Fluid-induced and vapor-sustained magma production at the Stromboli and Campania Province volcanoes (Southern Italy)

Fluid-induced and vapor-sustained magma production at the Stromboli and Campania Province volcanoes (Southern Italy) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Fluid-induced and vapor-sustained magma production at the Stromboli and Campania Province volcanoes (Southern Italy) Roberto Moretti This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8810254/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The Italian territory is heavily invested by widespread deep degassing characterized by huge emissions of carbon dioxide. The peri-Thyrrenian area of southern Italy is where these emissions are accompanied by active volcanism, with magmas displaying alkalic affinity and covering a wide compositional spectrum. It has long been debated whether such a production of CO 2 is related to decarbonation of crustal limestones or to other deep source features. However, a global look at melt inclusions and plume data from southern Italian volcanoes suggests that magmas are produced under CO 2 -rich and fluid-buffered conditions that initiate in the lithospheric mantle. Nevertheless, what we can “see” when “looking at” these data depend on the reliability of multicomponent saturation models for the volatile-melt systems. Using an accurate non-linear thermodynamic model, here I process relevant melt inclusion data and infer that volcanic plumbing systems are steadily infiltrated by deep mantle-derived fluids with a CO 2 /H 2 O ratio between 0.5 and 1 (molar), increasing southward. Furthermore, melt inclusions show variable degrees of CO 2 -enrichment (fluxing), related to 1) increasing CO 2 /H 2 O vapor (gas) ratio and flux in the ascending mantle-derived fluid or 2) cross-sectional variations of the ascending flow. The first scenario is related to variations occurring in the mantle source that are about its composition (mainly the alkali content) and the pressure/depth of early fluid release: The second scenario accounts for the fact that molten layers retain water and release a relatively CO 2 -enriched fluid upward, determining an effective H 2 O-CO 2 chromatography-like separation as long as melting goes on. The two scenarios are likely to operate in combination over time and space. Carbon dioxide water melt inclusions vapor/melt ratio mush rejuvenation Figures Figure 1 Figure 2 Figure 3 1 Introduction Two main interrelated endogenous processes distinguish the peri-Thyrrenian margin of Southern Italy: 1) the production of magma, episodically but recurrently erupted from volcanoes showing alkalic affinity (e.g., Etna, Stromboli, Vulcano, Somma-Vesuvius, Campi Flegrei, Ischia, Procida; Fig. 1 ), and 2) the deep CO₂ degassing, not only by direct volcanic emissions but also by diffuse degassing from regional extensional structures non volcanic per se [Peccerillo, 2005 ; Moretti et al., 2013 ; D’Antonio et al., 2013 ; Buono et al., 2023 ; Chiodini, et al., 2004 ; Fischer and Aiuppa, 2020 ; Tamburello et al., 2010; Frezzotti et al., 2010 ]. This degassing is a major source of natural carbon emissions, with some non-volcanic areas releasing amounts comparable to the global volcanic system [Chiodini et al., 2004 ; Fischer et al., 2020; Aiuppa et al., 2019 ; Burtono et al., 2013]. The degassing is linked to the region's geodynamic setting, where the African plate is subducting beneath the Eurasian plate [Peccerillo, 2005 ; Jolivet et al., 2008; Acocella and Funiciello, 2006 ] (Fig. 1 ). This process causes the recycling of subducted carbonates deep within the mantle, generating CO₂-rich fluids that migrate upward through deep lithospheric faults and fracture systems. The actively degassing structures, such as the Tuscanian-Roman and Campanian degassing structures [Chiodini et al., 2004 ; Fischer and Aiuppa, 2020 ], are then the fingerprint at the surface of the deep regional volcano-magmatic structures that, in association with extensional tectonics, have shaped the peri-Thyrrenian margin of the Italian peninsula (Fig. 1 ). CO 2 release is manifested in different ways such as 1) diffuse soil degassing, occurring as widespread, non-eruptive emissions through the soil in both volcanic and non-volcanic areas [Chiodini et al., 2004 ; Fischer and Aiuppa, 2020 ], 2) gas-rich springs, where CO₂ is often dissolved in cold and thermal groundwaters [Chiodini et al., 2004 ; Gambardella et al., 2024] and, of course, 3) direct emissions from active volcanoes like Stromboli, Etna and Campi Flegrei [Aiuppa et al., 2019 ; Burtono et al., 2013]. A major implication of the mutual interrelation between deep CO 2 degassing and volcanic-magmatic activity is then the imbalance that emerges when confronting the huge natural fluxes of volatiles (CO 2, particularly, but also H 2 O) versus the comparatively little production and extrusion of magmas from active Italian volcanoes [Edmonds et al., 2022 ; Moretti et al., 2018 ; Aiuppa et al., 2010 ; Pino et al., Moretti et al., 2013 b; Aiuppa et al., 2022 ; Ferlito, 2018 ]. In this context, volcanoes can be seen as deep boreholes that pierce the source areas of CO 2 and shed light on the role of CO 2 -rich fluids at crustal depths where volcanic plumbing systems extend downward. Most of the information comes from the study of melt inclusions (MI), which are droplets of melt trapped in phenocrysts [e.g., Roedder, 1984 ] during the volatile-saturated polybaric pathways of magma ascent and evolution from deep-crustal magma chambers to the surface. Briefly, MIs provide fundamental qualitative and quantitative constraints on magma storage conditions and pre-eruptive degassing and represent a fundamental complement to gas discharges vented at the surface [e.g., Buono et al. 2023 ; Moretti et al., 2018 ; Aiuppa et al., 2010 ; Pino et al., 2011 ; Moretti et al., 2013 b; Aiuppa et al., 2019 ]. In this study, I use MIs from Stromboli, Campi Flegrei, Ischia, and Somma-Vesuvius volcanoes to elucidate the role of CO 2 -rich fluids of mantle origin in driving the evolution of plumbing systems associated with these volcanoes and the geodynamic setting in which they are embedded. 2 Methods and selection criteria I have performed a literature survey of melt inclusions (MIs) from most mafic magmas erupted by active volcanoes of Southern Italy (Stromboli, Vesuvius, Ischia, Procida, and Campi Flegrei; Fig. 1 ) that run NW to SE along the peri-Thyrrenian margin of Southern Italy and surround the sector of interest here (semi-transparent round-rectangular area in Fig. 1 ). These volcanoes have been selected because their rock geochemistry shares common source features, such as trace element ratios and isotopes, strongly suggesting that Somma-Vesuvius and the adjoining Campi Flegrei, Procida, and Ischia volcanoes are the northern extension of the eastern Aeolian arc rather than the southern end of the so-called Roman Province [Peccerillo, 2001 , 2005 ]. Table 1 Average composition (wt %; CO 2 in ppm wt) and temperature of melts from studied volcanoes (Str: Stromboli; CF-Pr-Is: Campi Flegrei and Ischia; SV: Somma-Vesuvius). Str Shoshonite CF-Pr-Is shoshonite; latite; trachy-basalt SV (K-)tephrite T (°C) 1150 1120 1100 SiO 2 51.61 50.16 48.55 TiO 2 0.87 0.93 0.93 Al 2 O 3 16.22 17.35 14 Fe 2 O 3 1.87 2.93 4 FeO 6.55 3.96 4.12 MnO 0.16 0.18 0.13 MgO 6.64 3.57 7.6 CaO 10.61 9.37 13.38 Na 2 O 2.45 1.51 1.77 K 2 O 1.27 5.08 5.13 H 2 O (min-max) 0.8 — 3.84 0.2 — 4.25 0.37 — 6.05 CO 2 (min-max) 360 — 2264 56 — 4601 188 — 4432 Notes Aiuppa et al. ( 2010 ); Pino et al. ( 2011 ) and refs. therein; MIs are from the so-called “golden” low-porphyric magma Moretti et al. ( 2013 a); Mangiacapra et al. ( 2008 ); Mormone et al. ( 2011 ); one MI from Esposito et al. (2025) (sample RE.SC2.F4.M1) Marianelli et al. (20925) and refs. therein; data are from the 1637 CE–1944 CE period of Strombolian activity I focus on the mafic, less-differentiated magmas as they are representative of the deepest - or at least deeper - portions of the plumbing system of selected volcanoes. Therefore, they are the most suitable to extract information about the fluids in the deep magma-source(s). Based on MI analyses, I set average melt compositions and temperatures for each investigated volcanic setting (see Table 1 ), according to published data and elaborations (see Table 1 and Fig. 2 captions). Campi Flegrei (CF), Ischia (Is), and Procida (Pr) are considered together as they constitute the Phlaegrean Volcanic District (PVD), which is structurally and morphogenetically distinct from Somma-Vesuvius [Acocella and Funiciello, 2006 ; Moretti et al., 2013 a]. Using a well-established model for gas-melt saturation [Papale et al., 2006 a; Papale et al., 2022 ], I have then calculated the H 2 O and CO 2 saturation conditions, which form the basis for the forthcoming discussion of results and interpretation. Conditions for model initialization are reported in Table 1 . The reader should refer to Papale et al. ( 2022 ) for a full review of melt-fluid equilibria and the relevant techniques to compute H 2 O-CO 2 -melt conditions. 3 Results and data inspection Figure 2 reports H 2 O-CO 2 saturation isobars for melt compositions typified as in Table 1 . Entrapment pressures are up to ∼ 310 MPa for Stromboli, ∼ 450 MPa for Vesuvius, and ∼ 480 MPa for Ischia and Campi Flegrei. In the Str saturation plot (Fig. 2 a), datapoints are, with very good approximation, bounded by a CO 2 /H 2 O ≈ 1 (molar) isopleth on the right side (H 2 O-rich) of the diagram. In the SV saturation plot (Fig. 2 b), datapoints have been differentiated to better appreciate H 2 O-CO 2 pathways per event. Overall, they are better bounded on the right by a limiting isopleth [CO 2 /H 2 O] v ≈ 0.5. This ratio also applies to the rightward bounding isopleth in the CF-Pr-Is saturation plot (Fig. 2 c). In the SV case, a cluster of CO 2 -enriched datapoints is visible at about 100 MPa, with [CO 2 /H 2 O] v ratios exceeding those of the CO 2 -richest MIs from pressures > 200 MPa. In all cases, datapoint alignments emerge that follow nearly isobaric lineages (sketched by a yellow semi-transparent arrow in Fig. 2 ), along which CO 2 /H 2 O vapor ratios increase up to ∼ 20 (molar) at CF-Pr-Is (Fig. 2 c). Such arrays of CO 2 -enrichment (CO 2 -fluxing) appear at several pressures (hence depths) throughout plumbing systems and emerge clearly at the highest entrapment pressures, that is, at the bottom of each plumbing system. In each saturation plot, the location of the CO 2 /H 2 O bounding trend is not affected by computations performed within a ± 100°C interval around adopted temperatures (see Table 1 ). Furthermore, changing for each volcanic type redox conditions (i.e., the FeO and Fe 2 O 3 proportions in Table 1 ) or the average compositions (within the range of variations displayed by MIs), shifts the pressure range but produces minor changes in the bounding [CO 2 /H 2 O] v values. This is evident in panel d of Fig. 2 , where all MIs have been plotted on an H 2 O-CO 2 saturation diagram. For the sake of clarity, in Fig. 2 d, only isobars for SV and CF-Is-Pr have been plotted, because together they embrace the computed excursion of each isobar. Figure 2 d also displays the variation for the isopleths with [CO 2 /H 2 O] v i ratios of 0.5 and 1. Adopting compositions more or less differentiated than those in Table 1 would be another choice of reasonable conditions to compute saturation properties. However, this would not alter the arguments presented in the forthcoming Discussion section, which are based on the main result of this study: on the water-rich saturation diagram, each magmatic system within the geodynamic sector of interest (Fig. 1 ) displays a bounding vapor composition, with comparable amounts of CO 2 and H 2 O. Given that the CO 2 /H 2 O limiting isopleths, trends of magma decompression under closed- or open-system degassing that are generated by starting conditions along or close to the [CO 2 /H 2 O] vapor bounding lines cannot reproduce the limiting isopleth, as typified by the CUSAT line in Fig. 2 a. Deep magmas displaying [CO 2 /H 2 O] vapor > 1 can generate decompression paths under closed-system degassing conditions that cross the [CO 2 /H 2 O] vapor bounding line in the low-pressure field, such as the CUSAT line in Fig. 2 c. However, trends like the CUSAT ones in Fig. 2 a,c cannot reproduce the dissolved H 2 O and CO 2 patterns of the bounding volatile isopleths. To match vapor isopleths, the ascent paths of magma decompression under closed-system degassing conditions must have vastly high total (exolved + dissolved) volatile contents (e.g., the CFSAT trend in Fig. 2 a). Therefore, very high volatile-rich melts rising under closed-system degassing conditions have CO 2 /H 2 O vapor ratios corresponding to isopleth ones because the vapor/melt ratio is so high to make the vapor phase unaffected by the exchange of volatiles with the melt over the entire pressure range of interest and not only at low pressures, when all degassing trends deviate to converge to zero (Fig. 2 ). 4 Discussion Melt inclusions in Fig. 2 are all from alkalic mafic magmas erupted by the investigated volcanoes on the peri-Thyrrenian margin of southern Italy, genetically related to the subduction of the African plate beneath the Eurasian plate and the opening of the Tyrrhenian basin. However, important compositional differences characterize these volcanoes and their magmas, which are related to their sources and their differentiation histories. Such differences affect the composition of magmas, particularly their alkalic affinity, the relative proportions of H 2 O and CO 2 in their equilibrium fluid, and the oxidation state [D’Antonio et al., 2013 ]. It is then quite surprising to observe that for all studied volcanoes, MI data from the less differentiated magmas are limited by a vapor isopleth with a CO 2 /H 2 O ratio in the 0.5-1 range 1 (molar) (Fig. 2 ). This bounding condition applies over the entire pressure range of MIs’ entrapment (Fig. 2 d) and is unlike to be a casual feature, given that 1) studied magmas are genetically similar [Peccerillo, 2011] but on the other side, they have different physico-chemical and saturation properties (Fig. 2 ), and 2) widespread diffuse regional degassing of deep CO 2 occurs in the studied sector, pointing to a common mantle source accumulating volcanic and non-volcanic degassing [Chiodini et al., 2004 ; Fischer and Aiuppa, 2020 ; Tamburello et al., 2018 ; Frezzotti et al., 2010 ; Aiuppa et al., 2019 ; Burtono et al., 2013; Gambardella et al., 2004 ]. On the right side (H 2 O-rich) of each saturation plot (Fig. 2 ), the limiting isopleth goes across the entire plumbing system, embracing deep (P > 400 MPa or z > 15 km by using a melt/rock density of 2700 kg/m 3 ) and shallow melts, and defines an ascent path in which both the vapor volume and mass fractions vastly exceed the melt ones. In other words, magma ascent and decompression should occur under vapor-dominated conditions if the initial [CO 2 /H 2 O] vapor ratio is close to that of the bounding isopleth. Contrary to Str and SV, in the case of CF-Pr-Is (Fig. 2 c), no high-pressure (> 200 MPa ) MIs fall close or along the bounding isopleth, and one can assume that typical closed-system decompression trends may originate from high pressure for [CO 2 /H 2 O] vapor > 1, such as in the case of the CUSAT line in Fig. 2 c. The curvature that this trend must have not to cross the limiting isopleth depends on the total volatile content (sum of dissolved and exsolved H 2 O and CO 2 ), which is ∼ 8 wt% or ∼ 25% molar, with already > 20% of volume vapor fraction at 400 MPa, i.e., ∼ 16 km deep in the crust. These numbers require that deep magmas in the crust have a very low density, approaching that of the H 2 O-CO 2 fluid at very high vapor/melt ratios. Such magmas would be highly buoyant, hence unstable at their equilibrium depth and prone to ascend. This implies that deep, highly buoyant magma blobs overcome the surrounding confining pressure and rise from large crustal depths under closed-system degassing conditions to feed the magma chambers developed throughout the plumbing system. This is in line with the activity observed at open-conduit basaltic volcanoes and certainly works during the early extensional phases, building the volcanic system and its vertically extended plumbing system. However, the persistence of continuous transfer of extremely volatile-rich blobs appears counterintuitive at a stage when volcanic systems are well established. Volcanoes such as Somma-Vesuvius, Campi Flegrei, Procida, and Ischia tend to evolve as closed-conduit systems, and their less differentiated eruptive products have sampled deep MIs, recording pressures and depth conditions almost twice those of a currently open-conduit volcano such as Stromboli (Fig. 2 ). In the classical view (henceforth CV), the pre-eruptive evolution of plumbing systems is characterized by magma chamber refilling that occurs by the upstream of magmas, which are the “moving parts” coming from an often unspecified deep crustal or mantle reservoir. When these magmas emplace, they release partly or totally the vapor phase as they rise and equilibrate under fluid pressures that allow the magma batch to overcome the strength of the surrounding rocks, thus opening the path upward. Overall, the formation of magma chambers is seen as resulting from a sequence of episodes of volatile-rich magma injection through extensional structures under both closed- and open-system degassing conditions. In this view, melting and magma production occur in a deep-mantle source, and the subsequent history is characterized by stepwise magma migration and differentiation, including the separation and release of magmatic vapors. However, this view is adopted almost inevitably, even when volcanoes like those studied here are well-established and settled systems, in which plumbing systems result from long-standing, preceding activity of magma emplacement, differentiation, and extrusion. At such a stage, plumbing systems are mostly composed of crystal mushes [Moretti et al., 2013 a; Buono et al., 203; Esposito et al., 2018 ; Moretti et al., 2019 ], which can be pictured as huge piles or columns extending throughout the crust and plugging the extensional lithospheric structures through which earlier volcanoes were built. In this context, the very high volatile abundances emerging from this study conflict with the general applicability of the CV. Similar to what was done for Etna by Ferlito ( 2018 ), this conflict justifies my inference of a more suitable working scenario for volatile-flooded volcanic plumbing systems. Here, I enlarge the model presented in [Moretti et al., 2019 ] and propose that static mushy systems fluxed by H 2 O-CO 2 -rich fluids are similar to a chromatographic column. MIs could then represent locally re-equilibrated or newly molten mush pockets in which dissolved H 2 O-CO 2 contents are fixed by the upstream fluid flow rate, rather than snapshots of extremely volatile-enriched magma blobs that emplaced after closed-system ascent. The ascending fluids making that make up the background bounding isopleths (CO 2 /H 2 O from 0.5 to 1) in Fig. 2 are likely to be generated by deep mantle sources. A tempting scenario involves a single source common to all volcanoes, but buffered by pressure conditions that vary from North-West to South-East. The NW-to-SE increase in the [CO 2 /H 2 O] vapor ratio from 0.5 to 1 may have multiple explanations, which are beyond the scope of the current study. For example, it may result from the NW-SE deepening of the melt-H 2 O-CO 2 saturation surface of a hypothetical mantle magma reservoir. Overall, the CO 2 -enrichment and consequent melt dehydration (CO 2 -fluxing) along equilibrium isobars (Fig. 2 and refs. Moretti et al., 2013 a,b,2018, 2019; Edmonds et al., 2022 ; Aiuppa et al., 2009 , 2010 ; Pino et al., 2011 ; Mangiacapra et al., 2008 ; Mormone et al., 2011 ; Esposito et al., 2018 ; Marianelli et al., 2005 ; Allard et al., 2010) might then be a consequence of any shift in deep reservoirs that reverberates throughout the overlying layers. Because CO 2 -fluxing also occurs for MIs datapoints with the highest entrapment pressures (Fig. 2 ), there must be deep mechanisms of CO 2 magnification throughout the entire plumbing system, down to the pristine source region of ascending fluids. With reference to Fig. 3 , I propose that the (background) CO 2 -H 2 O fluid mixture on its ascent equilibrates with the melt batches in the plumbing system (left side, “ before ”, in Fig. 3 ), where magma differentiation (mainly via fractional crystallization) produces the crystals armoring melt inclusions in which dissolved H 2 O and CO 2 contents are imposed by the CO 2 /H 2 O ratio of the up streaming fluid mixture (Fig. 2 ). Then, to explain CO 2 -fluxing, two further scenarios can be pictured here: For open-conduit basaltic ( lato sensu ) volcanoes such as Stromboli, it was already shown that CO 2 -fluxing marks the arrival and emplacement in the shallow system of deep volatile-rich magma blobs. These blobs are pieces of magmatic foam in which the fluid phase is dominated by CO 2 (Aiuppa et al., 2009 , 2010 ; Allard et al., 2010; Pino et al., 2011 ). The magmatic foam forms at about 10 km depth, then collapses, and is released upward into the shallow system, which is directly connected to the crater [Allard et al., 2010; Pino et al., 2011 ]. This mechanism i ) requires melt addition from below, hence open pathways for the fast ascent of magma blobs, ii ) is in line with the CV about volcano functioning, and iii ) is tracked by significant increases of the plume CO 2 /SO 2 ratio and CO 2 fluxes, which in the short-term anticipate the so-called major and paroxysmal explosive eruptions [Aiuppa et al., 2009 , 2010 ; Allard, 2010 ]. Under this scenario, the shallow system that produces plume emissions via passive degassing is recurrently flooded by gas-melt slugs, that is, huge amounts of deep CO 2 -rich fluid carrying a relatively small amount of mafic magma. At Stromboli, eruptions occur shortly after (minutes to days; Aiuppa et al., 2009 ; Allard, 2010 ; Pino et al., 2011 ) the fluid invasion of the shallow system. The CO 2 -enriched (fluxed) melt inclusions (Fig. 2 a) have been trapped in olivines from the so-called “golden” low-porphyric magma, which has equilibrated with the incoming CO 2 -dominated ascending gas [Aiuppa et al., 2009 ; Allard, 2010 ; Pino et al., 2011 ]. A mechanism of fluid-induced magma remelting [Moretti, 2025] is instead more plausible at closed-conduit volcanoes. It does not require melt addition and produces MIs lineages that mirror magma ascent and degassing paths, characterized by very high vapor/melt ratios, which are unusual for the tensional state developed by a closed-conduit volcano. By fluid-induced re-melting, magmas “do not move”, and the high vapor/melt ratios are only apparent, reflecting the CO 2 and H 2 O content imposed by the flow rate of the vapor phase at the different pressures this encounters on its ascent through the mushy plumbing system. Following the energy budget reported in Morettu (2025), as well as the numerical framework in [Bachmann and Bergantz, 2006 ], a steadily upraising H 2 O-CO 2 flow can have the thermal potential to melt layers of the mushy plumbing system along the rock wet solidus. This re-melting process will be further favored if the bottom flux in conductive and/or advective heat increases. Figure 3 (rightward “ after ” side) sketches this fluid-induced melting process as related to the formation of a cross-sectional melt layer at some unspecified depth. There, most of the water from the ascending fluid is removed by dissolution into the forming melt. This changes the cross-sectional flow and, consequently, determines a chromatographic-like separation between CO 2 and H 2 O [Moretti et al., 2019 ] that enriches in CO 2 -the outgoing fluid-phase, which acquires CO 2 /H 2 O ratios up to 20 in the CF-Pr-s case SV case (Fig. 2 c). This, however, occurs without expanding the vapor/melt ratio to unreasonable values for a closed-conduit volcanic system. The now CO 2 -enriched upgoing fluids will then flux the overlying mush column and its magma pockets, being recorded by the entrapment of dehydrated and CO 2 -enriched melt inclusions, which can be armored by the same crystal already hosting more hydrous MIs equilibrated prior to the CO 2 -fluxing event and possibly recording the background [CO 2 /H 2 O] vapor value. In this respect, the increase of CO 2 fractions in fumarolic gas emissions vented at the surface of closed-conduit volcanoes is a strong indication that magma rejuvenation might be occurring in the plumbing system [Moretti, 2025; Moretti et al., 2025 ]. This process paves the road to future eruptive episodes of explosive nature but occurs without any apparent geophysical evidence of magma rise and chamber formation or replenishment at shallow depth. Summarizing, the CO 2 -enrichment scenario 1 demands a deep perturbation, leading to increased fluid — essentially CO 2 — and heat fluxes, eventually accompanied by the arrival of magma blobs rising in a very permeable conduit system characterized by an open-fracture architecture. On the other hand, scenario 2 suggests that CO 2 -enrichment results from deeper magma rejuvenation, induced by the infiltration of a deep-derived fluid whose flux and composition may also be constant. Notably, scenario 2 is likely to take place deep within a crustally extended plumbing system, prompting the conditions for scenario 1 to occur on top of it, in connection with the crater system. Most likely, a combination of both scenarios represents the working mode of the studied volcanic systems, all of which display a remarkable increase in CO 2 content and emissions prior to eruptions. Closed-conduit volcanoes such as Campi Flegrei or Vesuvius evolve from quiescence to eruption over long time-scales (decades or centuries), which are needed to disrupt the apparent stationarity and relax the crustal tension surrounding their plumbing systems. Variations in geophysical and geochemical observables are indeed relatively small, even during unrest periods of several years (e.g., INGV – OV, 2024), suggesting that these long-time intervals are spent building and/or reforming the crustal magma chambers that will feed future eruptions. This appears to be a slow, continuous process compatible with scenario 2, which will accelerate and inevitably escalate to eruption when deep CO 2 -rich magmas are remobilized, decompress, and ascend into the shallowest magma chambers (scenario 1, consistent with the CV). The well-known vapor/melt imbalance observed at Italian volcanoes and characterized by little magma production compared to emitted gas fluxes is a feature intrinsic in the evolution of their plumbing systems and can be explained by the chromatographic analogy: i ) plumbing systems made of mushy crystalline rocks can be assimilated to the static but reactive columns, persistently fluxed by H 2 O-CO 2 fluids ( 1 ≥ CO 2 /H 2 O ≥ 0.5, molar); ii ) fluid-induced melting of mushy portions produces volatile-rich batches and determines the CO 2 -enrichment of shallow portions within the plumbing systems; and iii ) deep batches can be remobilized, ascend and further nurture shallow magma chambers by mixing with the resident magma. In this respect, we cannot discard the hypothesis that the current Campi Flegrei unrest might be an example of ongoing CO 2 -fluxing associated with mush rejuvenation, given the continuous temporal decrease in the CO 2 /H 2 O ratio observed at fumaroles over the last 25 years [INGV-OV, 2024 ; Moretti et al., 2025 ]. On the same bottom line, I have already suggested that shallow (∼ 4 km depth) mush rejuvenation and remobilization could have generated the 1538 CE Monte Nuovo eruption [Moretti, 2025; Moretti et al., 2025 ], the most recent CF eruption [Piochi et al., 2005 and refs. therein]. The crystal-poor products of these eruptions [Piochi et al., 2005 ] could indicate melt extraction from a mush rejuvenated by the fluid-infiltration. Noteworthy, based on different premises, Rolandi et al. ( 2025 ) recently proposed a mechanism of magma rejuvenation due to bottom-up gas sparging as the origin of the Monte Nuovo eruption. In this study, I do not discuss the origin of ascending fluids and their CO 2 /H 2 O composition. However, it is quite well established that the widespread release of deep CO₂ in Southern Italy primarily originates from a combination of mantle degassing and the metamorphic breakdown of subducted carbonate rocks. The mantle beneath Southern Italy is anomalously rich in carbon, due to the combination of two main processes, likely related to each other: 1) the upwelling of carbon-rich melts from the deep mantle (70 to 130 km depth), which provides a significant, continuous source for the CO 2 cargo of crustal magmas [Frezzotti et al., 2009 ]; 2) ancient geological processes related to carbonatite metasomatism (e.g., Bragagni et al., 2022 ): infiltration by fluids and melts in the lithospheric mantle beneath Southern Italy has introduced large amounts of carbon, effectively creating carbon-rich domains or reservoirs. Extensive fluid infiltration also explains the huge amount of water observed in primitive magmas. In this framework, assimilation of shallow crustal carbonate material [Rittmann, 1933 ; Iacono-Marziano et al., 2008; Jolis et al., 2013 ; Mollo et al., 2010 ; Gozzi et al., 2014 ; Deegan et al., 2026 ] appears to be a minor or even negligible contribution: only in the case of Somma-Vesuvius shallow assimilation and/or thermometamorphic breakdown of limestones surrounding the magmatic chamber determines a clear CO 2 -gain at P ≤ 150 MPa bar [Dallai et al., 2011 ] (Fig. 2 b). It is worth noting that each studied magmatic system tends to display an upper bounding limit of [CO 2 /H 2 O] vapor on the hydrous-poor CO 2 -enriched leftward side (ratios are 3 for Str, 10 for SV, and 20 for CF-Is-Pr; Fig. 2 ). Different upper limits likely reflect the response of any plumbing system to the upstreaming fluid and its flow rate, which is controlled by permeability. How permeability is affected by magma composition and mush mineralogy, thermal gradients, and the tectono-structural setting is out of the scope of this paper. Beyond these speculations, I must precise that the upper bounding limit is here much less relevant than the lower CO 2 /H 2 O limit. The latter, in fact, carries the fluid signature of the mantle source and fixes the background levels of dissolved H 2 O and CO 2 in the melt at any pressure in the plumbing system. Finally, a caveat must be issued about the representativeness of H 2 O and CO 2 contents in MIs, in light of the multiple events of thermal re-equilibration that may occur in a mushy system [Esposito, 2020]. These phenomena could require compositional corrections for pot-entrapment diffusion and/or crystallization, which are here dismissed on the basis of the MI descriptions in the literature along with the available data (summarized in Table 1 ). Nevertheless, compositional corrections affecting the dissolved H 2 O and CO 2 contents of MIs (Fig. 2 ) would not alter the scenarios I have outlined here. First, the CO 2 amounts are high enough to require minimal magmatic vapor phases with comparable CO 2 and H 2 O abundances throughout the entire pressure range. Second, the imbalance between magmatic vapor fluxes over magma production is observed independently of MI data quality and, per se , provides a valid argument for shifting our view of how these plumbing systems work and evolve, as also shown for Etna in Ferlito ( 2018 ). 5 Conclusions The unexpected observation of a background vapor (fluid) composition (1 ≥ CO 2 /H 2 O ≥ 0.5) buffering the MIs in mafic melts from Campanian Volcanoes and Stromboli strongly suggests that magmatic systems evolve under vapor-buffered conditions since the large crustal depths of parental magmas. This observation is highly consistent with the huge amount of deep CO 2 release at the surface and may explain the well-known imbalance between gas fluxes at volcanic sites and magma extrusion rates. Under vapor-buffered conditions, plumbing systems may change their vertical cross-section by developing magma chambers via mush remelting induced by fluid infiltration. Because rejuvenated magmas must be relatively hydrous-rich, residual vapors enriched in CO 2 will flux the overlying plumbing system. The entire process may occur under a stationary CO 2 /H 2 O flux, depending on the associated advective heat carried along. However, it can be accelerated by an increase in deep fluid production and, consequently, in heat-advective release, related to geodynamic factors. I propose that, in the long term, magma production at Southern Italian volcanoes occurs under CO 2 -rich, vapor-buffered conditions and is modulated by variations of fluid and heat production that start at mantle depths and propagate upward in response to geodynamic factors. These findings can be extended to other worldwide volcanic systems characterized by high fluxes of volatile components, CO 2 particularly. Declarations Conflict of interest: The author declares no competing financial interests or personal relationships that could influence the work reported in this paper. Author Contribution As the sole author I conducted the whole study and reported in its present form Acknowledgments To be written Data Availability Data and results of melt-volatile computations are available upon request to the Author References Peccerillo A (2005) Plio-Quaternary volcanism in Italy: petrology, geochemistry, geodynamics. Springer Berlin Heidelberg, Berlin, Heidelberg, p 365 Moretti R, Arienzo I, Orsi G, Civetta L, D’Antonio M (2013) The deep plumbing system of Ischia: a physico-chemical window on the fluid-saturated and CO2-sustained Neapolitan volcanism (southern Italy). J Petrol 54(5):951–984 D’Antonio M, Tonarini S, Arienzo I, Civetta L, Dallai L, Moretti R, Orsi G, Andria M, Trecalli A (2013) Mantle and crustal processes in the magmatism of the Campania region: inferences from mineralogy, geochemistry, and Sr–Nd–O isotopes of young hybrid volcanics of the Ischia island (South Italy). Contrib Miner Petrol 165(6):1173–1194 Buono G, Caliro S, Paonita A, Pappalardo L, Chiodini G (2023) Discriminating carbon dioxide sources during volcanic unrest: The case of Campi Flegrei caldera (Italy). Geology. 10.1130/G50624.1 Chiodini G, Cardellini C, Amato A, Boschi E, Caliro S, Frondini F, Ventura G (2004) Carbon dioxide Earth degassing and seismogenesis in central and southern Italy. 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J Volcanol Geoth Res 182(3–4):221–230 Moretti R (2025) CO2 and Magma Remobilization: Implications for Campi Flegrei Caldera (Southern Italy). In: Çiner A et al (eds) Recent Research on Sedimentology, Stratigraphy, Paleontology, Tectonics, Geochemistry, Volcanology and Petroleum Geology. MedGU 2023. Advances in Science, Technology Innovation. Springer, Cham. https://doi.org/10.1007/978-3-031-87558-8_21 . Moretti R, De Natale G, Troise C (2025) Degassing, Deformation and the Phlaegrean Unrest: From Conflicting to Converging Interpretations of the Hydrothermal vs Magmatic Dilemma. In: Çiner A et al (eds) Recent Research on Sedimentology, Stratigraphy, Paleontology, Tectonics, Geochemistry, Volcanology and Petroleum Geology. MedGU 2023. Advances in Science, Technology Innovation. Springer, Cham. https://doi.org/10.1007/978-3-031-87558-8_28 Bachmann O, Bergantz GW (2006) Gas percolation in upper-crustal silicic crystal mushes as a mechanism for upward heat advection and rejuvenation of near-solidus magma bodies. J Volcanol Geoth Res 149(1–2):85–102 INGV-OV (2024) Il Monitoraggio dei Vulcani Campani 2024. INGV OV annual report ( https://www.ov.ingv.it/index.php/doclink/report-monitoraggio-vulcani-campani-2024/eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJyZXBvcnQtbW9uaXRvcmFnZ2lvLXZ1bGNhbmktY2Ft cGFuaS0yMDI0IiwiaWF0IjoxNzYwNjAyOTk0LCJleHAiOjE3NjA2ODkzOTR9.mT6X8PPsNxDivNAusPNUmf-eWlyg1SpxJ6dO_J5NwbM ; last accessed on 23 January 2026) Piochi M, Mastrolorenzo G, Pappalardo L (2005) Magma ascent and eruptive processes from textural and compositional features of Monte Nuovo pyroclastic products, Campi Flegrei, Italy. Bull Volcanol 67:663–678 Rolandi G, Troise C, Sacchi M, Di Lascio M, De Natale G (2025) The 1538 eruption at the Campi Flegrei resurgent caldera: implications for future unrest and eruptive scenarios. Nat Hazards Earth Syst Sci 25(9):3421–3453 Frezzotti ML, Peccerillo A, Panza G (2009) Carbonate metasomatism and CO2 lithosphere–asthenosphere degassing beneath the Western Mediterranean: an integrated model arising from petrological and geophysical data. Chem Geol 262(1–2):108–120 Bragagni A, Mastroianni F, Münker C, Conticelli S, Avanzinelli R (2022) A carbon-rich lithospheric mantle as a source for the large CO2 emissions of Etna volcano (Italy). Geology 50(4):486–490 Rittmann A (1933) Evolution und differentiation des Somma–Vesuv magmas. Zeitsch Vulkanol 15:8–94 Iacono Marziano G, Gaillard F, Pichavant M (2008) Limestone assimilation by basaltic magmas: an experimental re-assessment and application to Italian volcanoes. Contrib Miner Petrol 155(6):719–738 Jolis EM, Freda C, Troll VR et al (2013) Experimental simulation of magma–carbonate interaction beneath Mt. Vesuvius, Italy. Contrib Mineral Petrol 166:1335–1353. https://doi.org/10.1007/s00410-013-0931-0 Mollo S, Gaeta M, Freda C, Di Rocco T, Misiti V, Scarlato P (2010) Carbonate assimilation in magmas: a reappraisal based on experimental petrology. Lithos 114(3–4):503–514 Gozzi F, Gaeta M, Freda C, Mollo S, Di Rocco T, Marra F, Dllai L, Pack A (2014) Primary magmatic calcite reveals origin from crustal carbonate. Lithos 190:191–203 Deegan FM, Capriolo M, Troll VR, Weis FA, Callegaro S, Colucci S, Freda C, MisitiV, Aradi LE, Skogby H, Darmawan H, Geiger H (2026) CO 2 fluxing and carbon assimilation by arc melts during magma–limestone interaction. Chem Geol 704:123264. doi.org/10.1016/j.chemgeo.2026.123264 Dallai L, Cioni R, Boschi C, D'Oriano C (2011) Carbonate-derived CO2 purging magma at depth: influence on the eruptive activity of Somma-Vesuvius, Italy. Earth Planet Sci Lett 310(1–2):84–95 Esposito R, De Vivo B, Belkin HE (2020) G. Rolandi 141–174. Elsevier Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8810254","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":588823368,"identity":"96cedc05-6167-4c62-b12a-b516ee7e9895","order_by":0,"name":"Roberto Moretti","email":"data:image/png;base64,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","orcid":"","institution":"University of Campania \"Luigi Vanvitelli\"","correspondingAuthor":true,"prefix":"","firstName":"Roberto","middleName":"","lastName":"Moretti","suffix":""}],"badges":[],"createdAt":"2026-02-06 19:08:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8810254/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8810254/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102423992,"identity":"e5322d33-9b3f-4724-8a6f-160bcdba8a65","added_by":"auto","created_at":"2026-02-11 14:19:05","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1544078,"visible":true,"origin":"","legend":"\u003cp\u003eTraces of the subduction-related compression and extension fronts along the Italian peninsula. Is, Ischia; Pr, Procida; CF, Campi Flegrei; SV, Somma-Vesuvius; Vv, Vavilov; Str, Stromboli; Et, Etna; Pn, Pantelleria. Is, Pr, CF, SV, and Str are included because of their relevance to this study. The semi-transparent, round-rectangular orange area identifies the investigated sector, which runs along the subduction trace. Modified after [Acocella and Funiciello, 2006; Moretti et al., 2013a].\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8810254/v1/d489ec6bfe47633d4592fde6.png"},{"id":102423991,"identity":"354cc687-d802-4c96-8913-9db111fee1c0","added_by":"auto","created_at":"2026-02-11 14:19:05","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1302249,"visible":true,"origin":"","legend":"\u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e-melt saturation isobars for compositions in Tables 1, with reported H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e dissolved contents in MIs from the literature (see Table 1). Panel \u003cem\u003ea\u003c/em\u003e) Stromboli volcano (Str) in the Aeolian Islands. Panel \u003cem\u003eb\u003c/em\u003e) Somma–Vesuvius volcano, the Neapolitan Area (Strombolian activity, from year 1637 CE to year 1944 CE; Marinelli et al., 2005); black diamonds refer to shallow magmas further enriched in CO\u003csub\u003e2\u003c/sub\u003e by carbonate assimilation and/or breakdown. Panel \u003cem\u003ec\u003c/em\u003e) Campi Flegrei, Ischia and Procida volcanoes in the Neapolitan Area; MI data refer to less-differentiated magmas from Is (blue circles), CF (grey squares) and Pr (yellow triangles). Panel d) undistinguished MI data; two saturation isobars are plotted at each pressure (taken from panels b and c) to embrace model variations. The scales for the X- and Y-axes are the same across all panels.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8810254/v1/3a051a4523ea5a7be7ad8e21.jpeg"},{"id":102423990,"identity":"1e47747c-2da6-428e-a9d7-201fdefe69c1","added_by":"auto","created_at":"2026-02-11 14:19:05","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":272438,"visible":true,"origin":"","legend":"\u003cp\u003eIdeal evolution of a volatile flooded volcanic plumbing system, visualized as a nearly cylindrical magma feeding system in which fluids (gases) ascend along the entire vertical column. In the “before” panel, H\u003csub\u003e2\u003c/sub\u003eO (small filled black circles) and CO\u003csub\u003e2\u003c/sub\u003e (big empty circles) are nearly homogenously distributed throughout the plumbing system. In the “after” panel, the structural evolution is marked by the appearance of a laterally extensive sill-like magma layer in which water dissolves and concentrates. Above this layer, outgoing gases are CO\u003csub\u003e2\u003c/sub\u003e-enriched and ascend, re-equilibrating with local magma pockets (CO\u003csub\u003e2\u003c/sub\u003e-fluxing) and partially accumulating at shallower depths, possibly forming a concentrated, foamy structure. The lack of a vertical scale is by purpose, to avoid any association of depth intervals with steps described here and in the text.\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8810254/v1/1fc17c492a46f6198886d52d.png"},{"id":104834972,"identity":"c411624f-f625-463b-a396-3c9078a47f81","added_by":"auto","created_at":"2026-03-17 17:37:15","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4653333,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8810254/v1/4a2a10b4-3c2d-4d1f-8e2a-23a44cc65b28.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fluid-induced and vapor-sustained magma production at the Stromboli and Campania Province volcanoes (Southern Italy)","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eTwo main interrelated endogenous processes distinguish the peri-Thyrrenian margin of Southern Italy: 1) the production of magma, episodically but recurrently erupted from volcanoes showing alkalic affinity (e.g., Etna, Stromboli, Vulcano, Somma-Vesuvius, Campi Flegrei, Ischia, Procida; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), and 2) the deep CO₂ degassing, not only by direct volcanic emissions but also by diffuse degassing from regional extensional structures non volcanic \u003cem\u003eper se\u003c/em\u003e [Peccerillo, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Moretti et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; D\u0026rsquo;Antonio et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Buono et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Chiodini, et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Fischer and Aiuppa, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Tamburello et al., 2010; Frezzotti et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2010\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThis degassing is a major source of natural carbon emissions, with some non-volcanic areas releasing amounts comparable to the global volcanic system [Chiodini et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Fischer et al., 2020; Aiuppa et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Burtono et al., 2013]. The degassing is linked to the region's geodynamic setting, where the African plate is subducting beneath the Eurasian plate [Peccerillo, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Jolivet et al., 2008; Acocella and Funiciello, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2006\u003c/span\u003e] (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). This process causes the recycling of subducted carbonates deep within the mantle, generating CO₂-rich fluids that migrate upward through deep lithospheric faults and fracture systems. The actively degassing structures, such as the Tuscanian-Roman and Campanian degassing structures [Chiodini et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Fischer and Aiuppa, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e], are then the fingerprint at the surface of the deep regional volcano-magmatic structures that, in association with extensional tectonics, have shaped the peri-Thyrrenian margin of the Italian peninsula (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e release is manifested in different ways such as 1) diffuse soil degassing, occurring as widespread, non-eruptive emissions through the soil in both volcanic and non-volcanic areas [Chiodini et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Fischer and Aiuppa, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e], 2) gas-rich springs, where CO₂ is often dissolved in cold and thermal groundwaters [Chiodini et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Gambardella et al., 2024] and, of course, 3) direct emissions from active volcanoes like Stromboli, Etna and Campi Flegrei [Aiuppa et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Burtono et al., 2013].\u003c/p\u003e \u003cp\u003eA major implication of the mutual interrelation between deep CO\u003csub\u003e2\u003c/sub\u003e degassing and volcanic-magmatic activity is then the imbalance that emerges when confronting the huge natural fluxes of volatiles (CO\u003csub\u003e2,\u003c/sub\u003e particularly, but also H\u003csub\u003e2\u003c/sub\u003eO) versus the comparatively little production and extrusion of magmas from active Italian volcanoes [Edmonds et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Moretti et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Aiuppa et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Pino et al., Moretti et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003eb; Aiuppa et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Ferlito, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e]. In this context, volcanoes can be seen as deep boreholes that pierce the source areas of CO\u003csub\u003e2\u003c/sub\u003e and shed light on the role of CO\u003csub\u003e2\u003c/sub\u003e-rich fluids at crustal depths where volcanic plumbing systems extend downward.\u003c/p\u003e \u003cp\u003eMost of the information comes from the study of melt inclusions (MI), which are droplets of melt trapped in phenocrysts [e.g., Roedder, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1984\u003c/span\u003e] during the volatile-saturated polybaric pathways of magma ascent and evolution from deep-crustal magma chambers to the surface. Briefly, MIs provide fundamental qualitative and quantitative constraints on magma storage conditions and pre-eruptive degassing and represent a fundamental complement to gas discharges vented at the surface [e.g., Buono et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Moretti et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Aiuppa et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Pino et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Moretti et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003eb; Aiuppa et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this study, I use MIs from Stromboli, Campi Flegrei, Ischia, and Somma-Vesuvius volcanoes to elucidate the role of CO\u003csub\u003e2\u003c/sub\u003e-rich fluids of mantle origin in driving the evolution of plumbing systems associated with these volcanoes and the geodynamic setting in which they are embedded.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"2 Methods and selection criteria","content":"\u003cp\u003eI have performed a literature survey of melt inclusions (MIs) from most mafic magmas erupted by active volcanoes of Southern Italy (Stromboli, Vesuvius, Ischia, Procida, and Campi Flegrei; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) that run NW to SE along the peri-Thyrrenian margin of Southern Italy and surround the sector of interest here (semi-transparent round-rectangular area in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). These volcanoes have been selected because their rock geochemistry shares common source features, such as trace element ratios and isotopes, strongly suggesting that Somma-Vesuvius and the adjoining Campi Flegrei, Procida, and Ischia volcanoes are the northern extension of the eastern Aeolian arc rather than the southern end of the so-called Roman Province [Peccerillo, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2001\u003c/span\u003e, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2005\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAverage composition (wt %; CO\u003csub\u003e2\u003c/sub\u003e in ppm wt) and temperature of melts from studied volcanoes (Str: Stromboli; CF-Pr-Is: Campi Flegrei and Ischia; SV: Somma-Vesuvius).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eStr\u003c/p\u003e \u003cp\u003eShoshonite\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCF-Pr-Is\u003c/p\u003e \u003cp\u003eshoshonite; latite; trachy-basalt\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSV\u003c/p\u003e \u003cp\u003e(K-)tephrite\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eT (\u0026deg;C)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1150\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e51.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e50.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e48.55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.93\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e16.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFeO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMnO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMgO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e6.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e7.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCaO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e10.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e9.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e13.38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.77\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003csub\u003e2\u003c/sub\u003eO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eO (min-max)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.8 \u0026mdash; 3.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.2 \u0026mdash; 4.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.37 \u0026mdash; 6.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e (min-max)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e360 \u0026mdash; 2264\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e56 \u0026mdash; 4601\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e188 \u0026mdash; 4432\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNotes\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAiuppa et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e); Pino et al. (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) and refs. therein; MIs are from the so-called \u0026ldquo;golden\u0026rdquo; low-porphyric magma\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMoretti et al. (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003ea); Mangiacapra et al. (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2008\u003c/span\u003e); Mormone et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2011\u003c/span\u003e); one MI from Esposito et al. (2025) (sample RE.SC2.F4.M1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMarianelli et al. (20925) and refs. therein; data are from the 1637 CE\u0026ndash;1944 CE period of Strombolian activity\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eI focus on the mafic, less-differentiated magmas as they are representative of the deepest - or at least deeper - portions of the plumbing system of selected volcanoes. Therefore, they are the most suitable to extract information about the fluids in the deep magma-source(s).\u003c/p\u003e \u003cp\u003eBased on MI analyses, I set average melt compositions and temperatures for each investigated volcanic setting (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), according to published data and elaborations (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e captions). Campi Flegrei (CF), Ischia (Is), and Procida (Pr) are considered together as they constitute the Phlaegrean Volcanic District (PVD), which is structurally and morphogenetically distinct from Somma-Vesuvius [Acocella and Funiciello, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Moretti et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003ea].\u003c/p\u003e \u003cp\u003eUsing a well-established model for gas-melt saturation [Papale et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2006\u003c/span\u003ea; Papale et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e], I have then calculated the H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e saturation conditions, which form the basis for the forthcoming discussion of results and interpretation. Conditions for model initialization are reported in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The reader should refer to Papale et al. (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) for a full review of melt-fluid equilibria and the relevant techniques to compute H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e-melt conditions.\u003c/p\u003e"},{"header":"3 Results and data inspection","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e reports H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e saturation isobars for melt compositions typified as in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Entrapment pressures are up to \u0026sim; 310 MPa for Stromboli, \u0026sim; 450 MPa for Vesuvius, and \u0026sim; 480 MPa for Ischia and Campi Flegrei.\u003c/p\u003e \u003cp\u003eIn the Str saturation plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), datapoints are, with very good approximation, bounded by a CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO \u0026asymp; 1 (molar) isopleth on the right side (H\u003csub\u003e2\u003c/sub\u003eO-rich) of the diagram. In the SV saturation plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), datapoints have been differentiated to better appreciate H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e pathways per event. Overall, they are better bounded on the right by a limiting isopleth [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003ev\u003c/sub\u003e \u0026asymp; 0.5. This ratio also applies to the rightward bounding isopleth in the CF-Pr-Is saturation plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). In the SV case, a cluster of CO\u003csub\u003e2\u003c/sub\u003e-enriched datapoints is visible at about 100 MPa, with [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003ev\u003c/sub\u003e ratios exceeding those of the CO\u003csub\u003e2\u003c/sub\u003e-richest MIs from pressures\u0026thinsp;\u0026gt;\u0026thinsp;200 MPa.\u003c/p\u003e \u003cp\u003eIn all cases, datapoint alignments emerge that follow nearly isobaric lineages (sketched by a yellow semi-transparent arrow in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), along which CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO vapor ratios increase up to \u0026sim; 20 (molar) at CF-Pr-Is (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). Such arrays of CO\u003csub\u003e2\u003c/sub\u003e-enrichment (CO\u003csub\u003e2\u003c/sub\u003e-fluxing) appear at several pressures (hence depths) throughout plumbing systems and emerge clearly at the highest entrapment pressures, that is, at the bottom of each plumbing system.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn each saturation plot, the location of the CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO bounding trend is not affected by computations performed within a\u0026thinsp;\u0026plusmn;\u0026thinsp;100\u0026deg;C interval around adopted temperatures (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Furthermore, changing for each volcanic type redox conditions (i.e., the FeO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e proportions in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) or the average compositions (within the range of variations displayed by MIs), shifts the pressure range but produces minor changes in the bounding [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003ev\u003c/sub\u003e values. This is evident in panel d of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, where all MIs have been plotted on an H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e saturation diagram. For the sake of clarity, in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, only isobars for SV and CF-Is-Pr have been plotted, because together they embrace the computed excursion of each isobar. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed also displays the variation for the isopleths with [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003ev\u003c/sub\u003e i ratios of 0.5 and 1.\u003c/p\u003e \u003cp\u003eAdopting compositions more or less differentiated than those in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e would be another choice of reasonable conditions to compute saturation properties. However, this would not alter the arguments presented in the forthcoming Discussion section, which are based on the main result of this study: on the water-rich saturation diagram, each magmatic system within the geodynamic sector of interest (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) displays a bounding vapor composition, with comparable amounts of CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e \u003cp\u003eGiven that the CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO limiting isopleths, trends of magma decompression under closed- or open-system degassing that are generated by starting conditions along or close to the [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e bounding lines cannot reproduce the limiting isopleth, as typified by the CUSAT line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. Deep magmas displaying [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e \u0026gt; 1 can generate decompression paths under closed-system degassing conditions that cross the [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e bounding line in the low-pressure field, such as the CUSAT line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. However, trends like the CUSAT ones in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea,c cannot reproduce the dissolved H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e patterns of the bounding volatile isopleths. To match vapor isopleths, the ascent paths of magma decompression under closed-system degassing conditions must have vastly high total (exolved\u0026thinsp;+\u0026thinsp;dissolved) volatile contents (e.g., the CFSAT trend in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). Therefore, very high volatile-rich melts rising under closed-system degassing conditions have CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO vapor ratios corresponding to isopleth ones because the vapor/melt ratio is so high to make the vapor phase unaffected by the exchange of volatiles with the melt over the entire pressure range of interest and not only at low pressures, when all degassing trends deviate to converge to zero (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e"},{"header":"4 Discussion","content":"\u003cp\u003eMelt inclusions in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e are all from alkalic mafic magmas erupted by the investigated volcanoes on the peri-Thyrrenian margin of southern Italy, genetically related to the subduction of the African plate beneath the Eurasian plate and the opening of the Tyrrhenian basin. However, important compositional differences characterize these volcanoes and their magmas, which are related to their sources and their differentiation histories. Such differences affect the composition of magmas, particularly their alkalic affinity, the relative proportions of H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e in their equilibrium fluid, and the oxidation state [D\u0026rsquo;Antonio et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIt is then quite surprising to observe that for all studied volcanoes, MI data from the less differentiated magmas are limited by a vapor isopleth with a CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO ratio in the 0.5-1 range 1 (molar) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This bounding condition applies over the entire pressure range of MIs\u0026rsquo; entrapment (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed) and is unlike to be a casual feature, given that 1) studied magmas are genetically similar [Peccerillo, 2011] but on the other side, they have different physico-chemical and saturation properties (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), and 2) widespread diffuse regional degassing of deep CO\u003csub\u003e2\u003c/sub\u003e occurs in the studied sector, pointing to a common mantle source accumulating volcanic and non-volcanic degassing [Chiodini et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Fischer and Aiuppa, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Tamburello et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Frezzotti et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Aiuppa et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Burtono et al., 2013; Gambardella et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2004\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOn the right side (H\u003csub\u003e2\u003c/sub\u003eO-rich) of each saturation plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), the limiting isopleth goes across the entire plumbing system, embracing deep (P\u0026thinsp;\u0026gt;\u0026thinsp;400 MPa or z\u0026thinsp;\u0026gt;\u0026thinsp;15 km by using a melt/rock density of 2700 kg/m\u003csup\u003e3\u003c/sup\u003e) and shallow melts, and defines an ascent path in which both the vapor volume and mass fractions vastly exceed the melt ones. In other words, magma ascent and decompression should occur under vapor-dominated conditions if the initial [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e ratio is close to that of the bounding isopleth.\u003c/p\u003e \u003cp\u003eContrary to Str and SV, in the case of CF-Pr-Is (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), no high-pressure (\u0026gt;\u0026thinsp;200 MPa ) MIs fall close or along the bounding isopleth, and one can assume that typical closed-system decompression trends may originate from high pressure for [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e \u0026gt; 1, such as in the case of the CUSAT line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. The curvature that this trend must have not to cross the limiting isopleth depends on the total volatile content (sum of dissolved and exsolved H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e), which is \u0026sim; 8 wt% or \u0026sim; 25% molar, with already\u0026thinsp;\u0026gt;\u0026thinsp;20% of volume vapor fraction at 400 MPa, i.e., \u0026sim; 16 km deep in the crust.\u003c/p\u003e \u003cp\u003eThese numbers require that deep magmas in the crust have a very low density, approaching that of the H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e fluid at very high vapor/melt ratios. Such magmas would be highly buoyant, hence unstable at their equilibrium depth and prone to ascend. This implies that deep, highly buoyant magma blobs overcome the surrounding confining pressure and rise from large crustal depths under closed-system degassing conditions to feed the magma chambers developed throughout the plumbing system. This is in line with the activity observed at open-conduit basaltic volcanoes and certainly works during the early extensional phases, building the volcanic system and its vertically extended plumbing system. However, the persistence of continuous transfer of extremely volatile-rich blobs appears counterintuitive at a stage when volcanic systems are well established. Volcanoes such as Somma-Vesuvius, Campi Flegrei, Procida, and Ischia tend to evolve as closed-conduit systems, and their less differentiated eruptive products have sampled deep MIs, recording pressures and depth conditions almost twice those of a currently open-conduit volcano such as Stromboli (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the classical view (henceforth CV), the pre-eruptive evolution of plumbing systems is characterized by magma chamber refilling that occurs by the upstream of magmas, which are the \u0026ldquo;moving parts\u0026rdquo; coming from an often unspecified deep crustal or mantle reservoir. When these magmas emplace, they release partly or totally the vapor phase as they rise and equilibrate under fluid pressures that allow the magma batch to overcome the strength of the surrounding rocks, thus opening the path upward. Overall, the formation of magma chambers is seen as resulting from a sequence of episodes of volatile-rich magma injection through extensional structures under both closed- and open-system degassing conditions. In this view, melting and magma production occur in a deep-mantle source, and the subsequent history is characterized by stepwise magma migration and differentiation, including the separation and release of magmatic vapors. However, this view is adopted almost inevitably, even when volcanoes like those studied here are well-established and settled systems, in which plumbing systems result from long-standing, preceding activity of magma emplacement, differentiation, and extrusion. At such a stage, plumbing systems are mostly composed of crystal mushes [Moretti et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003ea; Buono et al., 203; Esposito et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Moretti et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e], which can be pictured as huge piles or columns extending throughout the crust and plugging the extensional lithospheric structures through which earlier volcanoes were built.\u003c/p\u003e \u003cp\u003eIn this context, the very high volatile abundances emerging from this study conflict with the general applicability of the CV. Similar to what was done for Etna by Ferlito (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), this conflict justifies my inference of a more suitable working scenario for volatile-flooded volcanic plumbing systems.\u003c/p\u003e \u003cp\u003eHere, I enlarge the model presented in [Moretti et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e] and propose that static mushy systems fluxed by H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e-rich fluids are similar to a chromatographic column. MIs could then represent locally re-equilibrated or newly molten mush pockets in which dissolved H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e contents are fixed by the upstream fluid flow rate, rather than snapshots of extremely volatile-enriched magma blobs that emplaced after closed-system ascent.\u003c/p\u003e \u003cp\u003eThe ascending fluids making that make up the background bounding isopleths (CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO from 0.5 to 1) in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e are likely to be generated by deep mantle sources. A tempting scenario involves a single source common to all volcanoes, but buffered by pressure conditions that vary from North-West to South-East. The NW-to-SE increase in the [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e ratio from 0.5 to 1 may have multiple explanations, which are beyond the scope of the current study. For example, it may result from the NW-SE deepening of the melt-H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e saturation surface of a hypothetical mantle magma reservoir.\u003c/p\u003e \u003cp\u003eOverall, the CO\u003csub\u003e2\u003c/sub\u003e-enrichment and consequent melt dehydration (CO\u003csub\u003e2\u003c/sub\u003e-fluxing) along equilibrium isobars (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and refs. Moretti et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2013\u003c/span\u003ea,b,2018, 2019; Edmonds et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Aiuppa et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Pino et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Mangiacapra et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Mormone et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Esposito et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Marianelli et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Allard et al., 2010) might then be a consequence of any shift in deep reservoirs that reverberates throughout the overlying layers. Because CO\u003csub\u003e2\u003c/sub\u003e-fluxing also occurs for MIs datapoints with the highest entrapment pressures (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), there must be deep mechanisms of CO\u003csub\u003e2\u003c/sub\u003e magnification throughout the entire plumbing system, down to the pristine source region of ascending fluids.\u003c/p\u003e \u003cp\u003eWith reference to Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, I propose that the (background) CO\u003csub\u003e2\u003c/sub\u003e-H\u003csub\u003e2\u003c/sub\u003eO fluid mixture on its ascent equilibrates with the melt batches in the plumbing system (left side, \u0026ldquo;\u003cem\u003ebefore\u003c/em\u003e\u0026rdquo;, in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), where magma differentiation (mainly via fractional crystallization) produces the crystals armoring melt inclusions in which dissolved H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e contents are imposed by the CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO ratio of the up streaming fluid mixture (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Then, to explain CO\u003csub\u003e2\u003c/sub\u003e-fluxing, two further scenarios can be pictured here:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eFor open-conduit basaltic (\u003cem\u003elato sensu\u003c/em\u003e) volcanoes such as Stromboli, it was already shown that CO\u003csub\u003e2\u003c/sub\u003e-fluxing marks the arrival and emplacement in the shallow system of deep volatile-rich magma blobs. These blobs are pieces of magmatic foam in which the fluid phase is dominated by CO\u003csub\u003e2\u003c/sub\u003e (Aiuppa et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Allard et al., 2010; Pino et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The magmatic foam forms at about 10 km depth, then collapses, and is released upward into the shallow system, which is directly connected to the crater [Allard et al., 2010; Pino et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2011\u003c/span\u003e]. This mechanism \u003cem\u003ei\u003c/em\u003e) requires melt addition from below, hence open pathways for the fast ascent of magma blobs, \u003cem\u003eii\u003c/em\u003e) is in line with the CV about volcano functioning, and \u003cem\u003eiii\u003c/em\u003e) is tracked by significant increases of the plume CO\u003csub\u003e2\u003c/sub\u003e/SO\u003csub\u003e2\u003c/sub\u003e ratio and CO\u003csub\u003e2\u003c/sub\u003e fluxes, which in the short-term anticipate the so-called major and paroxysmal explosive eruptions [Aiuppa et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2009\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Allard, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2010\u003c/span\u003e]. Under this scenario, the shallow system that produces plume emissions via passive degassing is recurrently flooded by gas-melt slugs, that is, huge amounts of deep CO\u003csub\u003e2\u003c/sub\u003e-rich fluid carrying a relatively small amount of mafic magma. At Stromboli, eruptions occur shortly after (minutes to days; Aiuppa et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Allard, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Pino et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) the fluid invasion of the shallow system. The CO\u003csub\u003e2\u003c/sub\u003e-enriched (fluxed) melt inclusions (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) have been trapped in olivines from the so-called \u0026ldquo;golden\u0026rdquo; low-porphyric magma, which has equilibrated with the incoming CO\u003csub\u003e2\u003c/sub\u003e-dominated ascending gas [Aiuppa et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Allard, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Pino et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2011\u003c/span\u003e].\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eA mechanism of fluid-induced magma remelting [Moretti, 2025] is instead more plausible at closed-conduit volcanoes. It does not require melt addition and produces MIs lineages that mirror magma ascent and degassing paths, characterized by very high vapor/melt ratios, which are unusual for the tensional state developed by a closed-conduit volcano. By fluid-induced re-melting, magmas \u0026ldquo;do not move\u0026rdquo;, and the high vapor/melt ratios are only apparent, reflecting the CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO content imposed by the flow rate of the vapor phase at the different pressures this encounters on its ascent through the mushy plumbing system. Following the energy budget reported in Morettu (2025), as well as the numerical framework in [Bachmann and Bergantz, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2006\u003c/span\u003e], a steadily upraising H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e flow can have the thermal potential to melt layers of the mushy plumbing system along the rock wet solidus. This re-melting process will be further favored if the bottom flux in conductive and/or advective heat increases. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e (rightward \u0026ldquo;\u003cem\u003eafter\u003c/em\u003e\u0026rdquo; side) sketches this fluid-induced melting process as related to the formation of a cross-sectional melt layer at some unspecified depth. There, most of the water from the ascending fluid is removed by dissolution into the forming melt. This changes the cross-sectional flow and, consequently, determines a chromatographic-like separation between CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO [Moretti et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e] that enriches in CO\u003csub\u003e2\u003c/sub\u003e-the outgoing fluid-phase, which acquires CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO ratios up to 20 in the CF-Pr-s case SV case (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). This, however, occurs without expanding the vapor/melt ratio to unreasonable values for a closed-conduit volcanic system. The now CO\u003csub\u003e2\u003c/sub\u003e-enriched upgoing fluids will then flux the overlying mush column and its magma pockets, being recorded by the entrapment of dehydrated and CO\u003csub\u003e2\u003c/sub\u003e-enriched melt inclusions, which can be armored by the same crystal already hosting more hydrous MIs equilibrated prior to the CO\u003csub\u003e2\u003c/sub\u003e-fluxing event and possibly recording the background [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e value. In this respect, the increase of CO\u003csub\u003e2\u003c/sub\u003e fractions in fumarolic gas emissions vented at the surface of closed-conduit volcanoes is a strong indication that magma rejuvenation might be occurring in the plumbing system [Moretti, 2025; Moretti et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2025\u003c/span\u003e]. This process paves the road to future eruptive episodes of explosive nature but occurs without any apparent geophysical evidence of magma rise and chamber formation or replenishment at shallow depth.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eSummarizing, the CO\u003csub\u003e2\u003c/sub\u003e-enrichment scenario 1 demands a deep perturbation, leading to increased fluid \u0026mdash; essentially CO\u003csub\u003e2\u003c/sub\u003e \u0026mdash; and heat fluxes, eventually accompanied by the arrival of magma blobs rising in a very permeable conduit system characterized by an open-fracture architecture. On the other hand, scenario 2 suggests that CO\u003csub\u003e2\u003c/sub\u003e-enrichment results from deeper magma rejuvenation, induced by the infiltration of a deep-derived fluid whose flux and composition may also be constant. Notably, scenario 2 is likely to take place deep within a crustally extended plumbing system, prompting the conditions for scenario 1 to occur on top of it, in connection with the crater system. Most likely, a combination of both scenarios represents the working mode of the studied volcanic systems, all of which display a remarkable increase in CO\u003csub\u003e2\u003c/sub\u003e content and emissions prior to eruptions.\u003c/p\u003e \u003cp\u003eClosed-conduit volcanoes such as Campi Flegrei or Vesuvius evolve from quiescence to eruption over long time-scales (decades or centuries), which are needed to disrupt the apparent stationarity and relax the crustal tension surrounding their plumbing systems. Variations in geophysical and geochemical observables are indeed relatively small, even during unrest periods of several years (e.g., INGV \u0026ndash; OV, 2024), suggesting that these long-time intervals are spent building and/or reforming the crustal magma chambers that will feed future eruptions. This appears to be a slow, continuous process compatible with scenario 2, which will accelerate and inevitably escalate to eruption when deep CO\u003csub\u003e2\u003c/sub\u003e-rich magmas are remobilized, decompress, and ascend into the shallowest magma chambers (scenario 1, consistent with the CV).\u003c/p\u003e \u003cp\u003eThe well-known vapor/melt imbalance observed at Italian volcanoes and characterized by little magma production compared to emitted gas fluxes is a feature intrinsic in the evolution of their plumbing systems and can be explained by the chromatographic analogy: \u003cem\u003ei\u003c/em\u003e) plumbing systems made of mushy crystalline rocks can be assimilated to the static but reactive columns, persistently fluxed by H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e fluids ( 1\u0026thinsp;\u0026ge;\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026ge;\u0026thinsp;0.5, molar); \u003cem\u003eii\u003c/em\u003e) fluid-induced melting of mushy portions produces volatile-rich batches and determines the CO\u003csub\u003e2\u003c/sub\u003e-enrichment of shallow portions within the plumbing systems; and \u003cem\u003eiii\u003c/em\u003e) deep batches can be remobilized, ascend and further nurture shallow magma chambers by mixing with the resident magma.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn this respect, we cannot discard the hypothesis that the current Campi Flegrei unrest might be an example of ongoing CO\u003csub\u003e2\u003c/sub\u003e-fluxing associated with mush rejuvenation, given the continuous temporal decrease in the CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO ratio observed at fumaroles over the last 25 years [INGV-OV, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Moretti et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2025\u003c/span\u003e]. On the same bottom line, I have already suggested that shallow (\u0026sim; 4 km depth) mush rejuvenation and remobilization could have generated the 1538 CE Monte Nuovo eruption [Moretti, 2025; Moretti et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2025\u003c/span\u003e], the most recent CF eruption [Piochi et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2005\u003c/span\u003e and refs. therein]. The crystal-poor products of these eruptions [Piochi et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2005\u003c/span\u003e] could indicate melt extraction from a mush rejuvenated by the fluid-infiltration. Noteworthy, based on different premises, Rolandi et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) recently proposed a mechanism of magma rejuvenation due to bottom-up gas sparging as the origin of the Monte Nuovo eruption.\u003c/p\u003e \u003cp\u003eIn this study, I do not discuss the origin of ascending fluids and their CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO composition. However, it is quite well established that the widespread release of deep CO₂ in Southern Italy primarily originates from a combination of mantle degassing and the metamorphic breakdown of subducted carbonate rocks. The mantle beneath Southern Italy is anomalously rich in carbon, due to the combination of two main processes, likely related to each other:\u003c/p\u003e \u003cp\u003e1) the upwelling of carbon-rich melts from the deep mantle (70 to 130 km depth), which provides a significant, continuous source for the CO\u003csub\u003e2\u003c/sub\u003e cargo of crustal magmas [Frezzotti et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2009\u003c/span\u003e];\u003c/p\u003e \u003cp\u003e2) ancient geological processes related to carbonatite metasomatism (e.g., Bragagni et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e): infiltration by fluids and melts in the lithospheric mantle beneath Southern Italy has introduced large amounts of carbon, effectively creating carbon-rich domains or reservoirs. Extensive fluid infiltration also explains the huge amount of water observed in primitive magmas.\u003c/p\u003e \u003cp\u003eIn this framework, assimilation of shallow crustal carbonate material [Rittmann, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1933\u003c/span\u003e; Iacono-Marziano et al., 2008; Jolis et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Mollo et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Gozzi et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Deegan et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2026\u003c/span\u003e] appears to be a minor or even negligible contribution: only in the case of Somma-Vesuvius shallow assimilation and/or thermometamorphic breakdown of limestones surrounding the magmatic chamber determines a clear CO\u003csub\u003e2\u003c/sub\u003e-gain at P\u0026thinsp;\u0026le;\u0026thinsp;150 MPa bar [Dallai et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2011\u003c/span\u003e] (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eIt is worth noting that each studied magmatic system tends to display an upper bounding limit of [CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003evapor\u003c/sub\u003e on the hydrous-poor CO\u003csub\u003e2\u003c/sub\u003e-enriched leftward side (ratios are 3 for Str, 10 for SV, and 20 for CF-Is-Pr; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Different upper limits likely reflect the response of any plumbing system to the upstreaming fluid and its flow rate, which is controlled by permeability. How permeability is affected by magma composition and mush mineralogy, thermal gradients, and the tectono-structural setting is out of the scope of this paper. Beyond these speculations, I must precise that the upper bounding limit is here much less relevant than the lower CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO limit. The latter, in fact, carries the fluid signature of the mantle source and fixes the background levels of dissolved H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e in the melt at any pressure in the plumbing system.\u003c/p\u003e \u003cp\u003eFinally, a caveat must be issued about the representativeness of H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e contents in MIs, in light of the multiple events of thermal re-equilibration that may occur in a mushy system [Esposito, 2020]. These phenomena could require compositional corrections for pot-entrapment diffusion and/or crystallization, which are here dismissed on the basis of the MI descriptions in the literature along with the available data (summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Nevertheless, compositional corrections affecting the dissolved H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e contents of MIs (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) would not alter the scenarios I have outlined here. First, the CO\u003csub\u003e2\u003c/sub\u003e amounts are high enough to require minimal magmatic vapor phases with comparable CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO abundances throughout the entire pressure range. Second, the imbalance between magmatic vapor fluxes over magma production is observed independently of MI data quality and, \u003cem\u003eper se\u003c/em\u003e, provides a valid argument for shifting our view of how these plumbing systems work and evolve, as also shown for Etna in Ferlito (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e"},{"header":"5 Conclusions","content":"\u003cp\u003eThe unexpected observation of a background vapor (fluid) composition (1\u0026thinsp;\u0026ge;\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026ge;\u0026thinsp;0.5) buffering the MIs in mafic melts from Campanian Volcanoes and Stromboli strongly suggests that magmatic systems evolve under vapor-buffered conditions since the large crustal depths of parental magmas. This observation is highly consistent with the huge amount of deep CO\u003csub\u003e2\u003c/sub\u003e release at the surface and may explain the well-known imbalance between gas fluxes at volcanic sites and magma extrusion rates.\u003c/p\u003e \u003cp\u003eUnder vapor-buffered conditions, plumbing systems may change their vertical cross-section by developing magma chambers via mush remelting induced by fluid infiltration. Because rejuvenated magmas must be relatively hydrous-rich, residual vapors enriched in CO\u003csub\u003e2\u003c/sub\u003e will flux the overlying plumbing system.\u003c/p\u003e \u003cp\u003eThe entire process may occur under a stationary CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO flux, depending on the associated advective heat carried along. However, it can be accelerated by an increase in deep fluid production and, consequently, in heat-advective release, related to geodynamic factors. I propose that, in the long term, magma production at Southern Italian volcanoes occurs under CO\u003csub\u003e2\u003c/sub\u003e-rich, vapor-buffered conditions and is modulated by variations of fluid and heat production that start at mantle depths and propagate upward in response to geodynamic factors. These findings can be extended to other worldwide volcanic systems characterized by high fluxes of volatile components, CO\u003csub\u003e2\u003c/sub\u003e particularly.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eConflict of interest:\u003c/h2\u003e \u003cp\u003eThe author declares no competing financial interests or personal relationships that could influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAs the sole author I conducted the whole study and reported in its present form\u003c/p\u003e\u003ch2\u003eAcknowledgments \u003c/h2\u003e \u003cp\u003eTo be written\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData and results of melt-volatile computations are available upon request to the Author\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003ePeccerillo A (2005) Plio-Quaternary volcanism in Italy: petrology, geochemistry, geodynamics. 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Elsevier\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Carbon dioxide, water, melt inclusions, vapor/melt ratio, mush rejuvenation","lastPublishedDoi":"10.21203/rs.3.rs-8810254/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8810254/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe Italian territory is heavily invested by widespread deep degassing characterized by huge emissions of carbon dioxide. The peri-Thyrrenian area of southern Italy is where these emissions are accompanied by active volcanism, with magmas displaying alkalic affinity and covering a wide compositional spectrum. It has long been debated whether such a production of CO\u003csub\u003e2\u003c/sub\u003e is related to decarbonation of crustal limestones or to other deep source features. However, a global look at melt inclusions and plume data from southern Italian volcanoes suggests that magmas are produced under CO\u003csub\u003e2\u003c/sub\u003e-rich and fluid-buffered conditions that initiate in the lithospheric mantle. Nevertheless, what we can \u0026ldquo;see\u0026rdquo; when \u0026ldquo;looking at\u0026rdquo; these data depend on the reliability of multicomponent saturation models for the volatile-melt systems. Using an accurate non-linear thermodynamic model, here I process relevant melt inclusion data and infer that volcanic plumbing systems are steadily infiltrated by deep mantle-derived fluids with a CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO ratio between 0.5 and 1 (molar), increasing southward. Furthermore, melt inclusions show variable degrees of CO\u003csub\u003e2\u003c/sub\u003e-enrichment (fluxing), related to 1) increasing CO\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO vapor (gas) ratio and flux in the ascending mantle-derived fluid or 2) cross-sectional variations of the ascending flow. The first scenario is related to variations occurring in the mantle source that are about its composition (mainly the alkali content) and the pressure/depth of early fluid release: The second scenario accounts for the fact that molten layers retain water and release a relatively CO\u003csub\u003e2\u003c/sub\u003e-enriched fluid upward, determining an effective H\u003csub\u003e2\u003c/sub\u003eO-CO\u003csub\u003e2\u003c/sub\u003e chromatography-like separation as long as melting goes on. The two scenarios are likely to operate in combination over time and space.\u003c/p\u003e","manuscriptTitle":"Fluid-induced and vapor-sustained magma production at the Stromboli and Campania Province volcanoes (Southern Italy)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-11 14:19:01","doi":"10.21203/rs.3.rs-8810254/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b3b61b64-3e54-484f-ad8f-74404a8efe66","owner":[],"postedDate":"February 11th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-18T15:24:39+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-11 14:19:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8810254","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8810254","identity":"rs-8810254","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}