New insights of the volcanic gas signature of the Central American Volcanic Arc

Volcanic gas emissions carry crucial information on pre- and syn-eruptive processes, and on behaviour of active volcanic systems. Gas variations arise from replenishment of magma storage zones with mafic magma, from magma ascent and evolution, and from interaction with hydrothermal systems and volcanic lakes. As such, volcanic gases represent “open windows” into genesis and release of volatiles from the Earth’s interior. Volcanic emission measurements allow understanding subsurface magmatic and hydrothermal processes, and contribute to eruption forecasting. Carbon dioxide and sulfur gas represent the most abundant and studied gas species. In particular, CO2, due his fast exsolution during magma decompression, represents an important parameter for volcanic hazard assessment. In addition, due to his greenhouse effect, quantification of CO2 budget from natural emissions at global scale represents a key scientific challenge. Indeed, active volcanic systems are able to release considerable quantities of gas. Volcanic arcs, in particular, release large amounts of CO2 due to carbon recycling from subducted sediments and crust, and due to fluid addition from the mantle wedge. Understanding and modelling these processes allow forecasting volcanic eruptions, quantifying the volcanos’ total volatile budgets, and contributing to climate change modelling. In the past years, the scientific community has made efforts in order to discriminate the role and contribution of subducted slab sediments and altered oceanic crust on the geochemistry of magmas in volcanic arcs worldwide. New geochemical tracers and isotopes, in tandem with other volcanological data, has allowed characterizing the mantle source. In spite of the efforts made, many active volcanic systems are still poorly studied and monitored, and an improved knowledge on dynamics of volcanic degassing and in global gas budgets are needed, with particular attention to carbon dioxide. Technological progress has brought significant advances in geochemistry. Unlike the past, new instrument allow to continuously monitoring volcanic gas, reducing drastically the risks for the operators. In this study (chapter 4, 5, 6), I took advantage of the recent advent of new techniques such as the Multi-GAS (Multi-component gas analyser system) and the Ultraviolet dual camera instruments. The Multi-GAS can perform gas measurements in a fully automatic way, while UV cameras allow measuring SO2 fluxes with high spatial/temporal resolution. Combining gas chemical composition (Multi-GAS) with SO2 flux (UV dual camera), it is possible to estimate the gas fluxes of other gas species. My PhD focused on characterizing gas compositions and fluxes from three most active volcanoes in the Central America Volcanic Arc (CAVA): Pacaya (Guatemala), Masaya (Nicaragua), and Rincón de la Vieja (Costa Rica). At Pacaya (Guatemala), I used a multi-disciplinary approach, combining geochemistry of gas and rocks, in the attempt to gain insights into the magmatic gas signature, on the volatile budget, and for characterizing the mantle source. Plume compositional data suggest Pacaya exhibits a H2O-poor (for an arc volcano) composition (80.5 mol. %), with a characteristic magmatic CO2/St ratio of ~1.0 to 1.5. Both the H2O-poor and low CO2/St ratio composition concur to suggest a limited slab-fluid contribution (at least compared to other volcanoes/arc segments; Aiuppa et al., 2017), and a dominant mantle-wedge derivation of the emitted volatiles. The 3He/4He ratios measured in fluid inclusions hosted in olivines (Ra) are the highest values in CAVA volcanism. This strongly supports that the mantle source beneath Pacaya lacks of any contamination of radiogenic 4He from the slab or the crust. At Rincón de la Vieja (Costa Rica), I combined gas geochemistry with volcano seismicity in order to derive constraints on the triggers of phreatic/phreatomagmatic explosions. For the first time, the composition of the gas released during discrete phreatic events (confirmed seismically) was resolved using Multi-GAS. The results demonstrate chemically distinct gas compositions during quiescent degassing versus explosive eruptive degassing. These results confirm (Christenson and Tassi, 2015) that the complex interplay between rising magmatic gases and sublimnic hydrothermal systems likely play a decisive role as eruption triggers. On Masaya (Nicaragua), I combined ground-based gas measurements and space-based thermal data in order to evaluate changes in gas composition associated with resumption of lava lake activity in 2015, and to derive novel information on the driving magmatic processes. Our results show that appearance of the lava lake in December 2015 was anticipated in mid- to late-November by a noticeable volcanic gas plume compositional change toward more CO2-rich compositions, and by a sizeable CO2 flux increase. Moreover, in view of the paucity of helium isotopes data for the northern part of CAVA, I collected rock samples on Pacaya and in 3 volcanoes in El Salvador (Santa Ana, San Miguel and Ilopango volcanic systems) to performe noble gas isotopes measurements in fluid inclusions. The data obtained here refine previous results obtained for the CAVA arc and suggest that 3He/4He along CAVA shows a regional trend as observed for other geochemical tracers (e.g., Ba/La, C/S, see chapter 7). The general of this work is to expand gas knowledge on poorly studied volcanic systems, improve the geochemical database of Central America, and contribute to a better understanding of magmatic and geochemical processes in convergent margins.