The quantities of carbon dioxide (CO 2 ) carried by magmas affect the style of volcanic eruptions and the evolution of Earth’s climate over geological time. There is growing evidence of volatile-rich primary magmas in intraplate domains such as oceanic islands and continental rifts, with CO 2 contents reaching several percent by weight. In most cases, these high CO 2 contents are inferred but not directly measured, because the volcanic products at the surface are strongly degassed. Here, we report the pre-eruptive CO 2 contents measured in olivine-hosted melt inclusions from Bas-Vivarais (Massif Central, France), a typical continental intraplate volcanic province. These basanitic melt inclusions have unusually high CO 2 concentrations of up to 4.8 percent by weight (3.6 percent by weight on average), among the highest ever measured to date. Such CO 2 -rich primary magmas appear to be the rule rather than the exception in continental intraplate settings and require carbon-enriched mantle sources (1475 to 2923 parts per million CO 2 for Bas-Vivarais). Despite relatively small magma volumes, eruptions of low-silica alkaline magmas in continental intraplate volcanic provinces release large amounts of CO 2 in the atmosphere. Thus, these provinces contribute much more to the global CO 2 flux and to the Earth’s short-term climate than their low magma production rates would suggest.

During years 2021–2022, an unusual seismic swarm was recorded at crustal level beneath the Monts Dore volcanic province (France). Complementary field and remote measurements were performed. Together with the time series recorded on the seismological and GNSS national networks, these measurements were fundamental for monitoring the evolution of the seismic swarm and deciphering its origin. Although a potential vertical migration of the seismic events is suggested, the complementary measurements presumably discard the hypothesis of magma intrusion at shallow crustal level. The ascent of a CO2-rich fluid originating from the mantle might instead have reacted with the hydrothermal system beneath the Monts-Dore since at least the summer 2021 leading to the reactivation of pre-existing tectonic structures with known associated seismicity. Feedback on the management of the 2021–2022 seismic swarm prompts for several recommendations that should be considered in future to better face and address at the national level the issues raised at long-dormant volcanic provinces in mainland France.

The recent installation of new broadband seismic stations in the French Massif Central (FMC) has resulted in the detection of a few “deep” earthquakes located near the crust‐mantle boundary beneath volcanic regions. Analysis of the spectral content of the respective waveforms has shown that the spectra of these “deep” earthquakes are significantly depleted in high frequencies. Based on these observations of anomalous depth and spectral content, these earthquakes can be classified as Deep Long Period (DLP) events. This is a specific class of volcanic seismicity observed beneath many active volcanoes around the World. While the exact physical origin of this type of earthquakes is still debated, they are often considered as indicators of the presence of magma near the crust‐mantle boundary. Therefore, observation of DLP earthquakes can bring new insights into understanding the state and the activity of the recent FMC volcanoes.

There is increasing evidence that the primary magmas at the origin of low-silica alkaline volcanism, such as basanites, are very rich in CO2 and that they can rise rapidly, directly from the mantle to the Earth’s surface. Such volcanic systems are numerous in intraplate oceanic and continental settings, including the French Massif Central, and some are remarkable for the abundance of large mantle-derived xenoliths. Although they usually represent relatively modest volumes of magma, their eruptions constitute a real and specific volcanic threat because of (1) their high ascent rate, with magmas capable of rising from mantle depths to the surface in less than a day to a few days, (2) the large volumes of CO2 emitted into the atmosphere at the time of eruption, and (3) the effusion of very fluid lava flows. The recent part of the Cézallier volcanic province, French Massif Central, offers nice examples of such low-silica alkaline volcanoes that erupted less than 200 ka ago.A study of fluid and melt inclusions has been carried out on three volcanoes from the recent part of the Cézallier volcanic province (Sarran, Mazoires, La Godivelle) in order to characterize the composition of primary magmas and to provide constraints on magma storage and ascent. Mg-rich olivine crystals (forsterite contents in the range of 83-89) were selected for the study of melt inclusions, while CO2-rich fluid inclusions were analyzed in olivine, pyroxene and amphibole crystals. After in-depth petrographic characterization, the melt inclusions were characterized using a series of analytical techniques, including: X-ray tomography (to characterize the shape and volume of melt inclusions and shrinkage bubbles); electron probe microanalysis (for major elements, Cl, F, S in glasses); Raman spectroscopy (to measure H2O and CO2 in glasses and to characterize the CO2-bearing phases in the shrinkage bubbles of the melt inclusions); and LA-ICP-MS (for trace elements). Microthermometry was used to measure CO2 densities in fluid inclusions, which were thereafter converted into pressures and into depths.The glass compositions of the melt inclusions plot into the fields of basanites, basalts and trachy-basalts. The glasses have particularly high CO2 contents: up to 1.8 wt% dissolved CO2. These values are minimum values, as CO2 is also present in the shrinkage bubbles as a fluid phase and as microcrystals of carbonates (Mg-calcite, nahcolite, ferromagnesite) covering the bubble walls. These high CO2 contents imply that the mantle sources at the origin of these magmas were enriched in carbon. CO2-rich fluid inclusions in olivine, pyroxene and amphibole crystals are all re-equilibrated and have thus lost their primary densities. At all three volcanoes, the CO2 density histograms show a major peak at 900 to 1090 kg/m3 (» 750 to 900 MPa), indicating a stage of magma storage at Moho level followed by rapid ascent to the surface. Work is in progress to reconcile the observation of large peridotite xenoliths (at Mazoires) with magma storage at Moho level.

Aircraft encounters with volcanic ash have caused significant damage over the past 40 years, resulting in particular attention being given to the issue. We analyzed the volcanic ash-aircraft encounter database published by the USGS. We added new volcanic eruptions and parameters such as eruption types, and dry–wet. Then, we applied standard and advanced statistical methods. Over 130 encounters have been documented in the mentioned database, with volcanic ash causing severe abrasions to the windshield, airframe, wings, and engine components. In nine cases, aircraft engines failed. We applied the binary regression analysis and some laboratory melting experiments on volcanic ash. Besides phreatomagmatism, we use the term external water in this work to describe meteoric water that enters volcanic plumes through precipitation or melting ice on ice-capped volcanoes. We demonstrated that engine failure occurs when our regression analyses undergo dry-to-wet conditions. In other words, statistically, there is a positive correlation between wet ash encounters with aircraft and engine failure incidents. Moreover, experiments conducted at 900 °C and under 40 bar pressure showed increased sintering in the dry sample, while melting textures were more prevalent in hydrated samples. We concluded that despite the various eruptive dynamics of volcanic ash, the introduction of external water into the volcanic plumes, probably causing instantaneous hydration of volcanic ash, is a common factor in engine failure incidents. Thus, we have identified the reasons behind engine failures during encounters between aircraft and volcanic ash and the specific damage that can occur depending on the type of eruption involved.

Carbonatites, carbon-rich magmatic rocks, are thought to form by low-degree partial melting of a relatively carbon-poor mantle followed by protracted differentiation and immiscibility. However, the nature of parental magmas and the characteristics of the early stages of differentiation that shape the subsequent crystal and liquid lines of descent remain poorly constrained. To provide new constraints, deep crustal cumulative xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatite magmas, were studied. We use major and volatile elements in primitive olivine-hosted melt inclusions, as well as major and trace elements in crystals, to reconstruct the conditions of formation and evolution of cumulates (pressure, temperature, composition). Xenoliths are composed of olivine, diopside, phlogopite, amphibole and accessory minerals. One remarkable feature is the presence of diopside and phlogopite oikocrysts enclosing roundish olivine chadacrysts. Melt inclusions do not have vapor bubble and have major element compositions resembling olivine nephelinite (7-10 wt % MgO after corrections for post-entrapment crystallization). The absence of vapor bubbles implies that the concentrations of volatile components (i.e. CO2, H2O, S) were not compromised by well-known post-entrapment volatile loss into the vapor bubble. Based on the melt inclusion study by SIMS, the volatile concentrations in olivine nephelinite magmas (early stage of differentiation) at Oldoinyo Lengai were 20-130 ppm S, 390-4500 ppm F, 50-540 ppm Cl, up to 6074 ppm CO2 and up to 1.5 wt % H2O. According to the calculated CO2-H2O saturation pressures and geophysical data, xenoliths from Embalulu Oltatwa document a mushy reservoir in the lower crust. Primitive olivine nephelinite melt inclusions have higher H2O contents than olivine nephelinite lavas from other further South volcanoes from the North Tanzanian Divergence (0.2-0.5 wt % H2O), suggesting that the lithospheric mantle source beneath the Oldoinyo Lengai is more hydrated than the mantle beneath the rest of North Tanzanian Divergence. We present a model in which resorption features observed in olivine chadacrysts, together with the LREE enrichments in olivine grains, are the consequences of reactive porous flows in a deep crustal mushy reservoir. We provide constraints on the major, trace and volatile element composition of the parental magmas of carbonatite series and demonstrate with Rhyolite-MELTS models that phonolites and related natrocarbonatites from Oldoinyo Lengai can be produced by protracted differentiation of olivine nephelinite melts.

Developing appropriate monitoring strategies in long-quiescent volcanic provinces is challenging due to the rarity of recordable geochemical and geophysical signals and the lack of experienced eruptive phenomenology in living memory. This is the case in the Massif Central (France) where the last eruptive sequence formed the Pavin's Group of Volcanoes, about 7 ka ago. There, current evidence of a mantle activity reminiscence is suggested by the presence of mineral springwaters, mofettes, and soil degassing. It appears fundamental as a prerequisite to decipher the evolution of the gas phase in the magmatic system at the time of the eruptive activity to understand the meaning of current local gas emissions. In this study, we develop an innovative approach coupling CO2 densimetry and geochemistry of fluid inclusions from products erupted by the Pavin's Group of Volcanoes. 3D imagery by Raman spectroscopy revealed that carbonate forming in fluid inclusions may lead to underestimation of CO2 density in fluid inclusions by up to 50% and thus to unreliable barometric estimates. Fortunately, we found that this effect may be limited by focusing on fluid inclusions with a small diameter (<4 & mu;m) and where no solid phase is detected on Raman spectra. The time evolution of the eruptions of the Pavin's Group of Volcanoes shows a progressive decrease of the pressure of magma storage (from more than 9 kbar down to 1.5-2 kbar) in parallel to magma differentiation (from basanites at Montcineyre to benmoreites at Pavin). The analysis of the noble gases entrapped in fluid inclusions yielded two main conclusions: (1) the helium isotope signature (Rc/Ra = 6.5-6.8) is in the range of values obtained in fluid inclusions from mantle xenoliths in the Massif Central (Rc/Ra = 5.6 & PLUSMN; 1.1, on average) suggesting partial melting of the subcontinental lithospheric mantle, and (2) magma degassing (4He/40Ar* from 4.0 to 16.2) mirrors magma differentiation and the progressive rise of the magma ponding zones of the Pavin's Group of Volcanoes. According to our modelling, about 80% of the initial gas phase would be already exsolved from these magmas, even if stored at mantle depth. Based on the results obtained from fluid inclusions, we propose a model of the evolution of the signature of noble gases and carbon isotopes from mantle depth to crustal levels. In this frame, gas emissions currently emitted in the area (Rc/Ra = 6.1-6.7 and 4He/40Ar* = 1.7) point to an origin in the lithospheric mantle. This study strongly encourages the establishment of a regular sampling of local gas emissions to detect potential geochemical variations that may reflect a change from current steady-state conditions.

The Regional Natural Park of the Monts d'Ardeche, located in south-eastern France, became the Monts d'Ardeche UNESCO Global Geopark in September 2014. This territory possesses significant volcanic features dating from the Miocene to the late Pleistocene. The UNESCO Global Geopark label helped to formalize a long-standing partnership with the University of Clermont Auvergne which includes support for scientific research, establishing conservation and protection priorities, establishing geosites for the public and involvement of local people and communities in geotouristic initiatives. Here we focus on some peculiar geosites that allow us to question magmatic processes, eruptive dynamics, morphological evolution of landscapes, the chronology of eruptions, relationships between humans and volcanoes during the Pleistocene, and highlight the delicate alliance between economic pressures, heritage conservation, and scientific tourism.

12 New series of alumino-silicate glasses spanning a wide range of chemical compositions 13 (basanites, tholeiitic basalts, calcalkaline andesites, peraluminous and peralkaline rhyolites) 14 and with water contents from 0.02 to 6.70 wt % were used for improving the method of 15 quantification of dissolved water with a highly confocal Raman micro-spectrometer. After 16 reconsideration of previously proposed methods for spectra acquisition and post-analysis data 17 treatment, we define the main critical steps that allow minimizing glass matrix effects. First, 18 we carefully assess the variation of Raman band intensities, in both water (~ 3000-3800 cm) 19 and alumino-silicate vibration (~ 200-1250 cm) regions with focus depth of the laser beam 20 inside the sample. Our results indicate that in the first 2-10 μm depth, the intensity increase in 21 the alumino-silicate region is twice as high as that in the water region. Optimal focus depths, 22 where the signal of the water band is maximum and the intensity ratio of the water band to 23 alumino-silicate band is minimum, vary with glass composition and confocal performance of 24 the Raman spectrometer. This influences both external and internal calibration slopes. 25 Second, this study recognizes critical parameters related with glass density, presence of ferric 26 iron and dissolved carbonates as mainly responsible for matrix effects on the internal 27

Experimental homogenization of olivine-hosted melt inclusions representative of near-primary basic and ultrabasic magmas is a powerful approach to investigate the nature of their source regions and the melting conditions in Earth's mantle. There is growing evidence that the total CO2 contents of olivine-hosted melt inclusions may reach values of the order of a single to several weight percent, especially in intraplate continental basalts. To be able to homogenize melt inclusions with such high CO2 contents, we developed a technique allowing for heat treating of the melt inclusions under hydrostatic pressures up to 3–4 GPa in a piston cylinder, using thick-walled Au80–Pd20 containers and molten NaCl as the surrounding medium for the inclusion-bearing olivines. We applied this technique to olivine phenocrysts from Thueyts basanite, Bas-Vivarais volcanic province, French Massif Central. Thueyts melt inclusions were chosen because of their high CO2 contents, as indicated by up to 1.19 wt % dissolved CO2 in the glasses and by the presence of shrinkage bubbles containing abundant carbonate microcrystals in addition to a CO2 fluid phase. The homogenization experiments were conducted at pressures of 1.5 to 2.5 GPa, temperatures of 1275 and 1300 ∘C, and run durations of 30 min. In all the melt inclusions treated at 2.5 GPa–1300 ∘C and half of those treated at 2 GPa–1300 ∘C, we were able to completely homogenize the inclusions, as indicated by the disappearance of the starting bubbles, and we obtained total CO2 contents ranging from 3.2 wt % to 4.3 wt % (3.7 wt % on average). In all the other melt inclusions (equilibrated at 1.5 or 2 GPa and 1300 ∘C or at 2.5 GPa–1275 ∘C), we obtained lower and more variable total CO2 contents (1.4 wt % to 2.9 wt %). In the inclusions with the highest total CO2 contents, the size of the shrinkage bubble was in most cases small (<5 vol %) to medium (<10 vol %): this is a strong argument in favor of an origin of these melt inclusions by homogeneous entrapment of very CO2-rich basanitic liquids (∼ 4 wt %) at pressures of 2 to 2.5 GPa. The lower total CO2 contents measured in some inclusions could reflect a natural variability in the initial CO2 contents, due for instance to melt entrapment at different pressures, or CO2 loss by decrepitation. An alternative scenario is heterogeneous entrapment of basanitic liquid plus dense CO2 fluid at lower pressures but still at least on the order of 1 GPa as indicated by dissolved CO2 contents up to 1.19 wt % in the glasses of unheated melt inclusions. Whatever the scenario, the basanites from the Bas-Vivarais volcanic province were generated in a mantle environment extremely rich in carbon dioxide.

The Bas-Vivarais (Ardèche, France) is one of the two most recent volcanic provinces in the French Massif Central. Very homogeneous basanites containing abundant crustal and mantle xenoliths were produced, but the magma genesis conditions remain unclear. Here, we bring textural, compositional and volatile clues to constrain the origin of the parental magmas of this volcanic province through the study of melt inclusions hosted in olivine phenocrysts of seven volcanoes from Bas-Vivarais. A peculiar feature of these inclusions is that, in addition to the silicate glass, they systematically contain CO 2 - rich bubbles whose walls are almost fully covered by microcrystals (mainly carbonates). We characterised the glass phase of the melt inclusions by electron probe microanalysis (major elements, Cl, F and S) and Raman spectrometry (H 2 O and CO 2 ), and obtained basanitic compositions with high volatile contents (ranging from 1.4 to 2.1 wt% H 2 O and up to 1.3 wt% CO 2 dissolved in glasses). These CO 2 values do not take into account the carbon dioxide contained in the bubbles (both as a fluid phase and as carbonate microcrystals). Two different techniques were used in order to estimate the total CO 2 content at the time of melt inclusion entrapment. First, two and three-dimensional Raman imaging was used to characterise the phases contained in the bubbles (CO 2 density, carbonates mineral species, etc.), and to estimate their volumes. From the volumes, densities and CO 2 contents of all phases in presence, including glass, we

The structural parameters and the thermal behavior of a complete series of Ca–Mg carbonates synthesized at high pressure and temperature (1–1.5 GPa, 1273–1373 K) in the range 0–50 mol% MgCO3 have been investigated by in situ powder synchrotron high-resolution X-ray diffraction at ambient and up to 1073 K under self-controlled CO2 partial pressure. The crystal structures are disordered Mg calcite in the range 1–41 mol% MgCO3, and Ca dolomite at 49 mol% MgCO3. New calibration curves of the cell parameters for the Mg content and thermal expansion from ambient to 1073 K are given. Short-range structural effects of cation substitution and ordering and their thermal behavior as a function of Mg content were identified from three sets of data: the peak broadening, the cell parameter strains and the Raman band enlargements. Both intra- and inter-crystalline levels of compositional heterogeneity are identified and allow splitting the Mg calcites into two groups: low- and high-Mg calcites. The low-Mg calcites (up to 22 mol% MgCO3) are homogeneous in Mg content with short-range ordering. High-Mg calcite (up to 41 mol% MgCO3) displays domains with different local ordering configurations and similar or slightly different Mg contents, and to which is added a compositional variation between crystals, as determined by EMP, of the order of ± 0.8 mol% MgCO3. The cation ordering in Ca-rich dolomites similarly occurs in high-Mg synthetic calcites. The role of (CO3)2− group ordering is shown to be an important factor in the formation of Ca–Mg carbonates.

We present a novel application of Raman microtomography for quantitative characterisation of glass inclusion-hosted bubbles, which allows for the simultaneous identification and volumetric quantification of mineral and fluid phases filling the bubble. The combination of Raman microtomography with synchrotron XRF mapping and scanning electron microscopy provides a complete compositional and textural characterisation of the bubble. In the studied samples, minerals are systematically present on the walls of the bubbles: dominantly carbonates in samples from continental intra-plate and hotspot volcanic provinces, and sulfates in the sample from subduction-related settings. Along with fluid CO2, carbonates sequester 65 to 84 % of the CO2 originally dissolved in the melt, while 18 to 60 % of the sulfur contained in the inclusion is stored in sulfides and/or sulfates. Thus, the total melt inclusion CO2 and S contents can be underestimated (by up to similar to 40 % and 60 %, respectively) if minerals in the bubbles are neglected. This study highlights the importance of 3D mapping of shrinkage bubbles hosted in glass inclusions for a better assessment of the bulk pre-eruptive contents of volatiles in magmas.

The accessibility of the continental crust (CC) sharply decreases with depth. The upper crust is relatively well-known but the geochemical composition of the deepest parts of the crust is harder to estimate. Our recent study combining the measurement of 138 La-183 Ce and 147 Sm-143 Nd systematics showed that the upper crust isotopic composition defined by loess measurements plots on the regression line that defines the mantle array [1]. Mass balance estimates for silicate reservoirs predict that (1) the bulk CC is off the ε Ce-ε Nd mantle array; (2) the lower crust plots in the lower left quadrant of the ε Ce-ε Nd diagram. The aim of this study is to better characterize the CC for the La-Ce systematics. We analyzed Hf, Nd and Ce isotopic composition of upper to lower crustal rocks from four locations: xenoliths from the French Massif Central; uplifted crust from the Southern Ivrea-Verbano zone (Italy); Paleoproterozoic to Archaean xenoliths from Udachnaya (Siberian craton); and composite samples from the Precambrian Canadian upper crust. Most of the samples are located along mantle arrays in the ε Ce-ε Nd and ε Hf-ε Nd isotopic plots, except Siberian samples whose isotopic compositions plots well below the ε Ce-ε Nd array. Siberian samples deviate from the mantle array as predicted in the calculations for the lower crust end-member. Such compositions have not been measured so far and contrast with results obtained on the Lu-Hf and Sm-Nd systematics