[1] The fumarolic gas output has not been quantified for any of the currently deforming calderas worldwide, due to the lack of suitable gas flux sensing techniques. In view of resumption of ground uplift (since 2005) and the associated variations in gas chemistry, Campi Flegrei, in southern Italy, is one of the restless calderas where gas flux observations are especially necessary. Here we report the first ever obtained estimate of the Campi Flegrei fumarolic gas output, based on a set of MultiGAS surveys (performed in 2012 and 2013) with an ad-hoc-designed measurement setup. We estimate that the current Campi Flegrei fumarolic sulphur (S) flux is low, on the order of 1.5-2.2 tons/day, suggesting substantial scrubbing of magmatic S by the hydrothermal system. However, the fumarolic carbon dioxide (CO 2 ) output is $4606160 tons/day (mean6SD), which is surprisingly high for a dormant volcano in the hydrothermal stage of activity, and results in a combined (fumaroles þ soil) CO 2 output of $1560 tons/ day. Assuming magma to be the predominant source, we propose that the current CO 2 output can be supplied by either (i) a large (0.6-4.6 km 3 ), deeply stored (>7 km) magmatic source with low CO 2 contents (0.05-0.1 wt%) or (ii) by a small to medium-sized ($0.01-0.1 km 3 ) but CO 2 -rich (2 wt%) magma, possibly stored at pressures of $100 to 120 MPa. Independent geophysical evidence (e.g., inferred from geodetic and gravity data) is needed to distinguish between these two possibilities.

A new eruption started at Stromboli on 6 August 2014, which had been preceded by 2 months of increased Strombolian activity and several lava overflows from the craters. The eruption was characterized by a lava effusion in Sciara del Fuoco from a fracture at 650 m above sea level that lasted until 13-17 November. Here we present the first geochemical observations of this eruption, based on the soil CO 2 flux in the summit area and on 3 He/ 4 He ratios in the thermal waters near Stromboli village. We infer that this eruption was triggered by the gradual replenishment of the feeding system by a CO 2 -and 3 He-rich magma at the end of 2013 and after June 2014, suggested by the increase in 3 He/ 4 He ratio before eruption, which reached its highest value since 2007. We thus infer that this eruption was unusual, and we finally speculate on the evolutionary scenario of posteruption.

The 2014-2015 Bárðarbunga fissure eruption at Holuhraun in central Iceland was distinguished by the high emission of gases, in total 9.6 Mt SO 2 , with almost no tephra. This work collates all ground-based measurements of this extraordinary eruption cloud made under particularly challenging conditions: remote location, optically dense cloud with high SO 2 column amounts, low UV intensity, frequent clouds and precipitation, an extensive and hot lava field, developing ramparts, and high-latitude winter conditions. Semi-continuous measurements of SO 2 flux with

In spite of its major role on the atmospheric volatile budget, climate, and tracking magmatic transfers, mantle (CO 2 ) degassing below volcanoes is still poorly understood. Most of the studies on this scientific topic lack constraint on the CO 2 concentration of primary melts, the depth at which it starts degassing, and the extent of this process in the mantle. In this study of Piton de la Fournaise (PdF) volcano, we couple geochemistry of low solubility gases (He, Ar, CO 2 , d 13 C) in fluid inclusions (FIs) and petro-chemistry of magmatic inclusions on a set of olivine and clinopyroxene crystals from basalts and ultramafic enclaves.We constrain basaltic melt degassing at PdF over a large pressure range (from 4 GPa up to the surface). Based on CO 2 -He-Ar systematics, we infer that extensive degassing occurs already in the upper mantle (4-1 GPa) and it is favored by multiple steps of magma ponding and differentiation up to the mantle-crust underplating depth (0.4 GPa). Thus, we calculate that basaltic melts injected at crustal depth (<0.4 GPa) have already exsolved $94 ± 5 wt% of their primary CO 2 content in accordance with (1) the evolved and degassed signature of erupted lavas and (2) the weakness of inter-eruptive gas emissions in the active area bearing low-temperature vapor-dominated fumaroles. Our results at PdF strongly contrast with previous findings on other ocean island volcanoes having a higher magma production rate and faster magma ascent, like Kilauea (Hawaii), whose basalts experience only limited extent of differentiation and degassing. We propose that extensive degassing already in the upper mantle can be a common process for many volcanoes of the Earth and is tightly dependent on the dynamics of magma ascent and differentiation across multiple ponding zones.Based on the modeling developed in this study, we propose a new estimation of the CO 2 content (up to 3.5 ± 1.4 wt%) in primary basaltic melts at PdF leading to a carbon content in the mantle source of 716 ± 525 ppm. This new estimation is considerably higher than the few previous calculations performed for Ocean Island Basalts (OIB) systems. Another implication of this work involves the possible bias between the d 13 C measured in volcanic gas emissions (<À6‰) and that of primary vapour phase (À0.5 ± 0.5‰) constrained in this work. This bias would confirm the early step of extensive CO 2 degassing within the upper mantle and could represent an alternative for the hypotheses of carbon recycling or mantle heterogeneity in support of the low d 13 C signature of some mantle reservoirs. This study bears significant implications on the global budget of volcanic ⇑ Corresponding author at: Observatoire Volcanologique du Piton de la Fournaise ((G. Boudoire). volatile emissions, chiefly regarding the contribution of past and future emissions of volcanic CO 2 to climate dynamics, and on volcanic gas monitoring.

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