# Melt inclusions reveal massive carbon dioxide emissions from continental intraplate volcanism

Roxane Buso, Didier Laporte, Federica Schiavi, Nicolas Cluzel

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Roxane Buso, Didier Laporte, Federica Schiavi, Nicolas Cluzel. Melt inclusions reveal massive carbon dioxide emissions from continental intraplate volcanism. Communications Earth & Environment, 2025, 6, 10.1038/s43247-025-02958-y. hal-05454328

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# Melt inclusions reveal massive carbon dioxide emissions from continental intraplate volcanism

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Roxane Buso ©<fnref n="1" />,², Didier Laporte ©<fnref n="1" />, Federica Schiavi ©<fnref n="1" /> & Nicolas Cluzel ©<fnref n="1" />

The quantities of carbon dioxide (CO₂) 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 CO₂ contents reaching several percent by weight. In most cases, these high CO₂ contents are inferred but not directly measured, because the volcanic products at the surface are strongly degassed. Here, we report the pre-eruptive CO₂ 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 CO₂ concentrations of up to 4.8 percent by weight (3.6 percent by weight on average), among the highest ever measured to date. Such CO₂-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 CO₂ for Bas-Vivarais). Despite relatively small magma volumes, eruptions of low-silica alkaline magmas in continental intraplate volcanic provinces release large amounts of CO₂ in the atmosphere. Thus, these provinces contribute much more to the global CO₂ flux and to the Earth's short-term climate than their low magma production rates would suggest.

Carbon is a volatile element that controls major geodynamic processes such as mantle melting and volcanic activity<fnref n="1" />. The volcanic flux of carbon from the deep mantle to the surface is a critical part of the global carbon cycle, with a strong impact on climate and therefore life on Earth². Due to its low solubility, most CO₂ is lost by degassing during magma ascent and eruption, making it difficult to measure pre-degassing CO₂ concentrations, and to calculate total CO₂ fluxes and carbon content of mantle sources³. Therefore, most estimates of the initial CO₂ contents in magmas come from indirect methods, such as measurements of volcanic emissions⁴,⁵, reconstructions from dissolved carbon content and isotope composition data⁶, use of an empirical relationship between melt CO₂ and SiO₂⁷,⁸, or use of refractory trace elements, such as Nb or Ba, as proxies to estimate the CO₂ content of mantle melts before degassing⁹. Unlike bulk rocks or matrix glasses, small droplets of magma enclosed in crystals as melt inclusions may escape degassing and preserve the volatiles they contain at the time of entrapment. Accordingly, analyzing melt inclusions trapped at mantle depths within the first crystals growing in a magma may provide direct access to the CO₂ concentrations of parental magmas prior to degassing¹⁰.

It is now well accepted that the generation of silica-undersaturated magmas in intraplate settings requires the involvement of a carbon-rich

metasomatized mantle source¹¹ and that these magmas release large amounts of CO₂ into the atmosphere upon eruption¹²⁻¹⁴. Indeed, CO₂ contents reaching several weight percent have been reported for primary magmas from oceanic islands, intracontinental volcanoes or continental rifts (Fig. 1 and Supplementary Data 1). However, these high CO₂ contents were obtained using the indirect methods mentioned above and have never been measured directly in glasses or melt inclusions before. So far, most of the analyzed melt inclusions have CO₂ contents of the order of a few hundred to a few thousand parts per million (ppm) (Fig. 1), with only a few studies reporting CO₂ concentrations ≥ 1 percent by weight (wt%) in olivine-hosted melt inclusions⁷,¹⁴⁻¹⁷ (Fig. 1). This can be attributed to two factors. Firstly, melt inclusions that have not undergone pre-entrapment degassing are very rare¹⁰,¹⁸. Secondly, melt inclusions are subject to various post-entrapment modifications, including formation upon cooling of a shrinkage bubble in which an important fraction of the CO₂ initially dissolved in the melt can be stored¹⁵,¹⁹. Indeed, twenty to ninety % of the CO₂ initially dissolved in olivine-hosted melt inclusions is sequestered as a fluid phase in the shrinkage bubble⁹,¹⁹,²⁰. In addition to the fluid phase, a considerable amount of carbon can be contained in carbonate microcrystals on bubble walls¹⁵,²⁰⁻²². Most of the previous studies have overlooked the carbon

<footnote n="1">¹Laboratoire Magmas et Volcans, OPGC, Université Clermont-Auvergne, CNRS, IRD, Clermont-Ferrand, France. ²Present address: Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK. e-mail: roxane.buso@manchester.ac.uk</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1** | Carbon dioxide contents of silica-undersaturated magmas in intraplate volcanic settings. Circles and triangles represent low-silica lavas in oceanic islands and continental intraplate settings, respectively. Each symbol is associated with measurements of CO₂ in melt inclusions and estimates of CO₂ in primary melts (see the compilation in Supplementary Data 1 for details and references). For CO₂ in melt inclusions, we report: (i) the maximum CO₂ content measured in the glass phase of</figcaption></figure>

the melt inclusions (in black); and (ii) the range of total CO₂ contents of melt inclusions calculated from the glass and bubble measurements (in red). Total CO₂ contents were obtained by taking into account the CO₂ stored in the bubble, either partially (only fluid CO₂) or totally (fluid CO₂ plus carbonate microcrystals), in addition to the CO₂ dissolved in the glass phase. The CO₂ contents of the primary melts and their standard deviations (SD) are reported in green.

contained in microcrystals, which certainly led to an underestimation of the total CO₂ contents of melt inclusions containing bubbles¹⁵,²³.

Here, we measure for the first time the CO₂ contents of near-primary melts in a continental intraplate volcanic province. Using a new high-pressure homogenization technique²³, we show that olivine-hosted melt inclusions from the Bas-Vivarais volcanic province (Massif Central, France) represent a rare occurrence of almost undegassed magmas that can contain up to 4.8 wt% CO₂. We discuss the implications of our results for the formation of these CO₂-rich magmas, the CO₂ content of mantle sources in continental intraplate environments, and the flux of CO₂ to the atmosphere.

# Results and Discussion

## Description of the melt inclusions and their bubbles

We analyzed 142 olivine-hosted melt inclusions from Bas-Vivarais volcanoes for major and volatile elements (Methods): 93 unheated inclusions and 49 homogenized at high-pressure. These inclusions are all basanitic (Supplementary Fig. 1) and hosted in Mg-rich olivines (forsterite 86 on average).

All the unheated melt inclusions contain one or more bubbles, occupying on average 7.8 ± 5.4 % (1 standard deviation [SD], *n* = 104) of the inclusion volume (Supplementary Fig. 2). There is a positive correlation between bubble size and inclusion size (Supplementary Fig. 3), consistent with an origin by homogeneous entrapment for most melt inclusions²³ (see Supplementary Discussion for details). The bubbles contain fluid CO₂ (in a gaseous and/or liquid state) and abundant microcrystals, which partially or totally cover bubble walls (Fig. 2d). These microcrystals are mainly carbonates with sulfides in subordinate quantities (see Supplementary Discussion).

## CO₂ concentrations of melt inclusions

The glass phase of unheated melt inclusions contains 1.5 ± 0.4 wt% H₂O on average (SD, *n* = 90) and 0.2 to 1.9 wt% CO₂ (0.8 ± 0.4 wt% on average; SD, *n* = 84). Concentrations of dissolved CO₂ in unheated melt inclusions show

no correlation with the concentrations of major or volatile elements (Supplementary Fig. 4) or with bubble volume (Fig. 3). Such elevated CO₂ concentrations have rarely been documented in the glass phase of melt inclusions and are among the highest reported so far (Fig. 1). Nevertheless, they represent only a small part of the total amount of CO₂ in the melt inclusions of our study, as demonstrated by the homogenization experiments at high pressure.

After homogenization at high pressure, melt inclusions no longer contain bubbles, but still have the same size and shape (Fig. 2a, b). The CO₂ contents measured after homogenization represent the total CO₂ contents of melt inclusions because the volatile contents of the bubbles have been dissolved in the glass. The CO₂ contents of melt inclusions homogenized at 2.0 to 2.5 GPa are extremely high, ranging from 1.6 wt% to 4.8 wt% (3.6 ± 0.7 wt% on average; SD, *n* = 47). This is highlighted in Fig. 2c, where the Raman spectrum of an unheated melt inclusion is compared with that of a homogenized inclusion. The peak of the CO₃²⁻ band at 1080 cm⁻¹ is particularly high and corresponds to a CO₂ content above 4 wt%. Based on the average CO₂ content measured in the glass phase of unheated melt inclusions (0.76 wt%) and the range of CO₂ contents measured in homogenized melt inclusions (1.60 to 4.83 wt%), we calculate that 52% to 84% of the total CO₂ of the Bas-Vivarais melt inclusions is initially trapped in the bubble, both as CO₂ fluid and carbonates.

Concentrations of dissolved CO₂ in homogenized melt inclusions show no correlation with major elements (Supplementary Fig. 4) or with bubble volume before homogenization (Fig. 3). It is remarkable that very high total CO₂ contents (between 4 and 5 wt%) are measured in both the melt inclusions containing medium-sized bubbles (>20 vol%) and those containing the smallest bubbles (<5 vol%).

## Carbon content of the mantle sources of Bas-Vivarais basanites

Olivine-hosted melt inclusions from Bas-Vivarais contain up to 4.8 wt% dissolved CO₂. These measured CO₂ contents are consistent with the CO₂
<figure><img src="image_5.png" /><figcaption>**Fig. 2** | Melt inclusions from Bas-Vivarais volcanic province (French Massif Central). Optical microphotographs of a melt inclusion before (a) and after (b) homogenization at high pressure (HOP no. 12b-MIa). The complete disappearance of the bubble after melt inclusion homogenization indicates that all the CO₂ initially stored in the bubble has been dissolved in the liquid phase. The black phases present in the melt inclusion after homogenization are sulfide grains. c Comparison of the Raman spectrum of an unheated melt inclusion (Ja2-Pt5-Gr4; 1.95 wt% CO₂) in blue with the spectrum of a melt inclusion after homogenization at high pressure</figcaption></figure>

(HOP11a-MIa; 4.83 wt% CO₂) in red. The larger the CO₃²⁻ band at 1080 cm⁻¹, the higher the CO₂ content of the glass. d Scanning electron microscope micro-photograph showing rhombohedral carbonate (magnesian calcite) microcrystals covering the bubble wall of a melt inclusion from Bas-Vivarais (microphotograph taken by Dr. Letian Li with a Helios G4 PFIB CXe system at the Thermo Fisher Scientific NanoPort in Eindhoven, the Netherlands). Scale bars: 20 µm in a and b, and 5 µm in d.

contents of primary magmas estimated using Sun and Dasgupta's²⁴ equation, which yields 4.3 ± 0.7 wt% CO₂ (SD, n = 76) for the SiO₂ contents of unheated melt inclusions (from 41.8 to 46.4 wt%) (Fig. 1). This suggests that the most CO₂-rich Bas-Vivarais inclusions trapped almost undegassed magmas with volatile contents close to those of primary magmas. In this case, the variability of measured CO₂ contents (from 1.6 to 4.8 wt%; Fig. 3) may result from melt inclusion entrapment at various depths²⁵. In comparison to other geodynamic settings, the Bas-Vivarais magmas are carbon-enriched relative to mid-ocean ridge basalts (MORBs, ~0.2 wt% CO₂²⁶) and basaltic arc magmas (0.3–1.3 wt% CO₂²⁷,²⁸), but comparable to primary magmas in other intraplate areas (Fig. 1).

To estimate the concentration of carbon in the mantle sources of Bas-Vivarais basanites, [CO₂] source, we consider that carbon mostly goes into the melt due to its high degree of incompatibility (D~0.00055 ± 0.00025²⁹) and that its concentration into the melt is almost inversely proportional to the melt fraction, F: [CO₂] melt = [CO₂] source/[D+F(1-D)] (1). For [CO₂] melt we use the maximum CO₂ content measured in homogenized melt inclusions as a proxy for undegassed magma. Here, we consider the case of a peridotite mantle, with melt fractions between 3 and 6 wt% (F = 0.03 to 0.06), in agreement with estimates of the degree of melting for monogenetic basic volcanoes⁵,⁸,¹⁶, and we obtain CO₂ contents in the source ranging from 1475 ± 160 to 2923 ± 304 ppm (2 SD) (i.e., 403 ± 44 and 798 ± 83 ppm C, respectively). These estimates of [CO₂] source are also valid in the case of a mixed peridotite and pyroxenite/hornblendite mantle source, for which similar degrees of melting are anticipated (F = 0.04)³⁰.

In comparison to the mantle sources of Bas-Vivarais basanites, the CO₂ content of the bulk mantle is much lower, ranging from 403 ± 147 ppm³¹ to 1235–1755 ppm³². Considering as reference the “convecting mantle” of Shimizu et al. ³³ (i.e., 697 ± 327 ppm CO₂), a minimum enrichment factor of 2.1 to 4.2 is necessary to reach the level of CO₂ estimated in the Bas-Vivarais mantle sources. Other evidence of carbon-rich mantle sources beneath the Bas-Vivarais, and more broadly beneath the whole French Massif Central, has already been highlighted and includes the occurrence of small volumes of carbonatites at the surface³⁴, the observation of CO₂-rich fluid inclusions in mantle xenoliths³⁵, and the abundance of bubbling springs whose CO₂ comes predominantly from the subcontinental lithospheric mantle³⁶. Overall, all Cenozoic volcanism in western and central Europe appears to be characterized by carbon-rich mantle source regions. Notably, the involvement of a source enriched by carbonatitic or carbonated-silicate melts has

been proposed to explain the genesis of Kaiserstuhl (Germany)³⁷ and Bohemian Massif³⁸ nephelinites, the geochemical features of mantle xenoliths from western and central Europe³⁹, and the presence of carbonatites from central Europe (Kaisertuhl and Eifel, Germany) to south-central Spain (Calatrava Volcanic Province), via France⁴⁰.

Although the mantle sources beneath the French Massif Central appear to be richer in carbon than the bulk mantle³¹,³², the Depleted MORB Mantle⁶,¹⁰,²⁹,³³ and most other mantle sources⁴,⁹,¹⁴,¹⁸, our estimates of CO₂ are within the very broad range of values inferred for the mantle sources of other intraplate settings: from 1210 to 2623 ± 1924 ppm CO₂ for OIBs⁵,⁸,¹⁶,⁴¹,⁴², and from 1850 ± 568 to 2451 ± 1722 ppm CO₂ in continental settings⁵. In line with recent proposals¹³, our results therefore suggest carbon enrichment of the mantle sources of continental intraplate volcanism and raise the question of how CO₂ becomes concentrated in these sources. According to experimental and petrological evidence, the most suitable metasomatic agents for enriching the mantle with carbon are carbonatitic melts⁴³, formed by incipient melting of carbonated peridotites¹ and/or subducted carbonated oceanic lithosphere¹¹.

Consequently, the very high CO₂ concentrations of the Bas-Vivarais magmas, the highest ever measured in basaltic and basanitic melt inclusions to date, confirm that carbon-rich mantle sources and primary magmas with high CO₂ contents are the rule rather than the exception in intraplate settings, and particularly in continental intraplate context⁴,⁵,⁷,⁸. In the past, the failure to consider the carbon contained in the bubble of melt inclusions has led to an underestimation of the CO₂ content of magmas and, therefore, of mantle sources in intraplate settings.

## Implications for CO₂ flux and volcanic hazards

The abundance of CO₂ in the Bas-Vivarais magmas has potential implications in terms of volcanic hazards, both in relation to magma ascent and explosivity, and to the impact of CO₂ on the environment. The critical role of CO₂ as a controlling factor in the rapid ascent and explosivity of mafic magmas has recently been highlighted⁷,⁴⁴,⁴⁵. Our observations in the Bas-Vivarais province reinforce this conclusion since we can relate the high CO₂ contents of the magmas with very fast magma ascent. Indeed, the Bas-Vivarais basanites contain large mantle xenoliths, the largest we know having an estimated mass of 120 kg⁴⁶. These large xenoliths are evidence of fast magma ascent rates, close to the maximum values reported for peridotite-bearing alkaline basalts (~10 km h⁻¹)⁴⁷⁻⁴⁹. The abundance of
<figure><img src="image_6.png" /><figcaption>**Fig. 3** | Total CO₂ content vs. relative bubble size in melt inclusions. Total CO₂ content measured in homogenized melt inclusions (wt%; in red) plotted as a function of relative bubble size (bubble volume divided by the inclusion volume, in vol%). The dissolved CO₂ content of the glass phase of unheated melt inclusions (in blue) is shown for comparison. For homogenized melt inclusions, bubble size corresponds to the relative volume of the bubble before homogenization.</figcaption></figure>

mantle xenoliths and the absence of differentiated terms in the Bas-Vivarais volcanic province indicate a direct transport of magma to the surface without intermediate storage in the crust. Considering an ascent rate of 1 to 10 km h⁻¹ and a mantle source located at a depth of 70-80 km, it would take between three days and less than ten hours for magmas to reach the surface, leaving little time for the detection of any precursor signal.

Our data also highlight that large amounts of CO₂ can be released by intraplate alkaline continental volcanism, both on the scale of an entire province and of a given eruption. To illustrate this point, we consider the case of Jaujac volcano, one of the youngest in the Bas-Vivarais province. Assuming an initial CO₂ content of 4.8 wt% and an eruptive volume of 0.11 km³, we estimate that the Jaujac eruption released at least 15 ± 3 Mt CO₂ (2 SD) in the environment (240 Mt for the whole Bas-Vivarais province, see Methods). This figure represents a considerable release of CO₂ compared to the emissions from other volcanoes² and suggests that even small volume basanitic eruptions can release large amounts of CO₂ into the atmosphere. For comparison, the 2014–2015 Holuhraun eruption of Bárðarbunga, the largest in Iceland for more than 200 years, emitted 1.6 ± 0.3 km³ of lava²⁰ and 5.9–7.2 Mt CO₂⁵¹. Thus, despite a much lower volume of magma emitted (only 7%), the mass of CO₂ released by Jaujac volcano is more than twice that of the Holuhraun eruption. It is beyond the scope of this work to discuss how these large quantities of CO₂ are dispersed in the atmosphere at the time of the eruption and the potential risks for the environment on a local and regional scale. Notably, the possibility that CO₂ may spread along the flanks of volcanoes and accumulate in topographic depressions, forming potentially lethal channeled flows, as reported in the case of phreatic eruptions²²,⁵³, deserves particular attention.

It is widely accepted that mid-ocean ridges and volcanic arcs are the main contributors to the global volcanic CO₂ flux²⁵,⁵⁴,⁵⁵. However, our results suggest that continental intraplate volcanic provinces can also contribute significantly. While most recent big volcanic eruptions in arc settings generated around 10 Mt of CO₂ per km3 of magma produced, small-volume intraplate volcanoes release up to ten times more CO₂ per unit volume of magma (Fig. 4). Notably, we estimate that the Jaujac eruption produced 134 Mt CO₂ per km3 of magma, a figure in agreement with recent estimates for the 2021 Tajogaite eruption (La Palma)⁵⁶. These estimates indicate that intraplate basic cinder cones can release much larger quantities of CO₂ than their small size would suggest.

These much higher estimations of CO₂ fluxes associated with eruptions in a continental intraplate context are direct consequences of our reassessment of the total CO₂ content of melt inclusions. This raises the question of the importance of continental intraplate magmatism for global CO₂ flux

and Earth's climate. This issue has already been addressed in the case of flood basalts and large igneous provinces⁵⁷,⁵⁸. In the case of Deccan traps, for instance, by reassessing the initial CO₂ contents of magmas (up to 1.3 wt% CO₂) and taking into account both intrusive and extrusive outputs, it has been shown that early Deccan magmatism could have been responsible for the 2-4 °C warming episode that preceded the Cretaceous-Paleogene boundary⁵⁹. Although the basalt extrusion rate per continental volcanic field is low (2.2 × 10⁻³ km3 yr⁻¹ on average, accounting for rift volcanism) compared to flood basalts (0.8 km3 yr⁻¹ on average)⁵⁹, this is offset by the potentially higher CO₂ content of the magmas and the number of continental volcanic fields active simultaneously worldwide. For example, 37 Quaternary volcanic fields have been reported in the southwestern USA alone⁶⁰. Assuming a similar distribution on all continents, Quaternary continental intraplate volcanic fields would correspond to an extrusion rate of 0.4 to 0.7 km³ yr⁻¹, which combined with an initial CO₂ content of 4.8 wt % in primary magmas, as observed in the Bas-Vivarais, results into a global CO₂ flux of 60 to 89 Mt yr⁻¹. This is in line with recent findings on CO₂ emissions from continental rifts¹²,¹³. Even higher fluxes can be expected during phases of high continental magmatic activity, particularly during supercontinent breakups. Thus, even if the contribution of continental intraplate volcanism to the total CO₂ budget is minor with respect to anthropogenic emissions, it has certainly influenced the Earth's climate, in the short and long term, at a level disproportionate to the volume of magma produced. Indeed, recent studies⁶¹,⁶² argued that phases of high continental alkaline magmatism activity are linked to periods of global warming. In the future, the contribution of continental intraplate volcanoes to global CO₂ flux and their potential to cause periods of warming will have to be reevaluated, taking into account our measurements of the primary magma CO₂ contents.

## Methods

### Samples

We collected olivine phenocrysts in basanitic tephras from the strombolian pyroclastic fallout of three Quaternary volcanoes in the Bas-Vivarais volcanic province (Ardèche, France): Thueyts, Jaujac and Souilhol (see Supplementary Discussion). The phenocrysts are generally between 650–2000 µm in size and usually contain several glassy melt inclusions. These melt inclusions are irregular polyhedra with concave faces, in general larger than 30 µm, and contain one or more bubbles (Supplementary Fig. 2). The melt inclusions and the bubbles were measured and photographed using an optical microscope.

The olivine grains used for this study were separated into two fractions. The first fraction was directly polished to expose the melt inclusions at the surface to analyze the glasses using electron probe microanalysis (major elements, S, Cl, and F) and Raman spectrometry (H₂O and CO₂). The olivines in the second fraction were subjected to high-pressure homogenization. After the experiments, the homogenized glasses were prepared and analyzed in the same way as the unheated melt inclusions. All the melt inclusions analyzed were far from the grain surface, not connected to the external melt, and randomly distributed.

### High-pressure homogenization experiments

Given the presence of CO₂-rich bubbles in the Bas-Vivarais melt inclusions, high-pressure homogenization was essential to recover the original CO₂ content of the trapped melts. We conducted homogenization experiments in a piston-cylinder apparatus at 2 to 2.5 GPa and at 1300 °C, using the method developed by Buso et al. ²³. We observed a complete homogenization of the melt inclusions after experiments, as evidenced by the absence of bubbles.

### Raman analysis

We analyzed the water and CO₂ contents in the glass phase of unheated and homogenized melt inclusions in back-scattered geometry using a Renishaw inVia confocal Raman micro-spectrometer at the Laboratoire Magmas et Volcans. This spectrometer is equipped with a 532.1 ± 0.3 nm diode-pulsed
<figure><img src="image_7.png" /><figcaption>**Fig. 4** | CO₂ emissions and erupted magma volume for recent eruptions. Amount of CO₂ emitted per unit volume of magma erupted (Mt km⁻³; circles, with the ordinate axis on the right) and volume of erupted material (km³; rectangular boxes, with the ordinate axis on the left) for a series of recent, historical and prehistoric eruptions, including the Jaujac eruption in the Bas-Vivarais province. See Supplementary Data 6 for details and references.</figcaption></figure>

solid-state laser (180 mW output power), a CCD detector of 1040 × 256 pixels, a Rayleigh rejection edge filter, and a Leica DM2500 M optical microscope with a motorized XYZ stage. We used a grating of 2400 grooves mm⁻¹, a 20 µm slit aperture (high-confocality setting), a × 100 microscope objective (numerical aperture of 0.9), and a 3 mW laser power on the sample. Daily calibration of the spectrometer was carried out using a silicon standard (520.5 cm⁻¹ peak). These analytical conditions result in a spectral resolution of ~0.5 cm⁻¹, while the lateral and vertical spatial resolutions were, respectively, 1 µm and 2–3 µm near the sample surface. The WIRE™ 4.4 software recorded the spectra in the ranges from ~100 to 1350 cm⁻¹ (aluminosilicate network region) and from ~2900 to 3800 cm⁻¹ (water region) for eight cycles of 30 s. As some spectra were contaminated by the signal of the host crystal, we corrected this effect by subtracting the pure olivine spectrum (measured in the same analytical conditions as for the melt inclusions) from the glass spectra.

The water and the CO₂ contents in melt inclusion glasses were determined at 1 µm depth using seven basanitic reference glasses with known volatile contents following the method described by Schiavi et al.⁶³ and Buso et al.²³, respectively. The uncertainty of water concentrations is calculated from the error related to the linear regression for 95% confidence interval according to Schiavi et al.⁶³, ~0.2 wt% (2 SD). The possible sources of error on CO₂ quantification highlighted by Morizet et al.⁶⁴ are related to the peak fitting procedure (estimating area and position of peaks), Raman scattering cross sections, and the linear regression describing the relationship between the total CO₂ content and the area of the spectral component corresponding to the CO₃²⁻ dissolved in the glass (see Fig. 3 in Buso et al.²³). The uncertainties associated with the linear regression defined by the seven basanitic reference glasses used are around 3%. As we used reference glasses with a composition close to that of the inclusions, the uncertainties associated with the potential dependence of the Raman scattering cross sections of carbonates on glass composition are negligible. Finally, the most significant uncertainties are linked to the fitting procedure. The reproducibility of the concentrations obtained for replicate fits on the same spectra varies with the CO₂ content. Thus, the total uncertainty associated with the quantification of CO₂ is less than or equal to 10% relative in the case of high CO₂ contents (>1.5 wt%) and greater than or equal to 15% relative for low CO₂ contents (≤1.5 wt%), in particular for CO₂ contents close to the limit of

detection by Raman spectroscopy (0.2 wt%). This is because the carbonate peak is higher and better defined in CO₂-rich glasses, which makes it easier to fit (Fig. 2c). All the water and CO₂ contents, and their associated errors, are listed in Supplementary Data 2–5, and described in Supplementary Discussion.

## Electron probe microanalysis (EPMA)

After Raman analysis, unheated and homogenized melt inclusion glasses, and their host olivines were analyzed for major and volatile (Cl, F, and S) elements with a CAMECA SXFive-TACTIS electron microprobe, at the Laboratoire Magmas and Volcans, using a 15 kV accelerating voltage. Major elements were analyzed using a 1 µm diameter focused beam and a current of 15 nA for olivines, or a defocused beam of 5–10 µm diameter and a current of 4–8 nA for melt inclusions. Volatiles were measured with a 5 µm beam size and a 40 nA current. Three analytical standards (scapolite, fluorite and VG-2 glass) were measured at the beginning of the session to calibrate the analysis of volatile elements. Details on the analytical procedures, including the counting times for each element and the calibration used for each glass analysis session, are given in Rose-Koga et al.⁶⁵. The raw compositions obtained with EPMA are listed in Supplementary Data 2 and 4. The relative analytical errors associated with the volatile element analysis are 15% for F, and 5% for S and Cl⁶⁵.

## Correction of post-entrapment modifications

The compositions of unheated and homogenized melt inclusions have been corrected for post-entrapment crystallization (PEC) or post-entrapment dissolution (PED) of the host olivine with the MIMiC software⁶⁶. These corrections involve adding/subtracting increments of olivine in the liquid until the Fe-Mg partition coefficient between olivine and liquid (K_D) reaches the equilibrium value (calculated using the model of Toplis⁶⁷). An oxygen fugacity of + 1 log unit relative to the quartz–fayalite–magnetite oxygen reference buffer (ΔQFM + 1) was used as determined by Jannot et al.⁶⁸ for the Chaîne des Puys (French Massif Central). This is in line with the value estimated by Gaborieau et al.⁶⁹ and corresponds to a Fe³⁺/ZrFe ratio of 0.19.

All the unheated melt inclusions experienced PEC, with a degree comprised between 1.3% and 13.5% (6.1% on average). For homogenized melt inclusions, the correction ranges from 14.9% olivine crystallization to 5.2% olivine dissolution (2% crystallization on average). It should be noted that the main results of this study do not depend on the PEC/PED correction, since the use of corrected and uncorrected compositions leads to similar conclusions. Unheated melt inclusions could have undergone some Fe loss, but this cannot be assessed because the FeO content of Bas-Vivarais bulk rocks is biased by systematic contamination by crustal and mantle xenocrysts. The PEC/PED-corrected compositions are listed in Supplementary Data 3 and 5, and described in Supplementary Discussion.

## Calculation of the quantity of CO₂ emitted

We estimated the mass of CO₂ released by the eruption of Jaujac volcano, MCO₂, using the equation:

$$M_{\text{CO}_2} = \rho V_{\text{DRE}} ([\text{CO}_2]_{\text{MI}} - [\text{CO}_2]_{\text{Matrix glass}}) \quad (1)$$

where ρ is the melt density, V_DRE is the volume of erupted magma (dense-rock equivalent), [CO₂]MI represents the initial weight fraction of CO₂ in undegassed magma, and [CO₂]Matrix glass represents the weight fraction of CO₂ remaining after degassing. Melt density (2777 ± 43 kg m⁻³; 2 SD) was calculated using the relationship between density and volatile content (H₂O + CO₂) of basanitic glasses developed by Buso et al.²³, considering the highest volatile contents measured in Jaujac melt inclusions. Volume V_DRE (0.11 ± 0.01 km³; 2 SD) is the sum of the volume of Jaujac scoria cone (0.09 km³) and the volume of the associated lava flow (0.02 km³; Vincent⁷⁰). For [CO₂]MI, we use 0.048, i.e., the maximum CO₂ weight fraction measured in Jaujac homogenized melt inclusions. As a first approximation, we set [CO₂]Matrix glass to zero because no CO₂ was detected by Raman spectroscopy in the matrix glass surrounding olivine crystals. The possible sources
of error on the quantification of the mass of CO₂ emitted by the Jaujac volcano (15 ± 3 Mt CO₂; 2 SD) are related to the uncertainties associated with the melt density calculation, the CO₂ quantification in melt inclusions and the Volume V<sub>DRE</sub> estimation. The mass of CO₂ released by the entire Bas-Vivarais province (240 Mt CO₂) was calculated by assuming that the volume of magma emitted by each of the sixteen volcanoes was of the same magnitude as that of Jaujac, so that the whole province mobilized 16 times the volume of Jaujac.

## Data availability

All data supporting the findings of this study are available within the article, its Supplementary Information, and Supplementary Data files. The Supplementary Information includes additional figures, discussion, and references. The Supplementary Data files contain the melt inclusion compositions, as well as the Excel spreadsheets used to produce Figs. 1 and 4. Raw Raman spectra used to produce Fig. 2 are also provided. The Supplementary Data files and Raman spectra are available on Figshare at https://doi.org/10.6084/m9.fishshare.29901638.

Received: 18 December 2024; Accepted: 28 October 2025;
Published online: 12 December 2025

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## Acknowledgements

The authors thank the following people for their help and technical assistance: Jean-Luc Devidal (electron probe microanalysis), Franck Poinçaud and Antoine Mathieu (management and maintenance of experimental facilities), and Jean-Louis Fruquière and Cyrille Guillot (machine shop). We also thank Pierre Condamine, Mickaël Laumonier, and Manon Hardiagon for scientific discussions and advice. This work was carried out within the framework of the VolcAURA project, which was funded by the Auvergne-Rhône-Alpes region (Pack Ambition Recherche; grant no. VolcAURA-LS172914). It also received financial support from the Geopark UNESCO des Monts d'Ardèche. Roxane Buso thanks the Auvergne-Rhône-Alpes region for funding her doctoral thesis. This is contribution no. 712 of the ClerVolc program of the International Research Center for Disaster Sciences and Sustainable Development of the Université Clermont Auvergne.

## Author contributions

Conceptualization: D.L., Investigation: R.B., D.L., F.S., N.C., Visualization: R.B., Funding acquisition: D.L., Writing—original draft: R.B., Writing—review and editing: R.B., D.L., F.S., N.C., and Validation: R.B., D.L., F.S., N.C.

## Competing interests

The authors declare no competing interests.
**Additional information**

**Supplementary information** The online version contains supplementary material available at
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**Correspondence** and requests for materials should be addressed to
Roxane Buso.

**Peer review information** *Communications Earth & Environment* thanks Thomas van Gerve and the other anonymous reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Feifei Zhang, Carolina Ortiz Guerrero, and Aliénor Lavergne. A peer review file is available.

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