A. Aiuppa

Magma-derived gas influx and water-rock interactions in the volcanic aquifer of Mt. Vesuvius, Italy

-European Union, -Ministero dell’Universita’ e della Ricerca Scientifica e Tecnologica; -CNR–Gruppo Nazionale per la Vulcanologia.

Turmoil at Turrialba Volcano (Costa Rica): Degassing and eruptive processes inferred from high-frequency gas monitoring

Abstract Eruptive activity at Turrialba Volcano (Costa Rica) has escalated significantly since 2014, causing airport and school closures in the capital city of San José. Whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material. Here we use high‐frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions. Pulses of deeply derived CO2‐rich gas (CO2/Stotal > 4.5) precede explosive activity, providing a clear precursor to eruptive periods that occurs up to 2 weeks before eruptions, which are accompanied by shallowly derived sulfur‐rich magmatic gas emissions. Degassing modeling suggests that the deep magmatic reservoir is ~8–10 km deep, whereas the shallow magmatic gas source is at ~3–5 km. Two cycles of degassing and eruption are observed, each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels. The magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system, modifying the gas compositions, contributing volatiles to the emissions, and reflecting complex processes of scrubbing, displacement, and volatilization. H2S/SO2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases, whereas the second did not. Massive degassing (>3000 T/d SO2 and H2S/SO2 > 1) followed, suggesting boiling off of the hydrothermal system. The gas emissions show a remarkable shift to purely magmatic composition (H2S/SO2 < 0.05) during the second eruptive period, reflecting the depletion of the hydrothermal system or the establishment of high‐temperature conduits bypassing remnant hydrothermal reservoirs, and the transition from phreatic to phreatomagmatic eruptive activity.

Intense magmatic degassing through the lake of Copahue volcano, 2013-2014

Here we report on the first assessment of volatile fluxes from the hyperacid crater lake hosted within the summit crater of Copahue, a very active volcano on the Argentina-Chile border. Our observations were performed using a variety of in situ and remote sensing techniques during field campaigns in March 2013, when the crater hosted an active fumarole field, and in March 2014, when an acidic volcanic lake covered the fumarole field. In the latter campaign, we found that 566 to 1373 t d−1 of SO2 were being emitted from the lake in a plume that appeared largely invisible. This, combined with our derived bulk plume composition, was converted into flux of other volcanic species (H2O ~ 10989 t d−1, CO2 ~ 638 t d−1, HCl ~ 66 t d−1, H2 ~ 3.3 t d−1, and HBr ~ 0.05 t d−1). These levels of degassing, comparable to those seen at many open-vent degassing arc volcanoes, were surprisingly high for a volcano hosting a crater lake. Copahues unusual degassing regime was also confirmed by the chemical composition of the plume that, although issuing from a hot (65°C) lake, preserves a close-to-magmatic signature. EQ3/6 models of gas-water-rock interaction in the lake were able to match observed compositions and demonstrated that magmatic gases emitted to the atmosphere were virtually unaffected by scrubbing of soluble (S and Cl) species. Finally, the derived large H2O flux (10,988 t d−1) suggested a mechanism in which magmatic gas stripping drove enhanced lake water evaporation, a process likely common to many degassing volcanic lakes worldwide.

The structure of a hydrothermal system from an integrated geochemical, geophysical and geological approach: the Ischia Island case study

The complexity of volcano-hosted hydrothermal systems is such that thorough characterization requires extensive and interdisciplinary work. We use here an integrated multidisciplinary approach, combining geological investigations with hydrogeochemical and soil degassing prospecting, and resistivity surveys, to provide a comprehensive characterization of the shallow structure of the southwestern Ischias hydrothermal system. We show that the investigated area is characterized by a structural setting that, although very complex, can be schematized in three sectors, namely, the extra caldera sector (ECS), caldera floor sector (CFS), and resurgent caldera sector (RCS). This contrasted structural setting governs fluid circulation. Geochemical prospecting shows, in fact, that the caldera floor sector, a structural and topographic low, is the area where CO2-rich (>40 cm3/l) hydrothermally mature (log Mg/Na ratios 150 g m−2 d−1), is clearly captured by electrical resistivity tomography (ERT) and transient electromagnetic (TEM) surveys as a highly conductive (resistivity 10,000 mg/l) and poorly conductive meteoric-derived (TDS < 4,000 mg/l) waters are observed, respectively. We finally integrate our observations to build a general model for fluid circulation in the shallowest (<0.5 km) part of Ischias hydrothermal system.

Spatially resolved SO2 flux emissions from Mt Etna

Abstract We report on a systematic record of SO2 flux emissions from individual vents of Etna volcano (Sicily), which we obtained using a permanent UV camera network. Observations were carried out in summer 2014, a period encompassing two eruptive episodes of the New South East Crater (NSEC) and a fissure‐fed eruption in the upper Valle del Bove. We demonstrate that our vent‐resolved SO2 flux time series allow capturing shifts in activity from one vent to another and contribute to our understanding of Etnas shallow plumbing system structure. We find that the fissure eruption contributed ~50,000 t of SO2 or ~30% of the SO2 emitted by the volcano during the 5 July to 10 August eruptive interval. Activity from this eruptive vent gradually vanished on 10 August, marking a switch of degassing toward the NSEC. Onset of degassing at the NSEC was a precursory to explosive paroxysmal activity on 11–15 August.

Conduit dynamics and post explosion degassing on Stromboli: A combined UV camera and numerical modeling treatment

Abstract Recent gas flux measurements have shown that Strombolian explosions are often followed by periods of elevated flux, or “gas codas,” with durations of order a minute. Here we present UV camera data from 200 events recorded at Stromboli volcano to constrain the nature of these codas for the first time, providing estimates for combined explosion plus coda SO2 masses of ≈18–225 kg. Numerical simulations of gas slug ascent show that substantial proportions of the initial gas mass can be distributed into a train of “daughter bubbles” released from the base of the slug, which we suggest, generate the codas, on bursting at the surface. This process could also cause transitioning of slugs into cap bubbles, significantly reducing explosivity. This study is the first attempt to combine high temporal resolution gas flux data with numerical simulations of conduit gas flow to investigate volcanic degassing dynamics.

A new sulfur and carbon degassing inventory for the Southern Central American Volcanic Arc: The importance of accurate time-series datasets and possible tectonic processes responsible for temporal variations in arc-scale volatile emissions

This work presents a new database of SO2 and CO2 fluxes from the Southern Central American Volcanic Arc (SCAVA) for the period 2015-2016. We present ∼300 SO2 flux measurements from 10 volcanoes and gas ratios from 11 volcanoes in Costa Rica and Nicaragua representing the most extensive available assessment of this ∼500 km arc segment. The SO2 flux from SCAVA is estimated at 6,240±1,150 T/d, about a factor of three higher than previous estimations (1972-2013). We attribute this increase in part to our more complete assessment of the arc. Another consideration in interpreting the difference is the context of increased volcanic activity, as there were more eruptions in 2015-2016 than in any period since ∼1980. A potential explanation for increased degassing and volcanic activity is a change in crustal stress regime (from compression to extension, opening volcanic conduits) following two large (Mw>7) earthquakes in the region in 2012. The CO2 flux from the arc is estimated at 22,500±4,900 T/d, which is equal to or greater than estimates of C input into the SCAVA subduction zone. Time-series datasets for arc degassing need to be improved in temporal and spatial coverage to robustly constrain volatile budgets and tectonic controls. Arc volatile budgets are strongly influenced by short-lived degassing events and arc systems likely display significant short-term variations in volatile output, calling for expansion of nascent geochemical monitoring networks to achieve spatial and temporal coverage similar to traditional geophysical networks.

Hydrothermal fluid venting in the offshore sector of Campi Flegrei caldera: A geochemical, geophysical, and volcanological study

The ongoing unrest at the Campi Flegrei caldera (CFc) in southern Italy is prompting exploration of its poorly studied offshore sector. We report on a multidisciplinary investigation of the Secca delle Fumose (SdF), a submarine relief known since antiquity as the largest degassing structure of the offshore sector of CFc. We combined high-resolution morpho-bathymetric and seismo-stratigraphic data with onshore geological information to propose that the present-day SdF morphology and structure developed during the initial stages of the last CFc eruption at Monte Nuovo in AD 1538. We suggest that the SdF relief stands on the eastern uplifted border of a N-S-trending graben-like structure formed during the shallow emplacement of the Monte Nuovo feeding dike. We also infer that the high-angle bordering faults that generated the SdF relief now preferentially allow the ascent of hot brines (with an equilibrium temperature of 179°C), thereby sustaining hydrothermal degassing on the seafloor. Systematic vertical seawater profiling shows that hydrothermal seafloor venting generates a sizeable CO2, pH, and temperature anomaly in the overlying seawater column. Data for the seawater vertical profile can be used to estimate the CO2 and energy (heat) outputs from the SdF area at ∼50 tons/day (∼0.53 kg/s) and ∼80MW, respectively. In view of the cause-effect relationship with the Monte Nuovo eruption, and the substantial gas and energy outputs, we consider that the SdF hydrothermal system needs to be included in monitoring programs of the ongoing CFc unrest. This article is protected by copyright. All rights reserved.

Shrinkage bubbles: the C-O-H-S magmatic fluid system at San Cristóbal volcano

New analytical results for the composition of shrinkage bubbles (0 9–7 0 vol. %) in olivine-hosted (Fo <80%) primary melt inclusions (MIs) have been incorporated into a novel geochemical model for San Cristóbal volcano, Nicaragua. The vapour, liquid, and mineral components found inside shrinkage bubbles may represent relics of early C–O–H–S fluids exsolved from a magmatichydrothermal system. This conclusion is supported by high-resolution Raman microspectroscopy revealing: (1) gaseous CO2 (d1⁄4 0 17–0 31 g/cm in 31 samples) coexisting with liquid H2O (in seven samples) at ambient temperature (<22 C) inside the shrinkage bubbles of naturally quenched inclusions; (2) several mineral phases (i.e. Fe, Cu-sulfides, Ca-sulfates and Mg-carbonates) formed along the bubble–glass interface, as confirmed by electron backscattered/energy-dispersive spectroscopy. The presence of liquid water was revealed by applying a novel subtraction method to fitted Raman spectra that isolated an isosbestic liquid-water band at 3460 6 60/cm (mean 6 SD). In MIs, the major oxide composition of glasses containing shrinkage bubbles were analysed by electron microprobe, whereas glass volatile contents were measured with nanoscale secondary-ion mass spectroscopy. According to the water content of the glass inclusions ( 3 3 wt %) and the presence of liquid water at the bubble–glass interface, only small amounts of water (0 3 wt %) appear to have migrated inside the bubbles. From pre-eruptive (up to 1200 C) to post-eruptive temperatures, aqueous fluids represent the principal agents for chemical reactions inside MI bubbles involving dissolved ionic species (e.g. SO4 , CO3 , and Cl) and major and/or trace elements from the inclusion glass (e.g. Mg, Fe, Cu, Si, Al, Na, and K). After the initiation of nucleation (1009– 1141 C), the volume of shrinkage bubbles expands and the surrounding glass contracts (at <530 C). The Fe–Mg–Cu-rich (vapour) shrinkage-bubble paragenetic mineral sequence formed during different cooling stages: (A) high-temperature sulfide precipitation at 500–700 C; (B) lowtemperature magnesite precipitation at hydrothermal conditions <350 C; and finally (C) low-toambient temperature precipitation of carbonates and sulfates in liquid water at <150 C. Our findings indicate that the C–O–H–S fluids in shrinkage bubbles can represent an ideal preserved/ closed magmatic-hydrothermal system evolving after the exsolution of magmatic fluids during cooling.

Sulfur Degassing From Steam-Heated Crater Lakes: El Chichón (Chiapas, Mexico) and Víti (Iceland)

The composition of the gases released by El Chichón (Chiapas, Mexico) and Víti (Askja volcano, Iceland) volcanic lakes is examined by Multi-GAS for the first time. Our results demonstrate that H2S and SO2 are degassed by these pH 2–3 lakes. We find higher CO2/H2S and H2/H2S ratios in the lakes’ emissions (31–5,685 and 0.6–35, respectively) than in the fumarolic gases feeding the lakes (13–33 and 0.08–0.5, respectively), evidencing that only a fraction (0.2–5.4% at El Chichón) of the H2S(g) contributed by the subaquatic fumaroles ultimately reaches the atmosphere. At El Chichón, we estimate a H2S output from the crater lake of 0.02–0.06 t/day. Curiously, SO2 is also detected at trace levels in the gases released from both lakes (0.003–0.3 ppmv). We propose that H2S supplied into the lakes initiates a series of complex oxidation reactions, having sulfite as an intermediate product, and ultimately leading to SO2 production and degassing. Plain Language Summary Volcanic lakes are the site of some of the most unpredictable, and therefore dangerous, volcanic eruptions in nature. Their activity is driven by a feeding volcanic gas phase supplied by the underlying hydrothermal/magmatic system. These volatile species, entering the lake bottom, are absorbed into lake water at different rates/degrees depending on their water solubilities and the lake physical and chemical characteristics. Hyperacidic crater lakes (pH <1) are degassing SO2, a gas that was earlier believed to be totally dissolved into the water. In this study, we investigate for the first time the presence of reactive S gases (SO2 and H2S) in the plumes of less acidic (pH 2–3) lakes El Chichón (Mexico) and Víti (Iceland). Our results demonstrate that H2S, coming from the sublimnic hydrothermal systems is only partially dissolved and oxidized by the lake water. In addition, we discover trace amount of SO2 coming off both lakes. We propose that SO2 is produced into the lake by H2S oxidation, with dissolved sulfite as an intermediate product. Our results thus open new piece of knowledge to our understanding and monitoring the activity of restless volcanic lakes.