dissolved in the crater lake varies from 2.7 to 3.6 mg kg−1 during the period of observation. REE in the water lake samples normalized to the average volcanic local rock (REEN-local rock) are depleted in light REE (LREE). On the contrary REEN-local rock in the solids precipitated (mainly gypsum/anhydrite), from lake water samples in laboratory at 22°C, are enriched in LREE. The low variability of (La/Pr)N-local rock and (LREE/ HREE)N-local rock ratios (0.92–1.07 and 0.66–0.81, respectively) in crater lake waters is consistent with the low phreatic activity (less than 10 phreatic eruptions in 2 years) observed during the period of observation. This period of low activity precedes the unrest started in 2015, thus, it could be considered as a pre-unrest, characterized by infrequent phreatic eruptions. No clear changes in the REE chemistry are associated with the phreatic eruption occurred at mid- 2013. The results obtained investigating water-rock interaction processes at theRincón de la Vieja crater lake show that rock dissolution and mineral precipitation/ dissolution are the main processes that control the variability of cations composition over time. In particular, precipitation and dissolution of gypsum and alunite are responsible for the variations of REE in the waters. Despite the low variations of (La/Pr)N-local rock and (LREE/HREE)N-local rock ratios, this study allows to suggest that REE can be used, together with major elements, as practical tracers of water-rock interaction processes and mineral precipitation/ dissolution at active hyperacid crater lakes over time, also during periods of quiescence and low phreatic activity.

The volatiles released by the volcanic structures of the world contribute to natural environmental pollution both during the passive and active degassing stages. The Island of Vulcano is characterized by solfataric degassing mainly localized in the summit part (Fossa crater) and in the peripheral part in the Levante Bay. The normal solfataric degassing (high-temperature fumarolic area of the summit and boiling fluids emitted in the Levante Bay area), established after the last explosive eruption of 1888–90, is periodically interrupted by geochemical crises characterized by anomalous degassing that are attributable to increased volcanic inputs, which determine a sharp increase in the degassing rate. In this work, we have used the data acquired from the INGV (Istituto Nazionale di Geofisica e Vulcanologia) geochemical monitoring networks to identify, evaluate, and monitor the geochemical variations of the extensive parameters, such as the SO2 flux from the volcanic plume (solfataric cloud) and the CO2 flux from the soil in the summit area outside the fumaroles areas. The increase in the flux of volatiles started in June–July 2021 and reached its maximum in November of the same year. In particular, the mean monthly flux of SO2 plume of 22 tons day−1 (t d−1) and of CO2 from the soil of 1570 grams per square meter per day (g m2 d −1) increased during this event up to 89 t d−1 and 11,596 g m2 d −1, respectively, in November 2021. The average annual baseline value of SO2 output was estimated at 7700 t d−1 during normal solfataric activity. Instead, this outgassing increased to 18,000 and 24,000 t d−1 in 2021 and 2022, respectively, indicating that the system is still in an anomalous phase of outgassing and shows no signs of returning to the pre-crisis baseline values. In fact, in the first quarter of 2023, the SO2 output shows average values comparable to those emitted in 2022. Finally, the dispersion maps of SO2 on the island of Vulcano have been produced and have indicated that the areas close to the fumarolic source are characterized by concentrations of SO2 in the atmosphere higher than those permitted by European legislation (40 µg m−3 for 24 h of exposition) on human health.