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.

A sharp increase in volatiles, especially SO2 fluxes from the solfataric plume and diffuse CO2 from the soils of the La Fossa crater area, started in June 2021, and subsequently from the Levante Bay area, suggests renewed unrest at Vulcano Island, Italy. This event has encouraged monitoring activities and stimulated new research activities aimed at understanding the recent evolution of the volcanic system. In this study, the chemical and isotopic composition of fumaroles, thermal waters, and soil gases from the main degassing areas of Vulcano Island with a special focus on sulfur isotopes, are used to investigate the fluid transfer mechanism inside the volcano. Sulfur is one of the most abundant volatile elements present in magmas and volcanic fluids from the La Fossa crater, where it mostly occurs as SO2 and H2S at variable relative concentrations depending on oxygen fugacity and temperature. The isotope composition and the chemical ratio of sulfur species depict a complex hydrothermal-magmatic system. In addition, we utilize the installed SO2 monitoring network that measures the total outgassing of SO2 with the UV-scanning DOAS technique. The SO2 fluxes from the La Fossa crater fumaroles, coupled with the SO2/CO2 and SO2/H2O ratios, were measured to evaluate the total mass of fluids emitted by the shallow plumbing system and its relationship with the status of volcanic activity. Combining the whole chemical composition of fumaroles analyzed with a discrete, direct sampling of high-temperature fumaroles located on the crater summit, the output of discharged water vapor has been estimated (5,768 t·d−1). On the basis of the water output, we estimated the total thermal energy dissipated by the crater during the last enhanced degassing activity (167 MW). This strong and sharp increase in energy observed during the current crisis confirms the long-growing trend in terms of mass and energy recorded in recent decades, which has brought the surface system of Vulcano Island to a critical level that has never been recorded since the last eruptive event of 1888–91.

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.

Since 2016, a UV camera records images of the Popocatépetl Volcano for continuous monitoring of the SO2 plume. Five band-pass filters (300a, 300b, 307, 310, 330) with the different bandwidth (BW) of 10 and 25nm with transmissions centered at 300nm (BW=10) nm), 300nm (BW=25 nm), 307nm (BW=25 nm), 310nm (BW=10 nm)and 330nm (BW=10 nm) are used for the analysis. In order to fully characterize the filters and improve our knowledge of the errors affecting the results, the UV camera was used together with a UV spectrometer for experiments and measurements. Gas cells with different concentrations were prepared to simulate field conditions and to calibrate the SO2 column images when measuring from a fixed site. Our study is focused on a) applying the required corrections due to vignetting by the instrumentation as well as flattening the light intensity gradients in the field of view, b) determining the effect on the results when using different interference filters, c) applying a simplified light dilution correction and d) performing an a posteriori calibration correction using four interference filters. Our results show that a permanently installed and well-characterized camera, even with the rather long distance from the crater (11.3 km), can provide valuable and reliable information of the SO2 emissions from the Popocatépetl Volcano.