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At volcanoes in unrest, the interpretation of geochemical time-series is a major issue for decrypting volcano dynamics and forecast eruptive scenarios. However, interpretation cannot be purely observational and demands the assessment of the main physicochemical features of the hydrothermal system. In the case of La Soufrière of Guadeloupe (FWI) andesitic volcano, a careful analysis of different techniques adopted historically for gas sampling and analysis by the local observatory has allowed us to model degassing and assess gas indicators from non-condensable species in the H2-N2-CH4-He-Ar system available since 2006. Here we report on the nature of discharged gases, resulting from the mixing of atmospheric component and a magmatic-hydrothermal gas evolving along a lineage connecting MORB-like upper mantle and arc-volcano components. We show that along this lineage we can track the hydrothermal build-up of pressure and temperature modulated by magmatic variations, particularly decompression. A careful analysis of inert gas fractionation allows recognizing two main regimes: one is about hydrothermal degassing conditions perturbed by the deep impulsive gas infiltration after magma refilling in a 4 to 8 km deep chamber; the other is determined by ascent of magma batches to a shallower (about 3 km deep) chamber. Further changes of the bulk permeability structure in the hydrothermal reservoir due to fracture sealing and clogging effect may exacerbate observed evolutions but do not represent the primary control of the degassing process. We also show that gas ratios in the H2-He-CH4 subsystem can effectively discriminate and anticipate such tendencies and, particularly, they can be turned into reliable precursors of magma-derived solicitations and set possible thresholds for next crises. The main test is made with reference to the 2013-2014 and 2018 episodes of accelerated unrest: we confirm that the latter is as an aborted phreatic eruption, triggered by the injection of hot magmatic fluids into the magmatic system. On the other hand, for the 2013-2014 period, poorly studied, we document for the very first time the ascent of a small batch of magma which refilled the 3 km deep shallow magma chamber. This triggered seismicity just on top of the brittle-ductile transition. Besides, our method reveals that in 2007-09 an unrest phase similar to the 2018 one occurred, although not marked by the same seismic activity likely because the volcanic system was more sealed and less fractured before the magmatic upward excursion of the 2013-14 phase. Our results and conclusions are suitable for all those volcanic systems at the hydrothermal stage and allow a better definition of unrest scenarios whenever sampling frequency of fumarolic fluids is compatible with the expected transit times of magmatic fluids from magma chambers to surface.

Passive volcanic degassing often occurs at active but not actively erupting volcanoes. Gases are emitted by fumaroles and through diffuse soil degassing. This results in the emission of toxic gases such as H2S at quasi-steady rates over long periods of time (months to years). Whilst less apparent than gas emissions during more vigorous and violent paroxysms, the long duration of emission and the fact that such gases are dispersed at low altitude (i.e. along the flanks of the volcano, often at human height) means that even the typically low concentrations (e.g. tens of ppb H2S) pose a significant hazard to human health in nearby habitations.La Soufrière de Guadeloupe has been undergoing an unrest phase since 1992 and it has one of the highest gas emission rates of any volcano in the Lesser Antilles. Gas emissions here are principally from three fumarolic sites at the summit though the typically high winds and low gas temperatures result in a laterally dispersed plume within a few metres of the ground. In this study, gas dispersion from the volcano over the period 2016–2021 was modelled using a numerical code that takes into account wind direction and strength, atmospheric stability, local topography and gas flux measurements; we used information from local meteorological stations, ECMWF Climate Reanalysis data and the gas flux dataset acquired by MultiGas measurements during the mentioned period. We ran c.100 individual simulations of the most frequently observed wind and gas flux conditions using a Monte-Carlo scheme. From the ensemble of results, we calculated the mean (i.e. most probable gas concentration values at any given location) and found that the most exposed zones are the hamlet of Matouba and the upper parts of St. Claude. We also simulated particular dates with strong H2S odours reported by local inhabitants via online surveys and we compared the model results with the Gwad’air agency’s air quality station located at St Claude, the closest town to the volcano. This allowed us to establish the prevalence of gases coming from La Soufrière in nearby cities and the accuracy of our models. The resulting maps of the areas potentially long-term (>8 years) exposed by gas emissions can be used to evaluate health risks for all people living around the volcano. In these locations, our results suggest that there is a 20% and 5% probability, respectively, for these areas of exceeding H2S guidelines for long-term gas exposure (70 ppb).

Le dégazage magmatique est l’une des principales manifestations de l’activité volcanique, avec des effets sur l'atmosphère terrestre. C'est un phénomène complexe car il dépend de la composition des magmas et de la solubilité des différentes espèces de gaz. La surveillance des gaz apporte des informations sur les systèmes magmatiques en profondeur et sur leur évolution. Ce chapitre décrit la composition, le comportement et les méthodes d'étude et de surveillance des gaz volcaniques.

La surveillance par télédétection en champ rapproché (sol, drone …) partage de nombreux points communs avec la télédétection spatiale, traitée dans un autre chapitre. Mais elle offre aussi une gamme d'approches plus riche en raison de la plus grande facilité de mise en œuvre et d'expérimentation et elle permet d'atteindre des résolutions très fines. Ce chapitre fait le point sur les principales méthodes actuelles de la télédétection en champ proche.

Signals of volcanic unrest have been used successfully to provide insights into the timing, magnitude, intensity, and style of future eruptions. However, in order to provide context for the subsequent activity, analysis of past eruptions is required. This provides useful information in order to understand processes of magma genesis, storage, evolution and ascent which lead to the onset of eruptions. Here, we examine basaltic-andesitic to andesitic deposits from La Soufrière de Guadeloupe Holocene eruptions, covering a range of explosive eruption styles, ages and magnitudes. Our work is timely given unrest at this system has increased over the last 25 years, with a potential eruption capable of directly impacting up to 80,000 people in Southern Basse-Terre and potentially thousands more indirectly on a regional scale. We report on the geochemistry of pre-eruptive magmas using detailed analyses of glass (melt inclusions and groundmass glass) from four Holocene explosive eruptions: 1657 Cal. CE (Vulcanian, VEI 2), 1530 Cal. CE (sub-Plinian, VEI 3), 1010 Cal. CE (Plinian, VEI 4), and 5680 Cal. BCE (Plinian, VEI 4). Major element concentrations vs SiO2 in whole rock (WR), groundmass glass (GM) and melt inclusions (MI) show a strong linear trend. MIs reveal a relatively homogenous melt composition from the first to the most recent eruptions, ranging from 63.6–78.7 wt% SiO2. Volatiles, including H2O (2.3–4.4 wt%), CO2 (35–866 ppm) and sulphur (30–202 ppm), are also consistent across the various eruptions. The major element and volatile compositional homogeneity across the eruptions indicates that composition and volatiles do not have a direct control on eruption explosivity at this system. Instead, we find differences in ascent rate, groundmass glass viscosity and microlite volume percentage, indicating that explosive eruptive style at La Soufrière is controlled by a combination of ascent rate and top-down controls affecting rock strength, stress distribution and the development of fluid overpressure. Rapid ascent in the absence of top-down controls (processes with a cause external to the magma but affecting the plumbing system) will result in explosive eruptions driven from the bottom-up (internal to magma dynamic response with varying pressure and temperature, e.g., 1010 Cal. CE in the case of very rapid ascent or 1657 Cal. CE in the case of rapid ascent). However, we also highlight the importance of top-down controls, such as conduit sealing which can promote the onset of explosive eruptions, even in the case of slow magma ascent (e.g., 5680 Cal. BCE). External effects (including ingress of water and rapid edifice unloading) can also favour explosive eruptions with flank collapses involved in some scenarios (e.g., 1530 Cal. CE). The multiple controls on explosive eruption style make this system more hazardous and complex to model and monitor. In order to improve early-warning system efficiency, forecast models, eruption scenario crisis response and long-term risk reduction planning, we stress that internal processes such as fracture and host-rock sealing (fluid pore pressure) as well as external processes such as water moving into the system and the mechanical stability of the edifice should be monitored and modelled closely.

International audience; The geochemical behavior of major and trace elements in fumarolic gas condensate samples collected between April 2017 and January 2021 was investigated at La Soufriere hydrothermal volcano. Samples collected from Cratère Sud and Napoleon Nord fumaroles (temperature varying from 93.9 to 108.4 °C) offer a unique opportunity to reveal the physicochemical processes that affect the deeper magmatic and shallow hydrothermal systems feeding fumaroles. Gas condensate samples exhibit abundant solid particles that suggest strong water-rock interaction and erosive processes in fumarolic conduits at La Soufriere. These particles were incorporated during sampling, except for rare Si-rich spherules that appeared to have precipitated during storage time. Gas condensate samples displayed abundant concentrations of S, halogens (Cl, Br, I), metals/metalloids (B, Ni, Cu, Zn, As, Mo, Ag, Cd, Sb, Pb, and Bi), as well as enhanced concentrations of rock-related lithophile and siderophile elements. The relative abundance of major and trace elements in gas condensates suggests the permanent contribution of volatile-rich magmatic fluids and strong water-rock interaction processes. Based on elemental correlations and the physical properties of chemical elements, it is postulated that most of them are discharged in the aerosol phase, except for the halogens and S, which can be found in the gaseous phase in low-temperature fumarolic emissions. Metals and metalloids (Sb, Bi, Zn, Mo, Ni, Ag, Cd, As, and Pb) enriched in gas condensates were transported within tiny aqueous droplets, while rock-related elements (including REEs) were transported in the aerosol phase. Trace element enrichments in La Soufriere's condensates are comparable with those found in worldwide magmatic-hydrothermal systems such as Kawaj Ijen (Indonesia), Lastarria (Chile), and White Island (New Zealand) volcanoes. The size of the hydrothermal system and the degree of water-rock interaction are thought to be important parameters in controlling the abundance of major and trace elements in low-temperature fumarolic discharge.

International audience; The volcanic-hydrothermal geo-diversity of the Basse-Terre Island of Guadeloupe archipelago (Eastern Caribbean, France) is a major asset of the Caribbean bio-geoheritage. In this paper, we use Guadeloupe as a representative of many small island developing states (SIDS), to show that the volcanic-hydrothermal geodiversity is a major resource and strategic thread for resilience and sustainability. These latter are related to the specific richness of Guadeloupe’s volcanic-geothermal diversity, which is de facto inalienable even in the wake of climate change and natural risks that are responsible for this diversity, i.e., volcanic eruptions. We propose the interweaving the specificity of volcanic-geothermal diversity into planning initiatives for resilience and sustainability. Among these initiatives research and development programs focused on the knowledge of geodiversity, biodiversity and related resources and risks are central for the long-term management of the water resource, lato sensu. Such a management should include a comprehensive scientific observatory for the characterization, exploration, and sustainable exploitation of the volcanic-hydrothermal geodiversity alongside planning for and mitigating geophysical risks related to sudden volcanic-induced phenomena and long-term systemic drifts due to climate change. The results of this exercise for Guadeloupe could typify innovative paths for similar SIDS around their own volcanic-hydrothermal geodiversity.

Whakaari is New Zealand’s most active volcano, and its last major magmatic eruption occurred during a protracted episode between 1976 and 2000. This eruption episode involved a sustained phase of heightened unrest, punctuated by regular, and small-volume, phreatic, magmatic and phreatomagmatic eruptions. We examine the scoria sample suite erupted throughout this period to develop a model for the storage conditions and the underlying cause of this long-lived activity. From our analysis of the major element composition of minerals, groundmass glass and phenocryst-hosted melt inclusions, along with the volatile content of melt inclusions, Whakaari magmas are relatively hot and dry arc andesites when compared globally. Based on a combination of plagioclase hygrometry, volatile contents of phenocryst-hosted melt inclusions and thermodynamic modelling, we conceptualize a distributed magmatic system that is dominated by andesite. We find that the 1976–2000 eruption episode was largely driven by volatile poor andesite-dacite magma that was periodically intruded by a relatively primitive mafic magma throughout this period. This andesite-dacite crustal magma reservoir (to at least 9 km) is capped by a shallow storage reservoir at ~1- to 2-km depth, where magmas stalled before erupting. Mafic magma/s periodically intruded the andesite-dacite reservoir providing heat, melt, volatiles, and mafic minerals, including olivine, into the system. Finally, we show that the volatile budget during this small-volume eruption episode released significant S and Cl into the atmosphere, similar in scale to large basaltic eruptions.

Fumarolic gas composition and temperature record deep processes that generate and transfer heat and mass towards the surface. These processes are a result of the emplacement, degassing and cooling of magma and the overturning of the above hydrothermal system. A reasonable expectation, and too often an unproved assumption, is that fumarole temperatures and the deep heat sources vary on similar timescales. Yet signals from deep and shallow processes have vastly different temporal variations. This indicates that signals arising from deep activity may be masked or modified by intervening hydrothermal processes, such as fluid-groundrock reactions in which secondary minerals play a major role. Clearly, this complicates the interpretation of the signals such as the joint variation of fumarole vent temperature and geochemical ratios in terms of what is occurring at depth. So what do the differences between the timescales governing deep and shallow processes tell us about the intervening transport mechanisms?At the volcanic dome of La Soufrière de Guadeloupe, the Observatoire Volcanologique et Sismologique de la Guadeloupe has performed weekly-to-monthly in-situ vent gas sampling over many years. These analyses reliably track several geochemical species ratios over time, which provide important information about the evolution of deep processes. Vent temperature is measured as part of the in-situ sampling, giving a long time series of these measurements. Here, we look to exploit the temporal variations in these data to establish the common processes, and also to determine why these signals differ. By fitting sinusoids to the gas-ratio time series we find that several of the deep signals are strongly sinusoidal. For example, the He/CH4 and CO2/CH4 ratios, which involve conservative components and mark the injection of deep and hot magmatic fluids, oscillate on a timescale close to 3 years. We also analyse the frequency content of the temperature measurements since 2011 and find that such long signals are not seen. This may be due to internal buffering by the hydrothermal system, but other external forcings are also present. From these data we build up a more informed model of the heat-and-mass supply chain from depth to the surface. This will potentially allow us to predict future unrest (e.g. thermal crises, seismic swarms), and distinguish between sources of unrest.

Fumarolic gas survey of dormant volcanoes is fundamental because the compositional and flux changes in gas emissions are recognised signals of unrest and may even be precursors of eruptions on several dormant volcanoes in hydrothermal unrest [1-5].Here we report on the chemical compositions (CO2, H2S, SO2, H2) and mass fluxes of fumarolic gas emissions from the low-temperature (from 97° to 104°C) volcanic-hydrothermal system of La Soufrière de Guadeloupe (Lesser Antilles). This present study covers the period 2016 to present, encompassing the peak activity of April 2018. Long-term trends are acquired from both portable MultiGAS measurements (performed monthly) and two permanent MultiGAS stations (4 automated 20’ measurements per day). These MultiGAS data are discussed along with other geochemical and geophysical parameters monitored at OVSG, such as complete fumarole chemistry via Giggenbach bottles, fumarole temperatures, volcanic seismicity and deformation in order to track the deep-sourced magmatic signal contribution compared to the one of the hydrothermal system and detect potential signs of unrest [6].Dealing with MultiGAS data from a low-T fumarolic system in a tropical environment is not straightforward due to external forcing effect of meteoric water on gas composition. Hence, interpretation of the observed chemical changes must consider (i) the role of water-gas-rock interactions and gas scrubbing processes by the hydrothermal system and the perched volcanic pond [7], which particularly affect sulphur precipitation and remobilization and (ii) how these processes vary with rainfall and groundwater circulation (i.e. rainy vs non-rainy seasons, extreme events).[1] Giggenbach and Sheppard, 1989; [2] Symonds et al., 1994; [3] Hammouya et al., 1998; [4] De Moor et al., 2016; [5] Allard et al., 2014; [6] Moretti et al., submitted; [7] Symonds et al., 2001

In recent decades, reliable computational models have significantly advanced, and now represent a valuable tool to make quantitative and testable predictions, supporting gas dispersal forecasting and hazard assessments for public safety. In this study, we carried out a number of tests aimed to validate the modelling of gas dispersal at La Soufrière de Guadeloupe volcano (Lesser Antilles), which has shown quasi-permanent degassing of low-temperature hydrothermal nature since its last magmatic eruption in 1530 AD. In particular, we focused on the distribution of CO2 and H2S discharged from the three main present-day fumarolic sources at the summit, using the MultiGAS measurements of continuous gas concentrations collected during March-April 2017. We implemented the open-source Eulerian code DISGAS-2.0 for passive gas dispersion coupled with the mass consistent Diagnostic Wind Model (DWM), using wind measurements and atmospheric stability information from a local meteorological station and the ECMWF-ERA5 reanalysis data. We found that model outputs are highly dependent on the resolution of the topographic data, which affect mainly the reliability of DWM meteorological fields, especially on and around the steep dome. Our results satisfactory reproduce the observed data, indicating the potential usefulness of DISGAS-2.0 as a tool for quantifying gas hazard and reproducing the fumarolic degassing and at La Soufrière de Guadeloupe.

Signals of volcanic unrest do not usually provide insights into the timing, size and style of future eruptions. However, analysis of past eruptions provides useful information in order to understand the evolution, magma storage and onset of future eruptions, Here, we examine basaltic-andesitic to andesitic eruption deposits from La Soufrière de Guadeloupe, covering a range of eruption styles, ages and magnitudes. Our work is timely given unrest at this system has increased over the last 25 years, with a potential eruption capable of directly impacting ~70,000 people in Southern Basse-Terre.Here, we study the geochemistry of pre-eruptive magmas and timescales of magmatic processes preceding four explosive eruptions: 1657 Cal. CE (Vulcanian), 1010 Cal. CE (Plinian), ca. 341 Cal. CE (Strombolian) and 5680 Cal. BCE (Plinian). Using diffusion timescale studies of orthopyroxene phenocrysts, we constrain the timing of magma injections into the La Soufrière de Guadeloupe magmatic reservoir. These range from 35 ± 0.37 to 848 ± 0.4 days before eruption. Diffusion timescales do not appear to correlate with eruption style/size, but may correlate with other parameters (e.g., magma interactions in the reservoir and/or volatile content of the magma).Major element concentrations in whole rock (WR), groundmass glasses (GM) and melt inclusions (MI) show a strong linear trend. However, this evolution cannot be resolved through fractional crystallisation alone, as there is no clear temporal trend. MIs reveal a relatively homogenous melt composition from the first to the most recent eruptions, ranging from 63.6 – 78.7 wt% SiO2. Volatiles, including H­2O (2.3-4.4 wt%), CO2 (35-866 ppm) and sulphur (30-202 ppm), are also consistent across the various eruptions. MIs are often more evolved than the GM, indicating they cooled prior to their entrainment. This, along with the different crystal populations observed, suggests a recharge magma has intruded through a mush system and entrained crystals stored there. Crystals in different regions of the mush therefore experience different interactions with magmatic processes.The major element compositional homogeneity across the eruptions indicates that composition does not have a large control on eruption style at this system. However, MI pre-eruptive volatile contents are more concentrated in the larger Plinian eruptions (e.g., CO­2 – 866 ppm) than the smaller Vulcanian and Strombolian eruptions (e.g., CO­2 – 674 ppm). Volatile emissions calculated through the petrologic method also differ, with higher total volatile emissions observed in the Plinian eruptions (12 Mt) than the smaller eruptions (0.1 Mt). The Plinian eruptions also have a faster magma ascent rate (0.3-22 m/s) than the vulcanian eruptions (3 m/s) as calculated from mass flux estimates. Though the composition of the La Soufrière de Guadeloupe system has remained constant over time, changes in eruption style can result from variations: (i) in the way magma interacts with the mush system, (ii) in the pre-eruptive volatile contents and (iii) in the ascent rates. Understanding the controls on eruption style is important, especially during the current phase of unrest, in order to improve early-warning system efficiency, forecast models, eruption scenario crisis response and long-term risk reduction planning.

International audience; Particularly in the presence of a hydrothermal system, many volcanoes output large quantities of heat through the transport of water from deep within the edifice to the surface. Thus, heat flux is a prime tool for evaluating volcanic activity and unrest. We review the volcanic unrest at La Soufrière de Guadeloupe (French West Indies) using an airborne thermal camera survey and in situ measurements of temperature and flow rate through temperature probes, Pitot-tube and MultiGAS measurements. We deduce mass and heat fluxes for the fumarolic, ground and thermal spring outputs and follow these over a period spanning 2000–2020. Our results are compared with published data and we performed a retrospective analysis of the temporal variations in heat flux over this period using the literature data. We find that the heat emitted by the volcano is 36.5 ± 7.9MW, of which the fumarolic heat flux is dominant at 28.3 ± 6.8 MW. Given a total heated area of 26 270 m2, this equates to a total heat flux density of 1366 ± 82 W/m2, which is amongst the highest established for worldwide volcanoes with hydrothermal systems, particularly for dome volcanoes. A major change at La Soufrière de Guadeloupe, however, is the development of a widespread region of ground heating at the summit where heat output has increased from 0.2 ± 1 MW in 2010 to 5.7 ± 0.9 MW in 2020. This change is concurrent with accelerating unrest at the volcano and the emergence of two new high-flux fumaroles in recent years. Our findings highlight the importance of continued and enhanced surveillance and research strategies at La Soufrière de Guadeloupe, the results of which can be used to better understand hydrothermal volcanic systems the world over.

Between 2008 and 2014, the Institut de Physique du Globe de Paris (IPGP) and the University of the West Indies, Seismic Research Centre (UWI-SRC) designed and built a regional seismic network across the Lesser Antilles. One of the goals of the network is to provide real-time seismic data to the tsunami warning centers in the framework of the Intergovernmental Coordination Group working toward the establishment of a tsunami and other coastal hazards early warning system (ICG-CARIBE-EWS) for the Caribbean and adjacent regions (McNamara et al., 2016). In an area prone to hurricanes, earthquakes, tsunamis, and volcanic eruptions, we chose different techniques and technologies to ensure that our cooperated network could survive and keep providing data in case of major natural hazards. The Nanometrics very small aperture terminal (VSAT) technology is at the heart of the system. It allows for duplicated data collection at the three observatories (Trinidad, Martinique, and Guadeloupe; Anglade et al., 2015). In 2017, the network design and implementation were put to the test with Saffir–Simpson category 5 hurricanes Irma and Maria that went, respectively, through the north and central portion of the Lesser Antilles, mainly impacting the sites operated by volcanological and seismological observatories of IPGP in Martinique (Observatoire Volcanologique et Sismologique de Martinique [OVSM]) and in Guadeloupe (Observatoire Volcanologique et Sismologique de Guadeloupe [OVSG]). Our concepts proved to be valid with a major data shortage of less than 12 hr and only two stations having sustained heavy damage. In this article, we review the strengths and weaknesses of the initial design and discuss various steps that can be taken to enhance the ability of our cooperated network to provide timely real-time seismic data to tsunami warning centers under any circumstances.

Co-auteur étranger; International audience; olivine-hosted melt inclusions (Mis) provide samples of magmatic liquids and their dissolved volatiles from deep within the plumbing system. Inevitable post-entrapment modifications can lead to significant compositional changes in the glass and/or any contained bubbles. Reheating is a common technique to reverse MI crystallisation; however, its effect on volatile contents has been assumed to be minor. We test this assumption using crystallised and glassy basaltic MIs, combined with Raman spectroscopy and 3D imaging, to investigate the changes in fluid and solid phases in the bubbles before and after reheating. Before reheating , the bubble contains CO 2 gas and anhydrite (caSo 4) crystallites. The rapid diffusion of major and volatile elements from the melt during reheating creates new phases within the bubble: So 2 , gypsum, Fe-sulphides. Vapour bubbles hosted in naturally glassy MIs similarly contain a plethora of solid phases (carbonates, sulphates, and sulphides) that account for up to 84% of the total MI sulphur, 80% of CO 2 , and 14% of FeO. In both reheated and naturally glassy MIs, bubbles sequester major and volatile elements that are components of the total magmatic budget and represent a "loss" from the glass. Analyses of the glass alone significantly underestimates the original magma composition and storage parameters. Olivine-hosted melt inclusions (MIs) provide insight into the nature of the magma mantle source, storage conditions , and pre-eruptive volatile contents 1. Following entrapment, MIs undergo compositional modifications due to growth of the host olivine along the MI walls, and to crystallisation of daughter minerals from the glass due to slow ascent rates, and/or cooling 1,2. Another modification is the nucleation of a vapour bubble in response to decompression during cooling and post-entrapment crystallisation 1,2 , further reducing the solubility of volatiles in the glass. Vapour, or shrinkage, bubbles produced by differential thermal contraction between the melt (glass) and the host crystal are considered to be inherent to the MI 1,2. However, pre-existing bubbles that formed externally in a vapour-saturated system may also become trapped inside MIs. Vapour bubbles may also form during MI leakage and decrepitation of the host crystal 3,4. Discriminating between various bubble types depends upon the size of the bubble relative to the total inclusion. Since the volumetric proportions of vapour bubbles depend on the cooling rate, volatile content and melt composition, cooling-related shrinkage and melt-saturated bubbles normally comprise 0.2 to 10 vol% of the inclusion 3,4. Bubbles with greater volumetric proportions are not considered inherent to the MI 3,4. Due to the strong pressure-dependency of CO 2 solubility, the contraction of the melt and the decrease of the internal pressure in response to cooling and post-entrapment crystallisation first leads to rapid CO 2 saturation of the melt, and consequently, the transfer of CO 2 gas into the bubble 5-7 , so that analyses of the glass yield erroneously low magmatic CO 2 concentrations. This poses significant problems as the CO 2 content is commonly used to infer the pressure of crystallisation and MI entrapment, as well as magmatic storage depths. By only considering the glass, these values are grossly underestimated. Many of the post-entrapment modifications that occur within olivine-hosted MIs can be corrected using well-constrained exchange coefficients and recently established methods to quantify the amount of volatiles, particularly CO 2 , sequestered by the bubble. These approaches include the use of trace element proxies, such as CO 2 / Nb, to determine the pre-eruptive CO 2 content of the undegassed melt 8 , reheating the MI to resorb the bubble

Volcanic gas emissions from the main low-temperature fumaroles (around 100°C) of one of the most dangerous volcanoes in the Lesser Antilles, La Soufrière de Guadeloupe, are measured routinely using direct sampling with NaOH bottles (for H2O, CO2, H2Stot, H2, CH4, CO, N2, He, Ar, O2), with P2O5 bottles (for H2O, CO2, H2S, SO2, H2) and MULTIGAS (for H2O, CO2, H2S, SO2, H2). This allows us to perform a study about an intercomparison of volcanic gas monitoring methologies. The results show some good overlap and similar temporal evolution between the dataset of the different methologies, which is of utmost of importance for monitoring purposes. However, there is no good agreement between the three methodologies for the same chemical ratio. The differences could be explained by a timedependant diffusive phenomenon that affects the concentrations of sampled gases for two methods of the three (P2O5 bottles and MULTIGAS). P2O5-based analyses are affected because effusion on rapid decompression and fast sampling affects the gas volume in the pipeline connecting the vent outlet to the sampling. Although MULTIGAS is a continuous, flow-through, sampling method, results are likely affected because of the sampling architecture involving filter and pump. Moreover, sulphur reaction (precipitation of elemental S upon exit from the fumarole, conversion of H2S to SO2, chemical reaction with the P2O5 powder that release H2S) and water condensation can also explain some discrepancies.

In the Central Volcanic Zone of the Andean cordillera (CVZ), thickening of the continental crust during the last 30 Ma led to the production of increasingly differentiated magmas. Better constrain on the interactions between ascending magmas and the thick continental crust are needed to understand the contamination processes and their implications on the geochemical diversity of arc magmas. Lascar volcano figures among the most active Quaternary volcano of the CVZ and yet geochemical studies on its volcanic products remain scarce. We lead major-, trace elements and Pb–Sr isotopes measurements on 10 new Lascar samples representative of the different petrologic types (andesitic to dacitic). 206Pb/204Pb (18.770–18.823), 207Pb/204Pb (15.636–15.655), 208Pb/204Pb (38.711–38.809) and 87Sr/86Sr isotope ratios (0.705731–0.706564) show small variations within the previously defined isotopic field of Lascar. In Pb–Sr or Pb–Pb isotope space, the data points show a triangular distribution that requires the contribution of at least three end-member components at the source of Lascar magmas. We built up a two-stages three-components assimilation fractional crystallization model (AFC) that reproduces the compositional range of Lascar magmas. It allows us to constrain the composition and nature of the three end-member components: (BA) a common mantellic source with low 87Sr/86Sr ( 0.70656) and in Pb (206Pb/204Pb > 18.82; 207Pb/204Pb > 15.647; 208Pb/204Pb > 38.79), (UC) a felsic upper continental crust (>65.1 wt% of SiO2) with similar Sr–Pb isotope composition as LC (87Sr/86Sr > 0.70643; 206Pb/204Pb > 18.823; 207Pb/204Pb > 15.655; 208Pb/204Pb > 38.809). Both BA and LC components occur in lavas with This model predicts that Lascar magmas assimilated ≤30% of lower crust and ≤29% of upper crust. Our conclusions are in good agreement with the CVZ literature that reports the contribution of three ubiquitous end-member components at the source of CVZ magmas, with similar natures and origins.

co-auteur étranger; International audience; Formation of secondary sulfate minerals during the reaction between volcanic gases and rocks modulates the compositionand flux of gaseous emanations. We report on the sub-surface formation of anhydrous alum (MIMIII(XVIO4)2with MI=-NH4+,Na+,K+;MIII=Al3+,Fe3+and XVI=S6+,Mo6+) in the 330°C fumaroles of the Lascar volcano (Chile). The alumoccurs as a few millimetres thick crust that grew internally by two-way diffusion of reaction gases and diffusive influx of rockcations within the crust. The average growth rate is estimated at ca. 0.3lm/day, based on the 19-year-long activity of thedegassing fracture hosting the crust. The growth rate is controlled by the slow migration of the rock cations and decreasestowards crust rim. The crust selectively concentrates Tl, V and Te (thousands oflg/g) and to a lesser extent Mo (hundredsoflg/g). The uptake of gaseous Tl, V and Mo is due to the possibility for these elements to enter the MI,MIIIand XVIsites ofalum, respectively. The process of Te uptake must be related to the incorporation of Tl and V with which Te tightly correlates.Extensive substitution of Tl, V and Te occurs at the surface of the crust where the supply of rock cations is the lowest. Suchsurface enrichment does not occur for Mo, because Mo substitutes for S, another element from the gas. These findings suggestthat the surface of mature alum crust has a high adsorption capacity for those gaseous metals able to compensate for the lackof rock-derived cations. Based on the composition of gases escaping from the fracture hosting the crust, it is estimated that thepartition coefficients of Tl (3.3107), V (1.1107) and Te (0.6107) between crust surface and gases are two to four ordersof magnitude higher than for other volatile metals and metalloids. It follows that gases equilibrating with anhydrous alumslose between 77 and 95% of their initial Tl content, but less than 1% of Pb. Given the Tl emission rate of Lascar volcano(5 g/day), between 17 and 104 g of toxic Tl would deposit every day if all Lascar gases were to equilibrate with anhydrousalums. This study suggests that anhydrous alums significantly immobilize Tl, V and Te in the ground of quiescent volcanoes,reducing the atmospheric emissions of these three elements.Ó2020 Elsevier Ltd. All rights reserved.

After 25 years of gradual increase, volcanic unrest at La Soufriere of Guadeloupe reached its highest seismic energy level on 27 April 2018, with the largest felt volcano-tectonic (VT) earthquake (M-L 4.1 or M-W 3.7) recorded since the 1976-1977 phreatic eruptive crisis. This event marked the onset of a seismic swarm (180 events, 2 felt) occurring after three previous swarms on 3-6 January (70 events), 1 st February (30 events, 1 felt) and 16-17 April (140 events, 1 felt). Many events were hybrid VI's with long-period codas, located 2-4 km below the volcano summit and clustered within 2 km along a regional NW-SE fault cross-cutting La Soufriere. Elastic energy release increased with each swarm whereas inter-event time shortened. At the same time, summit fractures continued to open and thermal anomalies to extend. Summit fumarolic activity increased significantly until 20 April, with a maximum temperature of 111.4 degrees C and gas exit velocity of 80 m/s, before declining to similar to 95 degrees C and similar to 33 m/s on 25 April. Gas compositions revealed increasing C/S and CO2/CH4 ratios and indicate hydrothermal P-T conditions that reached the critical point of pure water. Repeated MultiGAS analysis of fumarolic plumes showed increased CO2/H2S ratios and SO2 contents associated with the reactivation of degassing fractures (T = 93 degrees C, H2S/SO2 approximate to 1). While no direct evidence of upward magma migration was detected, we attribute the above phenomena to an increased supply of deep magmatic fluids that heated and pressurized the La Soufriere hydrothermal system, triggering seismogenic hydro-fracturing, and probable changes in deep hydraulic properties (permeability) and drainage pathways, which ultimately allowed the fumarolic fluxes to lower. Although this magmatic fluid injection was modulated by the hydrothermal system, the unprecedented seismic energy release and the critical point conditions of hydrothermal fluids suggest that the 2018 sequence of events can be regarded as a failed phreatic eruption. Should a similar sequence repeat, we warn that phreatic explosive activity could result from disruption of the shallow hydrothermal system that is currently responsible for 3-9 mm/y of nearly radial horizontal displacements within 1 km from the dome. Another potential hazard is partial collapse of the dome's SW flank, already affected by basal spreading above a detachment surface inherited from past collapses. Finally, the increased magmatic fluid supply evidenced by geochemical indicators in 2018 is compatible with magma replenishment of the 6-7 km deep crustal reservoir feeding La Soufriere and, therefore, with a potential evolution of the volcano's activity towards magmatic conditions.

Co-auteur étranger; International audience; The nature of the magmatic source beneath the Garibaldi Volcanic Belt (GVB) in NW Washington (USA) and SW British Columbia (Canada) has been debated both due to its classification as the northern equivalent of the High Cascades and the alkaline nature of northern basalts. Whole rock studies have shown that the GVB does not share the same magmatic source as the High Cascades (Mullen and Weis, 2013, 2015). Nonetheless, the presence of alkaline basalts in this arc raises questions about the exact source of mantle enrichment and whether it is related to the young age of the downgoing Juan de Fuca Plate (< 10 Ma) or the presence of a slab tear at the northern end of the arc. To gain insight into the source that underlies the GVB, we sampled olivine-hosted melt inclusions from each volcanic centre along the arc. Major, volatile and trace element data reveal a northward compositional trend from arc-typical calc-alkaline magma in the south to OIB-like melts in the north near the slab tear. Furthermore, contributions from the subducting slab are relatively high beneath the southern end of the arc (Cl/Nb> 80) but rapidly decreases to the north (Cl/Nb < 50). Finally, the significant differences in Zr/Nb from south to north (80 and 9, respectively) suggest two distinct mantle sources since one source cannot produce melts with such different ratios. As such, we propose the GVB should be segmented into the Northern and Southern groups, each having its own mantle source. Based on the geographic proximity, the enriched nature of the Northern group melt inclusions is likely controlled by the slab tear at the northern termination of the subducting Juan de Fuca Plate. Melt modelling results show that 3–7 % partial melting of the primitive mantle with a garnet lherzolite residue can reproduce the composition of the Northern group. Melt inclusions from the Southern group, on the other hand, imply a depleted MORB mantle that has been modified by fluids derived from the downgoing slab. Variability within the Southern group itself reflects the amount of hydrous fluids supplied beneath each centre and is correlated with slab age and subsequent degree of dehydration. This study addresses the compositional diversity along the arc and provides evidence that the age of the downgoing plate and the presence of a slab tear exert a strong compositional control over eruptive products within one arc.

Las Sierras-Masaya volcanic system is a persistently active basaltic caldera complex in Nicaragua. While there has been almost no juvenile material erupted since 1772, Masaya volcano has been persistently degassing for >150 years. An additional unusual behaviour for the Las Sierras-Masaya volcanic complex is its ability to produce large caldera-forming basaltic Plinian eruptions with the most recent occurring about 1800 years ago. Here we present melt inclusion analyses that provide constraints on the magmatic system over time. Melt inclusions hosted in plagioclase and olivine crystals were analyzed for major, trace and volatile elements (S, Cl, F, H2O). The data supports a consistent parental magmatic source with restricted compositional variability explained by simple fractional crystallization of plagioclase, olivine, clinopyroxene and magnetite at a nearly constant temperature. This broadly agrees with previous whole rock geochemical studies showing that the overall chemical signature of volcanic products at Masaya has remained largely unchanged for ~60,000 years and that both shallow fractionation and degassing processes dominate the whole evolution of the magmatic series. Based on volatile element in melt inclusions and gas composition and flux measurements, we determine the magmatic flux to be ~0.19 km3 yr−1 implying that up to 47 km3 of magma may have degassed since the last effusive eruption. As at other persistently active basaltic volcanoes (e.g., Mt. Etna, Italy; Kilauea, Hawaii, USA), this magmatic flux must involve significant endogenous storage which is likely accommodated by extensional tectonics. However, Masaya volcano differs in its apparent simplicity with respect to its stable chemistry and its fully interconnected magmatic system.

The 2014 Barðarbunga rifting event in Iceland resulted in a 6-month long eruption at Holuhraun. This eruption was characterized by high lava discharge rate and significant gas emission. The SO2 flux for the first 3 months was measured with satellite sensors and the petrologic method. High-resolution time series of the satellite data give 1200 kg/s that concurs with 1050 kg/s obtained from melt inclusion minus degassed lava sulfur contents scaled to the mass of magma produced. A high-purity gas sample, with elevated S/Cl due to limited chlorine degassing, reveals a similar degassing pattern of trace metals as observed at Kīlauea (Hawai'i) and Erta Ale (Ethiopia). This suggests a common degassing mechanism at mantle plume-related volcanoes. The trace metal fluxes, calculated from trace element to sulfur ratios in the gas sample and scaled to the sulfur dioxide flux, are 1–2 orders of magnitude stronger at Holuhraun than Kīlauea and Erta Ale. In contrast, volcanoes at convergent margins (Etna and Stromboli, Italy) have 1–2 orders of magnitude higher trace element fluxes, most likely caused by abundant chlorine degassing. This emphasizes the importance of metal degassing as chlorine species. Short-lived disequilibria between radon daughters, 210Pb-210Bi-210Po measured in the gas, suggest degassing of a continuously replenished magma batch beneath the eruption site. Earlier and deep degassing phase of carbon dioxide and polonium is inferred from low (210Po/210Pb) in the gas, consistent with magma transfer rate of 0.75 m/s.

International audience; The authors regret that an error occurred whilst converting the SO2 normalized mass ratios to trace gas fluxes. This implies that Lascar volcanic gas fluxes, shown in Table 6 and discussed in Section 5.1.2.2, are lower than previously estimated. The correct trace element emission rate estimates are now presented in the Table below. The average rates over the 2009-2012 period range from 0.1 g/day (In) to 324 g/day (As), suggesting that Lascar does not represent such a significant local source of pollutants for the environment.

International audience; Many active European volcanoes and volcano observatories are island-based and located far from their administrative “mainland”. Consequently, Governments have developed multisite approaches, in which monitoring is performed by a network of individuals distributed across several national research centers. At a transnational level, multinational networks are also progressively emerging. Piton de la Fournaise (La Réunion Island, France) is one such example. Piton de la Fournaise is one of the most active volcanoes of the World, and is located at the greatest distance from its “mainland” than any other vulnerable “overseas” site, the observatory being 9365 km from its governing body in Paris. Effusive risk is high, so that a well-coordinated and rapid response involving near-real time delivery of trusted, validated and operational product for hazard assessment is critical. Here we review how near-real time assessments of lava flow propagation were developed using rapid provision, and update, of key source terms through a dynamic and open integration of near-real time remote sensing, modeling and measurement capabilities on both the national and international level. The multi-national system evolved during the five effusive crises of 2014–2015, and is now mature for Piton de la Fournaise. This review allows us to identify strong and weak points in an extended observatory system, and demonstrates that enhanced multi-national integration can have fundamental implications in scientific hazard assessment and response during an on-going effusive crisis

The 2014 eruption at Piton de La Fournaise (PdF), la Reunion, which occurred after 41 months of quiescence, began with surprisingly little precursory activity, and was one of the smallest so far observed at PdF in terms of duration (less than 2 days) and volume (less than 0.4 Mm3). The pyroclastic material was composed of spiny-opaque, spiny-iridescent, and fluidal scoria along with golden pumice. Density analyses performed on 200 lapilli reveal that the spiny-opaque clasts are the densest (1600 kg/m3) and richest in crystals (54 vol%), and the golden pumices are the lightest (400 kg/m3) and poorest in crystals (14 vol%). The connectivity data indicate that the fluidal and golden (Hawaiian-like) clasts have more isolated vesicles (up to 40 %) than the spiny (Strombolian-like) clasts (0–5 %). These textural variations are linked to primary pre-eruptive magma storage conditions. The golden and fluidal fragments track the hotter portion of the melt, in contrast to the spiny fragments which mirror the cooler portion of the shallow reservoir. Progressive tapping of these distinct portions leads to a decrease in the explosive intensity from early fountaining to Strombolian activity. The geochemical results confirm the absence of new hot input of magma and confirm the involvement of a single, shallow, differentiated magma source, possibly related to residual magma from the November 2009 eruption. We found that the eruption was triggered by water exsolution, favoured by the shallow depth of the reservoir, rather than cooling and chemical evolution of the stored magma.

International audience; Cerro Negro, the youngest volcano along the Central American Volcanic Belt (CAVB), is a polygenetic cinder cone with relatively frequent basaltic eruptions. The neighbouring El Hoyo complex, of which Las Pilas is the dominant edifice, is a much larger and older complex with milder and less frequent eruptions. Previous studies have suggested a deep link beneath these two closely spaced volcanoes (McKnight, 1995; MacQueen, 2013). Melt inclusions were collected from various tephra samples in order to determine whether a connection exists and to delineate the features of this link. Major, volatile, and trace elemental compositions reveal a distinct geochemical continuum with Cerro Negro defining the primitive endmember and El Hoyo representing the evolved endmember. Magmatic conditions at the time of melt inclusion entrapment were estimated with major and volatile contents: 2.4 kbar and 1170 °C for Cerro Negro melts and 1.3 kbar and 1130 °C for El Hoyo melts with an overall oxygen fugacity at the NNO buffer. Trace element contents are distinct and suggest Cerro Negro magmas fractionally crystallise while El Hoyo magmas are a mix between primitive Cerro Negro melts and residual and evolved El Hoyo magma. Modelling of end member compositions with alphaMELTS confirms the unique nature of El Hoyo magmas as resulting from incremental mixing between Cerro Negro and residual evolved magma at 4 km depth. Combining all available literature data, this study presents a model of the interconnected subsurface plumbing system. This model considers the modern day analogue of the Lemptégy cinder cones in Massif Central, France and incorporates structurally controlled dykes. The main implications of this study are the classification of Cerro Negro as the newest conduit within the El Hoyo Complex as well as the potential re-activation of the El Hoyo edifice.

The solubility of sulphur in sulphide-saturated, H2O-bearing basaltic–andesitic and basaltic melts from Hekla volcano (Iceland) has been determined experimentally at 1,050°C, 300 and 200 MPa, and redox conditions with oxygen fugacity (logfO2) between QFM−1.2 and QFM+1.1 (QFM is a quartz–fayalite–magnetite oxygen buffer) in the systems containing various amounts of S and H2O. The S content of the H2O-rich glasses saturated with pyrrhotite decreases from 2,500 ppm in basalt to 1,500 ppm in basaltic andesite at the investigated conditions. Furthermore, the reduction of water content in the melt at pyrrhotite saturation and fixed T, P and redox conditions leads to a decrease in S concentration from 2,500 to 1,400 ppm for basaltic experiments (for H2O decrease from 7.8 to 1.4 wt%) and from 1,500 to 900 ppm (for H2O decrease from 6.7 to 1.7 wt%) for basaltic andesitic experiments. Our experimental data, combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas, indicate that the parental basaltic melts of Hekla were not saturated with respect to sulphide. During magmatic differentiation, the S content in the residual melts increased and might have reached sulphide saturation with 2,500 ppm dissolved S. With further magma crystallization, the S concentration in the melt was controlled by the sulphide saturation of the magma, decreasing from ~2,500 to 900 ppm S.

International audience; The Irruputuncu is an active volcano located in northern Chile within the Central Andean Volcanic Zone (CAVZ) and that has produced andesitic to trachy-andesitic magmas over the last similar to 258 +/- 49 ka. We report petrographical and geochemical data, new geochronological ages and for the first time a detailed geological map representing the eruptive products generated by the Irruputuncu volcano. The detailed study on the volcanic products allows us to establish a temporal evolution of the edifice. We propose that the Irruputuncu volcanic history can be divided in two stages, both dominated by effusive activity: Irruputuncu I and II. The oldest identified products that mark the beginning of Irruputuncu I are small-volume pyroclastic flow deposits generated during an explosive phase that may have been triggered by magma injection as suggested by mingling features in the clasts. This event was followed by generation of large lava flows and the edifice grew until destabilization of its SW flank through the generation of a debris avalanche, which ended Irruputuncu I. New effusive activity generated lavas flows to the NW at the beginning of Irruputuncu II. In the meantime, lava domes that grew in the summit were destabilized, as shown by two well-preserved block-and-ash flow deposits. The first phase of dome collapse, in particular, generated highly mobile pyroclastic flows that propagated up to similar to 8 km from their source on gentle slopes as low as 11 degrees in distal areas. The actual activity is characterized by deposition of sulfur and permanent gas emissions, producing a gas plume that reaches 200 m above the crater. The maximum volume of this volcanic system is of similar to 4 km(3), being one of the smallest active volcano of Central Andes.

During its last eruption in February 2000, Hekla volcano (Iceland) emitted a sub-Plinian plume that was condensed and scavenged down to the ground by heavy snowstorms, offering the unique opportunity to study the chemistry of the gaseous plume released during highly explosive eruptions. In this paper, we present results on trace element and minor volatile species (sulfates, chlorides, and fluorides) concentrations in snow samples collected shortly after the beginning of the eruption. The goal of this study is to better constrain the degassing and mobility of trace elements in gaseous emissions. Trace element volatility at Hekla is quantified by means of enrichment factors (EF) relative to Be. Well-known volatile trace elements (e.g., transition metals, heavy metals, and metalloids) are considerably enriched in the volcanic plume of Hekla. Their abundances are governed by the primary magmatic degassing of sulfate and/or halide compounds, which are gaseous at magmatic temperature. Their volatility is, however, slightly lower than in basaltic systems, most likely because of the lower magma temperature and higher magma viscosity at Hekla. More surprisingly, refractory elements (e.g., REE, Th, Ba, and Y) are also significantly enriched in the eruptive plume of Hekla where their apparent volatility is two orders of magnitude higher than in mafic systems. In addition, REE patterns normalized to the Hekla 2000 lava composition show a significant enrichment of HREE over LREE, suggesting the presence of REE fluorides in the volcanic plume. Such enrichments in the most refractory elements and REE fractionation are difficult to reconcile with primary degassing processes, since REE fluorides are not gaseous at magma temperature. REE enrichments at Hekla could be attributed to incongruent dissolution of tephra grains at low temperature by F-rich volcanic gases and aerosols within the eruptive plume. This view is supported by both leaching experiments performed on Hekla tephra and thermodynamic considerations on REE mobility in hydrothermal fluids and modeling of glass dissolution in F-rich aqueous solutions. Tephra dissolution may also explain the observed enrichments in other refractory elements (e.g., Th, Y, and Ba) and could contribute to the degassing mass balance of some volatile trace elements, provided they are mobile in F-rich fluids. It thus appears that both primary magmatic degassing and secondary tephra dissolution processes govern the chemistry of eruptive plumes released during explosive eruptions.

International audience; This study focuses on the chemical compositions of volcanic gases and aerosols emitted by Lascar volcano (Northern Chile). The sustained volcanic plume was sampled in April 2009, April 2011 and November 2012 through filter packs and analyzed for major gaseous species (SO2, HCl, and HF) and trace elements. During fieldwork, SO2 flux measurements were also performed by UV spectrometry (DOAS). The Lascar volcano is a significant and sustained emitter of SO2 (between 150 and 940 t/d), HCl (between 170 t/d and 210 t/d) and HF (up to 100 t/d). Combining the SO2-normalized trace element concentrations and the SO2 emission rates, we evaluate that the quiescent degassing of Lascar is an important local source of trace metals to the atmosphere with contributions to global volcanic fluxes generally less than 2%.Our data were used to infer the origin of the gas emitted at Lascar. Two major degassing sources were identified: a deep magmatic reservoir and a shallow hydrothermal system. The variable interaction between these two sources is the most likely scenario for explaining the compositional ranges in acid gases but also in a volatile and fluid–mobile trace element such as B. These variations are related to changes in volcanic activity: an extensive interaction between the hot ascending magmatic gases and the shallow aquifer occurred in 2009, possibly due to a long period of quiescence, before magmatic activity renewed in 2011 and 2012. Our study highlights, therefore, that filter-pack measurements may be used to study changes in subsurface processes that probably play a key role in triggering volcanic eruptions.

International audience; Sulfur concentrations have been measured in 28 melt inclusions (MIs) in plagioclase, clinopyroxene, and olivine crystals extracted from tephra produced during the explosive eruption of Grímsvötn in May 2011. The results are compared to sulfur concentrations in the groundmass glass in order to estimate the mass of sulfur brought to surface during the eruption. Satellite measurements yield order of magnitude lower sulfur (~0.2 Tg) in the eruption plume than estimated from the difference between MI and the groundmass glass. This sulfur "deficit" is readily explained by sulfur adhering to tephra grains but principally by sulfide globules caused by basalt-sulfide melt exsolution before degassing. A mass balance calculation reveals that approximately ~0.8 Tg of SO2 is present as globules, representing ~50% of the total sulfur budget. Most of the sulfide globules likely reside at depth due to their elevated density, for potential later remobilization by new magma or hydrothermal circulation.

[1] The 2010 eruptive activity at the Eyjafjallajokull volcanic system began 20 March with a basaltic flank eruption on a 300 m long fissure on the Fimmvorðuhals Pass, in between Eyjafjallajokull and Mýrdalsjokull volcanoes. The magma expelled from the fissure is olivine- and plagioclase-bearing mildly alkali basalt that exhibits uniform and rather primitive whole-rock composition. This event provides a rare opportunity to assess deep magmatic processes in Iceland. Melt inclusions (MIs) hosted in olivine phenocrysts were analyzed for their major, trace and volatile element concentrations to enable identification of magmatic source(s) for Eyjafjallajokull volcano and to better constrain processes occurring at depth. The MIs, in particular those in Mg-rich olivines, record primary magma composition before homogenisation and differentiation during magma ascent. The olivine phenocrysts hosting the MIs have a large compositional range, extending from Fo73 to Fo87, reflecting changes in the magma characteristics from the source to the surface. The MI compositions exhibit significant variations with MgO ranging from 5.2 to 7.2 wt%. This compositional range was caused by a binary mixing of two basaltic end-members followed by fractional crystallization process. The sources of these end-members are identical to those of Katla and Surtsey basalts, with a dominant role of the Katla source. Trace element characteristics of the Fimmvorðuhals MIs suggest important proportions of recycled oceanic crust in their mantle sources.

Lastarria volcano (25A degrees 10'aEuro parts per thousand S, 68A degrees 31'aEuro parts per thousand W; 5,697 m above sea level), located in the Central Andes Volcanic Zone (northern Chile), is characterized by four distinct fumarolic fields with outlet temperatures ranging between 80A degrees C and 408A degrees C as measured between May 2006-March 2008 and April-June 2009. Fumarolic gasses contain significant concentrations of high temperature gas compounds (i.e., SO(2), HCl, HF, H(2), and CO), and isotopic ratios ((3)He/(4)He, delta(13)C-CO(2), delta(18)O-H(2)O, and delta D-H(2)O) diagnostic of magmatic gas sources. Gas equilibria systematics, in both the H(2)O-H(2)-CO(2)-CO-CH(4) and alkane-alkene C(3) system, suggest that Lastarria fumarolic gasses emanate from a superheated vapor that is later cooled and condensed at relatively shallow depths. This two-stage process inhibits the formation of a continuous aquifer (e.g., horizontal liquid layer) at relatively shallow depth. Recent developments in the magmatic gas system may have enhanced the transfer and release of heat causing shallow aquifer vaporization. The consequent pressure increase and aquifer vaporization likely triggered the inflation events beginning in 2003 at the Lastarria volcano.