Roberto Moretti

Following the 2018–2020 Fani Maoré submarine eruption near Mayotte Island, Indian Ocean, multiple oceanographic expeditions provide unprecedented access to fresh alkaline volcanic glasses spanning basanite to phonolite compositions from the East-Mayotte Volcanic Chain (EMVC). We applied Fe and S K-edge X-ray Absorption Near-Edge Spectroscopy (XANES) to determine iron and sulfur oxidation states in 13 glass samples, providing the first comprehensive redox characterization of this submarine volcanic system. Fe3+/FeTOT ratios range from 0.19 to 0.51, while S6+/STOT ratios span 0.02–0.17, with more evolved compositions that tend to show higher oxidation states. Beam damage monitoring revealed significant photo-oxidation effects on sulfur measurements, requiring analysis of initial spectra only. Comparison of redox estimates from multiple independent oxybarometers based on the Fe3+/FeTOT and S6+/STOT as well as the olivine-spinel-ameltSiO2 reveals systematic discrepancies between Fe-based and S-based estimates, likely due to Fe-Ti nanolite contamination along X-ray beam paths. Integration of S6+/STOT measurements with the olivine-spinel-ameltSiO2 oxybarometer indicates ΔFMQ = +0.3 ± 0.2 for basanite to tephriphonolite magmas, suggesting slightly more oxidized conditions than previously estimated for EMVC. These results provide essential constraints for thermodynamic modeling of alkaline submarine volcanism and highlight the importance of multi-proxy approaches in determining magmatic redox conditions. The oxidation state is consistent with other regional volcanic systems in the SW Indian Ocean, supporting a moderately oxidized mantle source beneath the Comoros archipelago.

The stable chlorine isotopic composition (δ³⁷Cl) of gaseous HCl emitted by arc volcanoes provides insights into chlorine cycling in subduction zones and its interactions with hydrothermal systems. However, few studies have applied δ³⁷Cl data, and no standardized methodology exists for reliable measurements of fumarolic HCl. Here we present the first systematic comparison of two widely used sampling techniques for chlorine in volcanic gases discharged by fumarolic vents: water vapor condensates and NaOH-filled Giggenbach bottles (n = 38 and 42, respectively). Both sample types were collected simultaneously at fumaroles from La Soufrière de Guadeloupe (Lesser Antilles) and Vulcano Island (Aeolian arc, southern Italy).δ³⁷Cl was determined using a protocol ensuring quantitative chlorine recovery as CH3Cl(g) and isotopic conservation. External reproducibility was ±0.07 ‰ (1σ) for NaOH bottles and ±0.04 ‰ for condensates.Samples collected simultaneously at the same fumarole show discrepancies of up to 15 ‰ in δ³⁷Cl and one order of magnitude in Cl/H₂O ratios. Across both volcanoes, NaOH bottles generally yield higher δ³⁷Cl values and lower Cl/H₂O ratios than corresponding condensates. At La Soufrière, these discrepancies follow an open system trend with ΔHCl(g)-Cl-(aq)=1.5 ‰, consistent with partial dissolution of HCl(g) into condensed vapor within the sampling line, likely enhanced during NaOH bottle sampling by lower gas flux and heat loss. A relatively large range Cl/H2O ratios shown by data of samples from NaOH bottles at Vulcano likely reflect combined effects of condensation and HCl dissolution in the line, coupled with possible Cl₂(g) in fumarolic gases, which dissolves in NaOH but not in condensates. We therefore suggest to prevent vapor condensation as much as possible during NaOH bottles sampling.

Studying the behavior of dormant volcanoes is a major challenge in volcanology. Here, we conducted a 10‐year survey of hydrothermal mineralization at Campi Flegrei, an active volcano near Naples in Italy, which last erupted in 1538. Our survey revealed a significant output of boric acid at the hydrothermal sulfate discharge area of Pisciarelli, coinciding with a heightened volcanic unrest (so‐called bradyseisms). This unexpected output shyly began in 2019 before the vanishing of permanent hydrothermal springs and mud‐pools, in the form of transparent, colorless and glassy luster sassolite crystals found within the commonest mineralogical assemblage. High‐boron emissions peaked in July–August 2021, by spraying‐out whitish fine powders rich in sassolite, which has remained a prevalent feature through 2024 while we also measured a decline in NH ₄ ‐sulfates and a tenfold increase in mud boron content. Sassolite appearance parallels the increased H ₂ S flux at fumaroles, both marking the onset of recent acceleration in ground displacement and the accompanying increasing seismicity. Using new and literature mineral, chemical and isotope (δ ¹¹ B) data, we ascribe sassolite yields to the direct drainage of boron‐rich brines that ascend more efficiently from the deeper to the surface pathways of the geothermal system via fracturing activated by the escalating bradyseism. The shift from closed to open degassing may explain the appearance of sassolite, making it valuable for unraveling the unrest dynamics at Campi Flegrei. Additionally, our findings enlarge present views on traditional pegmatites associated with tourmaline systems.

Volcano observatories (VOs) around the world are required to maintain surveillance of their volcanoes and inform civil protection and aviation authorities about impending eruptions. They often work through consolidated procedures to respond to volcanic crises in a timely manner and provide a service to the community aimed at reducing the potential impact of an eruption. Within the International Airways Volcano Watch (IAVW) framework of the International Civil Aviation Organisation (ICAO), designated State Volcano Observatories (SVOs) are asked to operate a colour coded system designed to inform the aviation community about the status of a volcano and the expected threats associated. Despite the IAVW documentation defining the different colour-coded levels, operating the aviation colour code in a standardised way is not easy, as sometimes, different SVOs adopt different strategies on how, when, and why to change it. Following two European VOs and Volcanic Ash Advisory Centres (VAACs) workshops, the European VOs agreed to present an overview on how they operate the aviation colour code. The comparative analysis presented here reveals that not all VOs in Europe use this system as part of their operational response, mainly because of a lack of volcanic eruptions since the aviation colour code was officially established, or the absence of a formal designation as an SVO. We also note that the VOs that do regularly use aviation colour code operate it differently depending on the frequency and styles of eruptions, the historical eruptive activity, the nature of the unrest, the monitoring level, institutional norms, previous experiences, and on the agreement they may have with the local Air Transport Navigation providers. This study shows that even though the aviation colour code system was designed to provide a standard, its usage strongly depends on the institutional subjectivity in responding to volcano emergencies. Some common questions have been identified across the different (S)VOs that will need to be addressed by ICAO to have a more harmonised approach and usage of the aviation colour code.

Passive volcanic degassing results in the emission of toxic gases such as H2S at quasi-steady rates over long periods of time that pose a significant hazard to human health even in low gas concentrations. Currently, La Soufrière de Guadeloupe has one of the highest gas emission rates in the Lesser Antilles arc, with gas emitted mainly from low-temperature fumaroles. In this study, gas dispersion from the volcano between 2016–2021 was modelled using a numerical code that takes into account wind and atmospheric data, topography and gas flux measurements. We ran c.100 individual simulations of the most frequently observed wind and gas flux conditions using a Monte-Carlo scheme. Our results, validated using air-quality measurements and citizen-science surveys, show that the most exposed zones are the hamlet of Matouba and the upper St. Claude. These areas have 20% and 5% probability, respectively, of exceeding H2S guidelines for long-term gas exposure (70 ppb).