Abstract
The benign fuming activity of dormant volcanoes is punctuated by phases of escalating degassing activity that, on some occasions, ultimately prelude to eruption. However, understanding the drivers of such unrest is complicated by complex interplay between magmatic and hydrothermal processes. Some of the most comprehensively characterised degassing unrest have recently been observed at La Fossa cone on Vulcano Island, but whether or not these episodes involve new, volatile-rich ascending magma remains debated. Here, we use volcanic gas measurements, in combination with melt inclusion information, to propose that heightened sulphur dioxide flux during the intense fall 2021 La Fossa unrest is sourced by degassing of volatile-rich mafic magma. Calculations using a numerical model indicate observations are consistent with the unrest being triggered by the emplacement of ∼3·106 m3 of mafic magma at ∼4–5 km depth. Degassing of mafic magma is argued as a recurrent driver of unrest at dormant volcanoes worldwide.
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Introduction
Of the ~1500 volcanoes with known or postulated eruptive activity1 in the Holocene, only ~10% (or less) are erupting or persistently degassing at present2,3. The global volcano population is, therefore, dominated by dormant volcanoes that exhibit mild to feeble hydrothermal fuming activity3,4,5. A major challenge in volcanology is to detect and interpret the reactivation signs of these dormant volcanoes, many of which are close to densely populated regions1. However, the periods of volcanic unrest that precede volcano reawakening are often associated with complex, non-linear patterns in volcanological, geophysical, and geochemical monitoring parameters6,7,8,9,10, posing challenges for the timely recognition of eruption precursors11,12.
One recurrent sign of dormant volcanoes becoming restless is an escalation in surface degassing activity13,14, typically manifesting as accelerating outgassing rates2,15, increasing fumarole temperatures and/or extension of degassing areas16,17, and increasingly magmatic (and less hydrothermal) chemical characteristics of the emitted gases18,19,20,21,22,23. As not all degassing unrests ultimately culminate into eruption8, the fundamental challenge is to distinguish, from data streamed by monitoring networks, between pre-eruption magmatic unrests (those driven by magma ascent, and that will culminate into an eruption24,25), failed/aborted magmatic unrests (in which degassing magma is involved that will not ultimately erupt26), and hydrothermal unrests (in which no magma is involved but hydrothermal/phreatic explosions are still possible27). A quantitative understanding of the mechanisms underpinning periods of degassing unrest is required to interpret and manage an evolving unrest situation in real-time.
Some of the most puzzling degassing unrests have recently occurred at La Fossa volcano (Fig. 1), a relatively young (<6 kyr old28,29,30) tuff cone on Vulcano Island (Italy). Since its last31 vulcanian eruption in 1880–90, La Fossa has recurrently been in a state of unrest, motivating extensive geochemical work and leading to the acquisition of an uniquely robust gas dataset23. A common conclusion of these studies19,21,23 is that intense fumarolic activity is sustained by mixing between CO2-N2-He-rich magmatic gases and hydrothermal steam deriving from evaporation of seawater-fed hot and saline fluids. Periods of elevated unrest occurred in the 1920s (ref. 32) and in 1988-93 (ref. 19), this latter phase being thought33 to have started in late 1977. Lower intensity periods of unrest were detected in 1996, 1998, and 2004–2009 (refs. 23,34,35). These unrest periods have been characterised by volcanic gases becoming increasingly magmatic in composition19,21,23,36 (with CO2-N2-He concentrations increasing relative to H2O), by visible escalations in outgassing flux37,38 (Fig. 2a), by increasing fumarole temperatures (up to ~650 °C in 1991; ref. 19) and by increasing diffuse degassing and thermal output from the volcano’s periphery39. Notably, mechanisms proposed to explain these periodic, abrupt unrests include (i) achievement of some pressure threshold in the hydrothermal system, due to accumulation of exsolved fluids sourced from stationary magma cooling and crystallising at depth23,36,40, or (ii) increasing magmatic gas supply from intermediate (latitic) magma undergoing decompression and open-system degassing during shallow ascent21.
In mid-September 2021, a new abrupt, intense acceleration in the volcanic gas output from La Fossa was recorded41 (Fig. 1). Reports42,43 from Istituto Nazionale di Geofisica and Vulcanologia soon revealed that the vented gases were among the most magmatic (CO2-rich) ever detected, and that the gas output escalation was also accompanied by shallow long period seismicity and, for the first time on record, by centimetre-scale ground inflation centred on La Fossa crater. In the following weeks, degassing activity remained sustained at the summit fumaroles. At the same time diffuse CO2 release from the volcano’s periphery also escalated, raising concerns regarding potential asphyxiation hazards in the inhabited areas. Local authorities were forced to prohibit access to the Vulcano Porto village overnight, capturing media attention worldwide44. The unrest is still ongoing by the time of writing, with reduced intensity45.
During background activity, the relatively low (≤20 tons/day) sulphur dioxide (SO2) flux37,41 sustained by the high-temperature (∼300–400 °C) permanent fumaroles suggests pervasive hydrothermal circulation in the volcano’s subsurface. The 2021 unrest, as in 1988–93 (ref. 37), was associated with a remarkable acceleration41 in the SO2 flux up to >200 tons/day (Fig. 2a). Although these sin-unrest SO2 flux levels remain well below those charactering the strongest degassing volcanoes worldwide (where SO2 is typically discharged at rates of several thousands tons/day; ref. 2), they still require an excess sulphur source. However, the origin of sulphur (S) in La Fossa fumaroles has long been a matter of debate; previous interpretations invoke either a magmatic46,47, hydrothermal48 or mixed19 magmatic/hydrothermal origin. These interpretations have distinct volcanic hazard implications and therefore interpreting the cause for the elevated SO2 flux is central to defining a coherent model for the initiation, and evolution, of La Fossa degassing unrests.
Here, we use volcanic gas records (in situ plume compositions and fluxes from remote sensing) to resolve the magmatic versus hydrothermal contributions to the increased S output in 2021. By comparing observations with results of a degassing model, initialised at various P-T-X conditions relevant to Vulcano Island volcanism, we propose that degassing of mafic magma—either due to ascent and decompression at depth or through the release of previously accumulated gas at the roof of a ~5 km deep reservoir underneath La Fossa—triggered the 2021 unrest.
Results and discussion
SO2 emissions
We quantify the SO2 flux from La Fossa cone during the 2021 unrest from ground-based remote dual UV Camera measurements (Supplementary Fig. 1; see Methods). Although discontinuous, our UV Camera data (Supplementary Table 1) record an escalation in SO2 release during fall 2021 (Fig. 2a, b). Our measurements do not constrain the exact onset of the degassing unrest (fixed at September 13 based on geodetic evidence43) but instead bracket the transition period from background to unrest regimes. We demonstrate that, in June 2021, the SO2 flux was low at 10–16 tons/day (Fig. 2b); this is very close to the background SO2 emission levels from La Fossa during 2008–2016, as inferred from the mean (18 tons/day) and median (22 tons/day) fluxes derived from regular observations with a permanent scanning spectrometer49,50 (Fig. 2a). We find that, in mid-October 2021, the SO2 flux increased by a factor >10 (190–256 tons/day on October 12), and then stabilised at a flux well above previous background levels (73–282 tons/day) in late October-December 2021 (Fig. 2b). A similar SO2 escalation has recently been reported from results of a network of permanent UV scanning spectrometers41. Our results constrain the average SO2 flux during the unrest (October-December 2021) at 143 ± 60 tons/day. For comparison, the mean SO2 flux during the 1988–93 unrest was ~88 tons/day (inferred from averaging data in refs. 37,51,52). Similarly, the peak SO2 emission was 155–240 tons/day in June 1991 (ref. 51) and subsequently declined since 1992, with background emissions being restored by 1995 (Fig. 2a). We therefore consider the median SO2 flux of ∼23 ± 15 tons/day for the 1995–2020 period, in which the La Fossa showed only mild and short-lived degassing unrests19,34,35,53, as representative of the background La Fossa emissions (Fig. 2a).
Changes in gas composition
Changes in the H2O-CO2-SO2-H2S composition of the near-source atmospheric plume emitted by the La Fossa crater fumaroles, as measured by a portable Multi-Component Gas Analyser System (Multi-GAS) (Supplementary Fig. 2; see Methods), indicate that - in addition to the increasing SO2 flux (Fig. 2a, b)—the major element composition of the atmospheric plume changed markedly in October-December 2021, relative to the 1995-early September 2021 period (Supplementary Table 2; Fig. 2b–d).
We find that fumarolic emissions in October-November 2021 have anomalously low molar CO2/ST ratios (where ST = SO2 + H2S; Fig. 2b, c) compared to the typical background composition. The range in molar CO2/ST in October-November 2021 is large, from 4.2 to 38.6, reflecting both temporal variability (Supplementary Table 2; Fig. 2b, c) and, more importantly, the compositional heterogeneity of the fumarolic field (Fig. 3). Walking Multi-GAS surveys conducted in November 16–18 demonstrate the spatial diversity within the La Fossa fumarolic field, in which both S-rich (CO2/SO2 ratio <10) and S-poor (CO2/SO2 ratio >30) fumaroles coexist (Fig. 3). Figure 3 also highlights, however, a core of low CO2/SO2 ratio (and CO2/ST ratio) fumaroles in the central segment of the outer rim fumarolic field, extending along the F0 fumarole and partly including the FA-F5AT-F11 fumaroles (Fig. 3). In the context of previous Multi-GAS (circles) and direct sampling (crosses) observations (Fig. 2c), these October-November 2021 compositions correspond to some of the lowest CO2/ST ratios measured at La Fossa, comparable only to those seen during the 1988-93 unrest (Figs. 2c, 4a; refs. 19,36).
Our Multi-GAS results also indicate that SO2 prevails over H2S at all fumaroles during October-December 2021, with relatively uniform H2S/SO2 ratios averaging 0.09 ± 0.03 (1σ; range, 0.05–0.18; Supplementary Table 2 and Fig. 2d). Thus, the decrease in CO2/ST ratio (Fig. 2c) essentially reflects increasing SO2 in the fumarolic emissions, consistent with the SO2 flux increase observed (Fig. 2a). In addition to a low CO2/ST signature, the October-November 2021 emissions are also particularly H2O-poor, with measured molar H2O/CO2 ratios (1.2–4.1) being the lowest on record (Fig. 4a). Multi-GAS data from December 2021 exhibit more variable composition than in previous months (Fig. 4b), together with a gradual progression toward higher CO2/ST ratios (11.6-70; Supplementary Table 2, Figs. 2b, 4a, b) at similarly low H2O/CO2 ratios (of 1.5–4.7).
Overall, the October-December 2021 results identify a compositional field that overlap the magmatic gas compositional end-member identified by directly sampled 1988–91 fumaroles19 (Fig. 4a).
An especially SO2-rich volcanic gas in fall 2021
Our gas observations provide evidence for a particularly SO2-rich fumarolic composition in fall 2021. The October-December 2021 plume samples plot at the high SO2-low H2S end (Fig. 4c) of the range exhibited by the 1979–2004 La Fossa fumaroles19,36,40, which encompasses the 1988–93 unrest. They are also substantially more SO2-rich than the typical background composition during the 2005–2020 quiescent phase, measured using the same Multi-GAS technique used here, and with direct fumarole sampling23 (see Supplementary Table 2 for data sources; Fig. 4c). Further, the fall 2021 plume data cluster in a relatively narrow compositional domain of unusually high ST and low H2O (Fig. 4b). Notably, the October–November 2021 fumarolic emissions are similar, or even less hydrous than, the compositions observed at the climax of the intense 1988–93 unrest19,36 (Fig. 4a).
Together with the coeval SO2 flux increase41, these atypical gas compositions highlight an escalation in SO2 transport within—and release from—the La Fossa fumarolic system, and require an additional source of S in excess to that sustaining the background SO2 emission of ∼23 ± 15 tons/day (Fig. 2a). The fumarolic gases released from La Fossa have been long been attributed to binary mixing between a CO2-N2-He-rich magmatic endmember and H2O-rich hydrothermal vapours deriving from evaporation of hot (300–400 °C), seawater-derived brines19,21,23,36. The origin of S has been more difficult to recognize but geochemical models suggest a mixed magmatic-hydrothermal derivation19,48. CO2 and ST concentrations were found48 to be highly correlated in contemporaneously sampled fumaroles during 1988-91, indicating that S was primarily of magmatic origin during that time. However, during the final stage of the same degassing unrest between 1992 and 1995, modelling instead supports a prevalent hydrothermal S origin48. From a hazard perspective, it is imperative to determine whether the enhanced transport and release of S as SO2 during fall 2021 is driven by hydrothermal or magmatic processes.
Hydrothermal sulphur remobilization
Sulfides, especially pyrite, are common in the sub-soil of La Fossa cone54. Thus, when hot CO2-rich magmatic fluids are supplied during an unrest phase19, it is possible that S is remobilized into the gas phase by either steam-driven oxidation19 or brine-mediated dissolution48 of previously-deposited pyrite. Remobilization of mineral- (pyrite-) hosted hydrothermal S is, at least in principle, a viable mechanism to explain the SO2 escalation seen during degassing unrests (e.g., in 1988–91 and in fall 2021; Fig. 2). Here, we evaluate this potential S source in the context of our gas compositional data.
The gas molar H2S/SO2 ratio can in principle be used27 to resolve the S fraction produced in the hydrothermal envelope. This is because, in the prevailingly reducing hydrothermal conditions (relative to more oxidised magmas) H2S is favoured over SO2 (Fig. 4c; ref. 27). The H2S/SO2 ratio has shown large fluctuations at La Fossa over the last 45 years (Fig. 2d). However, using this ratio to resolve magmatic vs. hydrothermal gas provenance is hampered at La Fossa by the relatively oxidized state of the hydrothermal system (thought to be buffered by coexisting hematite and magnetite; i.e., at the Hematite-Magnetite [HM] buffer48). In dry (steam-only) conditions, and at such high redox condition, calculations show that the hydrothermal gas H2S/SO2 ratio, buffered by pyrite oxidation, can lie anywhere between ~160 (at 10 MPa and ~650 °C) and ~0.1 (at 0.1 MPa and ~700 °C) (ref. 19) (Fig. 4c). Similarly, in models assuming the La Fossa hydrothermal reservoir is a bi-phase (brine+steam) system at ~20 MPa and 300–400 °C (ref. 48), the equilibrium (HM-buffered) H2S/SO2 gas ratio would vary between 150 (300 °C) and 0.4 (400 °C), well above the ratios measured in the surface emissions during the 2021 unrest (0.05–0.18; Fig. 4c). However, if hydrothermal HM buffering prevails until the vapour phase is discharged at the surface (i.e., at 0.1 MPa) then the equilibrium H2S/SO2 gas molar ratios vary from 0.77 (300 °C) to 0.02 (400 °C), which encompass the observed fumarolic range (Fig. 4c). Therefore, in both models (dry or two-phase), an oxidised La Fossa hydrothermal system can buffer H2S/SO2 in the gas at values comparable to those obtained for a magmatic gas buffered by shallow (<50 MPa) magma at the Nickel-Nickel Oxide buffer (NNO; ref. 48) (Fig. 4c). Consequently, as previously noted55, the H2S/SO2 molar gas is an unsuitable tracer to resolve hydrothermal from magmatic contributions at La Fossa, and the relatively low ratios measured in October–December 2021 (Fig. 2c) cannot be taken in favour for the prevalence of any of the two potential S sources.
The S yield of pyrite hydrolysis reactions is strongly temperature-dependent19. Therefore, if increasing gas-phase S remobilization during magmatic-gas driven heating of the hydrothermal system is underpinning the enhanced S transport observed during the 2021 unrest (Figs. 2, 4) then a relationship between temperature and fumarolic S content is expected. However, an inspection of published fumarole compositions from La Fossa (1978–2010; Fig. 5) shows that neither the fumarolic molar CO2/ST, nor the total S content (ST), correlate with fumarole discharge temperature (Fig. 5 and Supplementary Fig. 3). Rather, we highlight a clear temporal lag between the ST peak (occurring in the 1988–90) and the temperature peak in 1993 (at FA fumarole), when ST was already decreasing (Fig. 5). Similarly, the SO2 emission flux reaches its maximum nearly two years before (in June 1991) the 1993 FA temperature peak (Fig. 5).
In summary, while we do not question that hydrothermal reactions can source some, if not all, the background SO2 flux from La Fossa (when the CO2/ST is also high; Fig. 2a, b), we see no evidence in the published volcanic gas record that the elevated SO2 transport during the 1988-93 unrest was linked to heating of (and S remobilization from) the hydrothermal system. Ultimately, considering that the fumarolic temperature increase in fall 2021 is more limited (<100 °C; refs. 42,43) than in 1988–93, and that temperatures stabilised, or even started to decrease, by December 2021 (ref. 43) while the SO2 flux remained elevated, we consider it unlikely that the enhanced transport of SO2 (and corresponding decrease in CO2/ST) are controlled by temperature-dependent hydrothermal reactions to any major extent.
SO2 release from La Fossa magmas
We next test the hypothesis that the elevated SO2 emissions at La Fossa during fall 2021 are sourced from magmatic degassing. We model S partitioning from silicate melt into the magmatic gas phase using a H-O-C-S volatile saturation model56,57 (see Methods) initialised at P-T-X conditions relevant to the La Fossa magmatic system. As it is becoming increasingly evident at arc volcanoes worldwide58, the Vulcano Island magmatic plumbing system (Fig. 6a) is complex and vertically-elongated, with multiple magma ponding zones distributed over a range of depths/pressures31,35,59,60,61. This polybaric system is thought to have evolved in time during the >100 kyr history of the volcano28 and can be described schematically by (i) a deep storage zone(s) at mid (17–12 km) to lower (18–21 km, Moho limit) crustal depths, occupied by mafic (basaltic) magma; (ii) an intermediate storage reservoir at ∼3–5 km (the level of neutral buoyancy of ascending magmas), where intermediate (latitic) magma develops from basalt differentiation (fractional crystallization and assimilation), and (iii) a very shallow (1–2 km depth), heterogeneous (trachytic to rhyolitic) storage zone beneath La Fossa (Fig. 6a).
To account for this trans-crustal magmatic system, we initialise the model runs by ad-hoc sampling the Vulcano Island melt inclusion (MI) population (Supplementary Table 3) to cover the full spectrum of magma compositions (basaltic-latitic-trachytic-rhyolitic; Fig. 7). The model run conditions are summarised in Supplementary Table 4. Runs B1–B6 (Supplementary Table 4 and Supplementary Fig. 4) simulate the decompression-driven, closed-system degassing of basaltic magma (represented by the La Sommata scoriae erupted at ~42 ka; ref. 62) on ascent from the deep Vulcano Island magma storage zone (initial pressure of 300 MPa). We investigate isothermal decompression paths from a range of starting conditions, including redox conditions from NNO-1 to NNO + 1 and initial CO2 contents between 1 and 2 wt. % (Figs. 6, 7, Supplementary Fig. 4). The L1 and T1 runs refer to latitic (represented by Vulcanello I tephra, ~X century AD; ref. 30) and K-trachytic (PAL-D eruption, ~XIII century AD; ref. 30) magmas, respectively (each run with distinct initial S and H2O contents to cover the spread of volatile contents in MIs; Fig. 7), degassing in open-system conditions23 upon their ascent from the intermediate magma storage level (initial pressure, 50 MPa). Lastly, the R1 run shows the degassing path for rhyolitic magma (represented by PAL-B eruption occurred at ~XI century AD; ref. 30) undergoing open-system degassing upon decompression from 40 to 0.1 MPa (see Supplementary Table 4 and Methods). An open-system degassing mode is favoured for these runs as implied by the relatively shallow pressure (depth) conditions of magma emplacement, as in previous study23.
Modelled decompression paths predict a strong pressure-dependence for the dissolved S content in magmas (Fig. 6b) and the gas CO2/ST ratio in equilibrium with the melt (Fig. 6c). Dissolved volatile compositions in melts during degassing runs B1/B4 (at NNO-1) and B2/B5 (at NNO) reproduce the H2O-S degassing trends recorded in basaltic MIs (see model curves in Fig. 7b) more closely than the B3/B6 runs (at NNO + 1). Further, the model runs initiated under more oxidising conditions (B3/B6) predict limited S partitioning to the gas phase at high pressures (Fig. 6b) that is not reflected by the MI dataset (Fig. 7b). These results suggest that redox conditions between NNO (refs. 48,63) and NNO-1 (Fig. 7b), rather than NNO + 1, prevail in the magmatic system. The results of the B3/B6 model runs are thus considered for illustrative purposes only in the following discussion (Figs. 4, 8; Supplementary Fig. 4). The L1, T1 and R1 model runs (Fig. 6b) match well the low H2O-S end of the MI populations (Fig. 7b).
The theoretical magmatic gas compositions coexisting in equilibrium with the modelled melts—for all melt compositions explored—evolve from being CO2-rich at initial pressures (300 to 50–40 MPa, depending on the runs) to H2O- and S-rich at 0.1 MPa (Fig. 4b, Supplementary Fig. 4), reflecting the pressure-dependent solubilities of the major volatile species in silicate melts64,65. This compositional evolution is reflected in the decreasing CO2/ST ratio in the fluid phase with decreasing pressure (Fig. 6c).
Magmatic versus hydrothermal mixing proportions, and a pressure estimate for the magmatic source
The predicted magmatic gas compositional range circumscribed by the B1/B4, B2/B5 and L1 modelled degassing paths overlap with the range of volcanic gas compositions measured between October and December 2021 (Figs. 4b, 8 and Supplementary Fig. 4). The T1 and R1 runs produce, instead, modelled gas compositions that are more hydrous than the observed fumaroles (Figs. 4b, 8).
This agreement between modelled (B1/B4, B2/B5 and L1) and measured gas compositions is strong evidence that the unusually H2O-poor (and S-rich) gas composition in October–December 2021 reflects venting of predominantly magma-derived fluids (see also Fig. 4a). However, the positive relationship between H2O/ST and CO2/ST defined by the October–December 2021 array (Fig. 8) also implies some degree of mixing between magmatic and hydrous (and S-poor) hydrothermal fluids19,21,23,36.
We estimate the composition of the magmatic gas end-member by averaging the most H2O-poor (i.e., those with the smallest hydrothermal component, if any) plume samples in our dataset. This end-member magmatic gas (white star labelled M in Fig. 8) contains ~68.4 mol.% H2O, ~28.7 mol.% CO2, and ~2.9 mol.% ST, and is less hydrous than estimated previously for the 1988–93 unrest (see purple star labelled M in Fig. 8; ref. 19).
Identifying the composition of the hydrothermal component is more challenging, as previous work has found this component to vary in composition over time48. The inferred hydrothermal compositional range for the 1970s–1990s (refs. 19,48) is illustrated in Fig. 8; however, mixing (see grey dashed lines in Fig. 8) between such hydrothermal fluids and our magmatic end-member cannot reproduce the October–December 2021 data array. Instead, the 2021 array, together with the 1999–2010 fumarole compositions23, suggest the involvement of a more S-poor hydrothermal component (e.g., with higher H2O/ST and CO2/ST ratios than in the 1970s–1990s) between 1999 to present. Although determining the exact hydrothermal fluid signature is beyond the scope of this study, we identify several possible end-member compositions based on the samples exhibiting the most hydrothermal (H2O-rich, S-poor) signatures and derive the corresponding magmatic-hydrothermal mixing curves (see black dashed curves in Fig. 8). From these mixing curves, and considering the compositional variation of fumarolic plume samples, we infer that between 30% and 100% of the S released by La Fossa fumaroles during the October–December 2021 unrest is of magmatic origin (Fig. 8). These calculations suggest that the fumarole-specific magmatic vs. hydrothermal mixing proportions control the spatial heterogeneity of the fumarolic field (e.g., the CO2/ST variability seen in each survey; see Figs. 2a, b and 3), as previously demostrated21,23. Further, based on average plume compositions, we estimate that the magmatic sulphur contribution reduces from ~80% during October-November 2021 to ~50% in December 2021 (Fig. 8).
If, as the above modelling suggests, fumarolic emissions from the October–November 2021 degassing unrest are indeed prevalently magmatic, then their CO2/ST ratio—at least that of the H2O-poorest samples, in which the magmatic gas proportion is estimated to be >80%—can be used to infer the pressure of final gas-melt equilibration (i.e., the pressure at which gas separates from melt) based on the distinct pressure-dependent solubilities of the major volatile species in silicate melts64,65,66. We show how the CO2/ST ratio in the exsolved fluid phase is predicted to vary as a function of pressure, for the various model starting conditions for La Fossa (Fig. 6c). The least hydrous (most magmatic) La Fossa plume samples yield CO2/ST ratios between 7.5 and 11.5, with an average of 9.9 ± 1.0 (see Fig. 6c). This mean ratio translates into a range of possible saturation pressures depending on the chosen model run conditions shown in Table 1. For a basaltic magma starting composition, we estimate equilibrium pressures ranging from ~55 ± 5 MPa (B1) to ~20 ± 2 MPa (B2 and B4) and ~10 ± 1 MPa (B5) (Fig. 6c). For a latitic magma, the magmatic gas CO2/ST ratio of 9.9 suggests gas-melt equilibrium pressures of ~19 ± 2 MPa (L1). Lastly, decompression of a K-trachyte and rhyolite magma would imply degassing pressures of <5 MPa and ~22 ± 2 MPa, respectively (Fig. 6c).
Constraints on the magmatic source from magma budget calculations
Our comparison between modelled and observed gas compositions, although clearly supporting the involvement of magmatic volatiles in the unrest, does not allow us to unambiguously resolve the nature (composition) and storage conditions (emplacement/storage depth) of the magmatic source. The T1 and R1 modelled gases exhibit hydrous compositions, and therefore do not match the observed gas compositions in fall 2021 (Figs. 4, 8). In addition, trachytic to rhyolitic magmas, such as those erupted during the ∼1200 AD and 1888–90 La Fossa eruptions, are volatile-poor (Fig. 7) and have more radiogenic (3He/4He ratios (R) normalised to the air ratio (Ra) of ∼3.2 to ∼4.8 Ra/Ra) He isotopic signature67 than present-day fumaroles (∼6 R/Ra), making them unsuitable candidates as potential sources of the 1978-to-present La Fossa fumarolic activity21,23,68,69. Instead, fluid inclusion studies demonstrate that only more deeply-stored magmas, such basalts to latites (Fig. 6), have He isotopic signatures consistent with those of present-day gas emissions67; we therefore focus on these two magma compositions in the subsequent discussion.
The question of whether mafic or intermediate (latitic) magma is currently feeding recent (1970s to present) degassing activity at La Fossa is critical to future volcanic hazard assessment35; however, no consensus has thus far been reached. Volcanic gas geochemical modelling suggests that the temporal evolution of fumarole composition (both chemical and isotopic) is best explained by degassing of a shallow (<40 MPa), vertically-elongated latitic magma reservoir23. However, volatile contents within melt inclusions indicate that shallow-stored (latitic to rhyolitic) magma underneath La Fossa is typically S-poor (see Fig. 7) and therefore that degassing of this magma source is insufficient to sustain the long-term La Fossa SO2 (and CO2) outgassing budget46. Consequently, ascent and degassing of deep-derived (>50 MPa) mafic (basaltic) melt has been invoked to explain the increased SO2 output during the 1988–93 degassing unrest46.
Together with the results of model calculations (Figs. 6–8), our 2021 SO2 flux time series (Fig. 2a) offers insight into the source characteristics of the parent magma. The time-averaged SO2 flux during October–December 2021 is 143 ± 60 tons/day. At this rate, we estimate that a total SO2 mass of 1.6 × 104 tons was outgassed between the unrest onset (September 13, 2021; ref. 43) and December 31. If we assume, conservatively, that 80% (September–November) and 50% (December) of the degassed SO2 is magmatic (Fig. 8), then the total magmatic SO2 mass reduces to 1.1 × 104 tons (Table 1). Using the typical S yield (mg of S released per kg of magma) for the various magma types, as inferred from difference between initial (from melt inclusions) and final (at emplacement depth/pressure; from model run calculations; Fig. 6b) melt S contents, we show that the volume of degassing magma required to explain the surface SO2 emissions during the period of unrest ranges from 1.8·106 (B4) to 7.2·106 (L1) m3 (see Table 1 for full mass balance calculations).
The mass balance calculations presented above assume that the increased S emissions, and ultimately the degassing unrest itself, are triggered by decompression-driven volatile exsolution and degassing of a magma batch, ascending from reservoir conditions (Fig. 6a) to final emplacement pressure (Fig. 6c), where gas-melt separation finally occurs. In this interpretation, we can use a simple Mogi modelling approach70 to calculate the expected uplift at barycentre (e.g., at La Fossa crater) from magma emplacement volume and depth (see Table 1). Although simplistic in that magma compressibility is not considered in the calculations71 and that the deformation is assumed to be a simple point source, the calculated uplift ranges from ≥50 cm (B5, L1) to ~3.6 cm (B1). The measured uplift during the unrest was ~3 cm (refs. 42,43), thus suggesting that only the B1 scenario is geologically plausible.
In summary, we conclude that our gas observations, combined with geodetic information (refs. 42,43), can consistently be explained by the emplacement of ∼3·106 m3 of mafic magma at ∼4–5 km depth (Model B1; Table 1). We stress that the calculations above are in no way meant to quantitatively model the deformation pattern at La Fossa – they should merely be intended as back-of-the-envelope estimates of the expected order-of-magnitude uplift in the different magma scenarios explored.
Migrating or stationary magma?
Models involving the movement of magma masses21 beneath La Fossa, as implied by our calculations (Table 1), have recently been challenged in light of the lack of supporting geophysical evidence23,72,73,74. During hydrothermal background degassing activity, such as between 1999 and 2013, modelling74 of ground deflation and relatively minor (infrequent) volcano-tectonic (VT) seismicity indicate the absence of magma intrusion events, and instead suggest a deflating magma reservoir at ∼3–5 km depth. In contrast, during degassing unrests the frequency of VT seismicity increases and is typically accompanied by ground uplift74, as observed in the 1980s–1990s (refs. 75,76) and in fall 2021 (refs. 42,43). However, even during these unrest periods, seismicity and deformation are confined mostly within the shallow edifice and are therefore interpreted as pressurisation caused by the accumulation of magmatic gas within the shallow hydrothermal system74,75,76,77,78. Similarly, pressure build-up in the ∼3–5 km magma storage zone in the late 1980s has been attributed to increased magmatic fluid pressure rather than to magma emplacement74. In summary, geophysical evidence points to a key role for the migration of magmatic fluids (likely controlled by changes in the regional stress field), rather than magma itself, in generating the observed geophysical monitoring signals74.
To reconcile geophysical observations with geochemical evidence, a plumbing system model has been proposed23 in which temporal changes in magmatic gas composition are explained by time-varying gas contributions from multiple stationary, compositionally-distinct magma storage reservoirs, located at various depths within the vertically-elongated La Fossa plumbing system (Fig. 6a). In this stationary magma scenario, degassing unrest reflects gas accumulation in the roof zone of a magma reservoir (with gas bubble deriving from deeper in the magmatic system, and/or through exsolution driven by cooling and crystallisation65,79), until such a time as this volatile-rich cap is destabilised, either by some external forcing or once some threshold pressure is reached. Following this, accumulated fluids are released abruptly and ascend through the shallow magmatic-hydrothermal system, causing unrest23.
With our data alone, we cannot unambiguously resolve between the ascending magma or stationary magma hypotheses. Indeed, our modelling results would be entirely consistent with a model of stationary magma, in which escalating fumarolic gas fluxes in 2021 are fed by a pulse of magmatic gas released from either (i) basaltic magma at ∼4–5 km or (ii) latitic magma at ∼2 km (Fig. 6c and Table 1). We argue, however, that latitic magma is relatively poor in volatiles (Fig. 7) and is therefore unlikely to have sustained S (ref. 46) and CO2 (ref. 53) discharges over a degassing unrest that, although varying in intensity, has lasted for more than 40 years (ref. 35). Using the time-averaged La Fossa SO2 flux of 23 ± 15 tons/day as characteristic of background periods (1978–1987 and 1994–12 September 2021), and considering the increased SO2 flux during the two major unrest periods (1988–1993 and fall 2021), we derive a total SO2 release of ~0.5 Mtons over 44 years (1978 to 2021). If we assume conservatively that, during background degassing, only ~10% of the degassed SO2 is magmatic in origin and that this fraction increases to ~80% during unrest (Fig. 8), then the cumulative magmatic SO2 release reduces to ~0.19 Mtons. Using this total mass, and assuming the extreme case of complete S loss from a latitic magma with 0.1 wt.% to 0.05 wt.% (Fig. 7b) initial S, we find that ~5·107 to 108 m3 of latitic magma is required to have degassed and ultimately crystallised, since 1978. Notably, these estimated volumes are minimal as they do not consider degassing during previous, poorly-documented unrest periods (e.g., 1920s) since the previous 1888–90 eruption. Although tomographic studies at La Fossa have been limited to the upper (<1.5 km) portion of the volcano80, it is unlikely these relatively large melt volumes would have remained undetected by geophysics74. Therefore, we conclude that, even in a stationary magma scenario, deep mafic magma must be implicated as the magmatic gas source fuelling the unrest.
Implications for the unrest
The La Fossa chronostratigraphic record is surprisingly difficult to interpret, in view of the high number of tephra layers of similar characteristics, limited thickness, and fine grain-size (favouring erosion by wind and water). This led to controversies and different interpretations in eruptive history reconstructions28,29. However, recent integrated analysis of tephrochronological and radiometric-paleomagnetic results30 has found that, since medieval time, volcanic activity has been concentrated within two main eruptive clusters (~850–1300 AD and 1420–1890 AD), in which eruptions occurred every 35 years on average (at least between AD 850 and 1550). Volcanic hazard assessment exercises35 may thus need to be scaled up to consider that historical eruptive activity has been more frequent than previously thought28.
The geological history of La Fossa provides copious evidence that the ascent of mafic magma has been a recurrent eruption trigger31,46,62. Thus, tracking any mafic recharge event within the magmatic plumbing system is critically important to future hazard assessment scenarios. Our results presented here indicate that a large fraction of the elevated SO2 emissions in October–December 2021 is magmatic in origin, and requires a mafic magma source. SO2 flux emissions waned since January 2022 (Fig. 9) but, by the time of writing, remain well above background, with a novel phase of increase in May–June 2022 (Fig. 9), in correspondence with an abrupt phase of hydrothermal fluid venting in the Baia di Levante area81. Likewise, fumaroles’ CO2/ST ratios in 2022 are higher (e.g., more hydrothermally controlled; Fig. 8) than in October–November 2021, but still remain at lower levels than during the background (June-September 2021) phase (Fig. 9). These observations suggest that the unrest has waned in 2022 but not stopped, and that the mafic magma source may therefore still be actively degassing (even if at reduced rate) below Vulcano Island. Although a stationary magma scenario (in which gas is sourced by destabilisation of previously accumulated foam layer in the roof zones of a magma reservoir) cannot be excluded, modelling (Fig. 6) and calculations (Table 1) indicate that the case of mafic magma ascent (in the 300 to 50 MPa pressure range) and emplacement (at ∼4–5 km depth) should at least be considered as a possibility. For this ascending magma case, basaltic magma is likely to be relatively reduced (NNO-1) and moderately CO2-rich (1 wt.%) (the conditions of B1 model run). We emphasise that, according to calculations, the degassing magma volume required to explain the S budget of the 2021 unrest period (e.g., that would have emplaced at ∼4–5 km depth) is relatively small (~3.6·106 m3; Table 1) and may therefore be difficult to resolve from seismic-geodetic records, especially if basement rocks at 4–5 km depth are hot and plastic. Therefore, although any shallower (<4 km) magma ascent is arguably excluded by our calculations (see the unacceptably large inflation predicted by model scenarios B2, B4 and B5; Table 1), the likely presence of mafic magma at depth, combined with the pervasive hydrothermal circulation existing, make the volcano potentially unstable27. For example, the reduced SO2 flux in 2022 may reflect a combination of reduced gas supply from depth and reduced permeability (sealing)—a risky conditions at hydrothermal volcanoes27. Ultimately, volcano monitoring efforts should prioritise the detection of any (further?) basaltic magma ascent and, for this, volcanic gas monitoring should remain central to detect any further escalation in magmatic S release.
Broader implications
Mafic magma as the S source, and trigger of unrest, is not unique to Vulcano Island but is instead common to many volcanoes worldwide. However, a remarkable aspect of unrest at Vulcano Island is its periodic nature and sudden onset: several degassing escalations have repeated in pulses over the last 45 years, each starting abruptly (as demonstrated by the 2021 example, in which degassing activity was still at background levels in early September, only a few days prior to the 13 September start of the degassing crisis). Such pulsatory degassing behaviour is perhaps only comparable to that observed82 at White Island volcano, New Zealand, prior to its 1976–2000 eruptive sequence, in which repeated injection of mafic magma into shallow-stored andesitic/dacitic magma fuelled the volatile output and provided the trigger for the subsequent eruption83. Comparison with unrest phases elsewhere, interrupting decades to centuries of volcano dormancy, is complicated by different volcanic setting, but still brings evidence for mafic magma involvement. For example at Turrialba volcano in Costa Rica, the hydrothermal-to-magmatic gas transition (as highlighted by increasing SO2 proportions in fumarolic emissions combined with escalating fluxes22,24) has been more gradual, occurring over timescales of decades rather than weeks/months (as observed at Vulcano), but still fuelled by deeply rising magma22. It should be cautioned, however, that the Vulcano Island gas record is unique for its longevity and continuity; any rapid magmatic gas pulse may have remained undetected at the many quiescent volcanoes (including Turrialba) where gas observations have remained sporadic and discontinuous, at least until the advent of near-continuous instrumental monitoring24. For example, mafic magma has been strongly implicated in sustaining excess S degassing during the long-lived (1995–2010) Soufrière Hills eruption in Montserrat island84,85 and yet the volcano’s fumarolic fields had been sampled on only five occasions in the 30 years preceding eruption onset, potentially precluding detection of any mafic magma-related precursory signal86. An important corollary of our study is that, in addition to regular monitoring, modelling S degassing behaviour in magmas87 is critically important to detect the presence, and chemical identity, of any magmatic contribution to degassing unrests at dormant volcanoes. Combining field and petrological observations (i.e., volcanic gas and melt inclusions88,89), modelling65 and thermodynamic/experimental constraints90 in a multi-parametric geochemical approach is therefore recommended to interpret the evolution of magmatic-hydrothermal unrest at volcanoes globally.
Methods
UV Camera
We use the dual UV Camera technique91,92,93 to quantify the SO2 flux release from the La Fossa fumarolic system. Measurements were initially conducted during campaigns (Supplementary Fig. 1a) with a portable UV Camera unit similar to the one described in ref. 94. The instrument was powered by a 12 V battery and commanded via a Toughbook PC using the Vulcamera software95. We acquired data for several consecutive hours per day (at 0.5 Hz rate) and the mean daily recorded SO2 flux is listed in Supplementary Table 1 (data illustrated in Fig. 2a). On November 23, a permanent UV Camera system was field deployed in the Vulcanello area, ~2 km away from La Fossa (see Supplementary Fig. 1a for location). This fully autonomous UV camera system, similar to those in operation at nearby Stromboli96, is powered by 12 V battery kit and solar panels, and is commanded by an embedded mini-PC (Jetway NF531M) using a set of ad-hoc designed algorithms93 that allow the system to acquire 5 h per day and store data in external hard-drives. The data listed in Supplementary Table 1 correspond to daily averages calculated over temporal windows with optimal viewing conditions (clear sky, high plume visibility, un-condensed plume), while unsuitable periods (cloudy, rainy, condensed plume) are discarded. Both units (portable and fixed) are equipped with two JAI CM-140GE-UV cameras sensible to UV-radiation, fitted with two distinct band-pass optical filters (both of 10 nm Full Width at Half Maximum) with central wavelengths of 310 (SO2 absorption) and 330 nm (no SO2 absorption) (refs. 93,95). The cameras acquire images (520 × 676 pixels at 10-bit resolution) at 0.5 Hz that are later post-processed using standard techniques93,94,95,96. In brief, sets of co-acquired images are combined into sequences of absorbance images (Supplementary Fig. 1c) that are then converted into slant column amount (SCA) images using calibrations derived from either calibration cells or a co-aligned Ocean-Optics USB2000 + Spectrometer. Finally, Integrated Column Amounts (ICA) timeseries are obtained by integrating the sequences of SCA images along a cross-section perpendicular to plume transport (Supplementary Fig. 1c). A SO2 flux time-series is finally derived by multiplying ICA by plume speed (derived by tracking the motion of plume gas fronts in image sequences, using an optical flow algorithm93). Uncertainty in the derived SO2 fluxes is evaluated at ~ ±30% (ref. 91) at the low SO2/ash in-plume contents observed at Vulcano.
Multi-GAS
The compositions of the La Fossa fumaroles’ atmospheric plumes are characterised from periodic field surveys using a portable Multi-Component Gas Analyser System97 (Multi-GAS). As in previous work at La Fossa97,98,99, the Multi-GAS is used in the walking traverse mode. During operations, a light (<3 kg) portable unit is moved by an operator through the field while continuously acquiring (at 1 Hz rate), and with its position recorded by a synchronised GPS. During the measurements, the operator performs a sequence (typically 2) of walking transects through the fumarolic field with the Multi-GAS handheld, and with its inlet exposed to the fumarolic gas effluents at ~50 cm distance above each vent (in order to avoid sensors’ signal saturation). Each fumarolic vent is monitored for a few minutes, and the various fumaroles are sequentially analysed to cover the entire field in circa 1 h. During October-December 2021, degassing activity was concentrated on the La Fossa outer crater rim, while the inner crater fumaroles were only weakly degassing (and relatively colder). Our walking traverses, therefore, concentrated on the former.
In October, the very vigorous degassing activity prevented us for completing a walking traverse, and only some distal bulk plume observations were obtained (Supplementary Table 2). Walking traverses of the fumarolic field were realised in the November-December surveys. On November 7, two transects were performed in the outer crater rim area stretching between F0 and F11 fumaroles, two of the most systematically monitored fumaroles on Vulcano23,34. The various fumaroles analyses all exhibited a pretty homogeneous gas signature on that occasion (Supplementary Fig. 2). On November 16–18 and December 14, a wider area was investigated, instead (see Fig. 3), also including a number of newly opened (fall 2021) fumarolic vents to the north of the F0-F11 fumarole clusters. The fumarolic field was found more compositionally heterogeneous in these occasions (Fig. 3 and Supplementary Fig. 2), as indicated by the ratio results for the individual fumarolic vents (GPS coordinates listed in Supplementary Table 2). Our Multi-GAS dataset is also complemented by a series of measurements carried out during routine monitoring surveys in 2009 to June 2021. These unpublished results are also listed in Supplementary Table 2. Interpretation of our Multi-GAS dataset is assisted by integration and comparison (Figs. 2, 4, 5, 8) with results of direct sampling of the fumaroles from the literature (Supplementary Fig. 3).
The Multi-GAS records in-plume gas concentrations using a sensor kit consisting of (i) one Gascard nondispersive infrared (NDIR) spectrometer from Edinburgh Sensors (for CO2) (ii) two specific electrochemical sensors (both from City Technology) for SO2 and H2S, and (iii) one KVM3/5 Galltec-Mela sensor for temperature (T) and relative humidity (Rh), which were converted into in-plume H2O concentrations98 using the Arden Buck equation. The most recent configuration of the Multi-GAS, including details on sensor’s accuracy and repeatability, calibration ranges and gas standards, is described in ref. 100). Our Multi-GAS version, assembled at UniPa, uses a Campbell CR6 datalogger to log simultaneously at 1 Hz the signals outputted from all sensors. We calculate ratios between SO2 and other volatiles in the fumarolic plumes of individual fumaroles (listed in Supplementary Table 2) by post-processing the acquired dataset using a routine processing protocol implemented in the Ratiocalc software99. To this aim, sub-sets of data obtained at individual fumaroles are initially selected, and gas ratios are then derived from the gradients of the best-fit regression lines in gas (either H2O, CO2 or H2S) vs. SO2 scatter plots. The obtained ratios are listed in Supplementary Table 2. At the high gas concentrations observed at La Fossa (tens to hundreds of ppm SO2; Supplementary Fig. 2), error on the ratios is typically <10%. This traditional processing routine97 is complemented, in the case of the 16–18 November 2021 Multi-GAS walking traverse results, by a supplementary post-processing methodology101 that is aimed at resolving (and mapping; Fig. 3) the spatial heterogeneity of the fumarolic field with high temporal/spatial resolution. This processing routine explores the field acquired time series of GPS-localised gas concentrations, searching for periods of high temporal coherence between CO2 and SO2 concentrations in 200 s long moving windows. Within each of the sequentially scanned 200 s long temporal intervals, the routine outputs a CO2/SO2 ratio if (i) the Pearson correlation is ≥0.7, (ii) the SO2 concentration threshold of 20 ppm is reached. The obtained GPS-localised CO2/SO2 ratios are interpolated with multi-level B-splines102 to obtain the contour map of Fig. 3. This figure explores the spatial heterogeneity of the fumarolic field at finer spatial resolution, and is thus complementary to the individual fumarole results listed in Supplementary Table 2.
Melt inclusions
The pre-eruptive volatile content of La Fossa magmas is inferred from information derived from analysis of silicate melt inclusions and by the application of mineral-melt equilibria. We rely on an extensive dataset (reported in Supplementary Table 3) assembled from collection of published major and volatile elements melt inclusion analyses31,61,62,103,104,105,106. The compilation is complemented with unpublished and new results obtained in this work (data all listed in Supplementary Table 3).
Silicate melt inclusions (MIs) are hosted in phenocrysts picked from pumiceous lapilli of pyroclastic deposits, which guarantee quenching of the trapped melt upon eruption. The sampled eruptions span the last 1000 years of activity of La Fossa and Vulcanello (Vulcanello 1, Palizzi eruptive period, Lower Pietre Cotte, 1888–90 eruption; ref. 30; Eruptive Epoch 8 in ref. 28). The dataset covers the entire basalt-to-rhyolite range (Fig. 7a). Since only few basaltic melt inclusions have been found in recent La Fossa products (in Fo90 olivine and diopside antecrysts in 1888–90 eruption62), the melt inclusions of La Sommata (about 42 ky; Eruptive Epoch 5 in ref. 28) are taken as proxy for the basalts feeding the La Fossa plumbing system, based on the similar composition and the petrogenetic reconstructions62. Available data from other Vulcano eruptions (Punta Luccia shoshonite, Mastro Minico scoriae latite) of the last 50 ky are also reported28,30.
The Vulcano Island basalts have measured H2O contents of 3–4 wt%, and contain 1500–3000 ppm S (Fig. 7), and 2300–4000 ppm Cl; an H2O content of up to 6 wt% at the time of MI trapping has been postulated considering H diffusion through the host olivine107. Our B1-B6 model runs are therefore initialized at 3.9 wt.% H2O and 2500 ppm S (Supplementary Table 4; Figs. 6, 7). A marked decrease in H2O and S, and a moderate Cl decrease, is registered at the transition from basalt to shoshonite composition (Supplementary Table 3 and Fig. 7). The shoshonitic and latitic MIs exhibit ≼3 wt% H2O, and the majority of them have H2O contents between 1 and 2 wt.% (Fig. 7b). S is typically at ≤1000 ppm and Cl at 2000–3000 ppm. The K-trachyte melt inclusions have H2O contents of 1.8–2.5 wt%, S mostly below 500 ppm, and Cl in the 3000–4000 ppm range; the maximum water content of K-trachytes is confirmed by the application of plagioclase-liquid hygrometer (up to 2.54 ± 0.57 wt%, ref. 108). Our L1 and T1 model runs are designed as to cover the compositional range of latites and K-trachytes, respectively, and they are initialized at 2 and 2.5 wt% H2O and 1000 and 400 ppm S, respectively. Rhyolitic melt inclusions are only found in the rhyolitic pumiceous lapilli of the Palizzi period and show maximum H2O contents of 2.5–3 wt% (1.99 ± 0.30 according to the application of pl-lq hygrometer108), S < 100 ppm and Cl up to 5000–7000 ppm. Our R1 model run is therefore initialized at 2.5 wt.% H2O and 80 ppm S. CO2 is not detected during FTIR analysis of MIs of any composition (50 ppm is the FTIR detection limit), and low CO2 values are detected by SIMS in La Sommata basaltic melt inclusions (<184 ppm, ref. 105). No CO2 partitioning into the shrinkage bubble is found using Raman spectroscopy, suggesting CO2 loss before MI entrapment. Our model runs are therefore initialized at 1 to 2 wt.% CO2 using information109 from nearby Stromboli volcano. The rhyolitic run R1 is initialized at lower CO2 contents (0.1 wt.%), but the effect of higher CO2 (1 wt.%) is also explored (in run R2, see Supplementary Table 4 and Supplementary Fig. 4).
Literature information is complemented with novel melt inclusion information for selected compositions (rhyolitic melt inclusions) that have received little or no attention in previous studies (Supplementary Table 3). Major elements, S and Cl were measured by EMPA at the LaMA Laboratory of the Department of Earth Sciences (University) and CNR-IGG of Florence, with a JEOL Superprobe JXA-8230. The instrument is equipped with 5 wavelength dispersion spectrometers (WDS) operating with different analyser crystals and different detectors (gas flow and sealed Xe-type). The working conditions for glasses are 15 kV of accelerating voltage, with variable beam current intensity depending on the different matrix material under analysis and 5 nA beam current. The beam diameter varies from 5 μm to 10 μm for glasses to reduce the diffusion effect on alkali. Variable counting times are set for major and minor elements: 15 s on peak and 7 s on both backgrounds for major elements; 30–40 s on peak and 15–20 s on both backgrounds for minor elements, except for Na that is measured for 10 s on peak and 5 on both backgrounds. The acquisition order starts with Si, Na, K, Cl and Fe, followed by Mg, Ca, Mn, F, S and Al, Ti and P and, eventually Sr and Ba. A selection of natural phases are used as primary standards for the elemental calibration (Astimex Albite for Si and Na, Plagioclase for Al, Olivine for Mg, Diopside for Ca, Sanidine for K, Bustamite for Mn, Apatite for P, Celestine for Sr and S, Tugtupite for Cl, Barite for Ba, Fluorite for F and Smithsonian ilmenite for Ti and Fe). H2O and CO2 contents are measured by FTIR using a Nicolet Magna-IR560 spectrometer coupled with a NicPlan infrared microscope (at Dipartimento di Scienze della Terra, Pisa, Italy). At least three spectra for different areas were collected for each MI. The concentrations (c, wt.%) were calculated according to the Beer–Lambert law: c = 100 × A × M/[ε × ρ × d], where A (absorbance) is the height of the IR absorption peak, M is the molar mass (g·mol−1, 18.02 for total H2O, molecular H2O, and OH and 44.00 for CO32−), ε is molar absorptivity (l · mol−1 cm−1), ρ is the density of glass (g·cm−3), and d is the thickness of the inclusion. Total dissolved H2O is measured from the intensity of the broad, asymmetric peak at 3550 cm−1, which corresponds to the fundamental OH− stretching vibration110. We did not use the peaks at 4500 and 5200 cm−1 because they were strongly superimposed by interferences fringes, which make interpretation of the spectra difficult. Absorbance, A, of the band of interest is calculated from the spectra by averaging at least two to three different height measurements of the absorption band peak. Glass density (ρ) is estimated based on MI composition at 2400 g/cm3. Thickness (d) of each inclusion is calculated by averaging results of three distinct measurements made using a Mitutoyo digital micrometer in three different points before the analysis, with an error of 2–3 μm. We use a molar absorptivity of 56 for total dissolved H2O at the 3550 cm−1 band. No C peak is observable above the background, indicating that CO2 is always below the detection limit (~50 ppm).
Magma degassing model calculations
We use the volatile (C-O-H-S system) saturation model of Moretti et al.57 to calculate (i) dissolved volatile contents (Fig. 7) and (ii) equilibrium composition of magmatic fluids (Figs. 4, 6, 8; Supplementary Fig. 4) at T-P-X conditions relevant to Vulcano Island magmas. Full details and model theoretical background and functionalities can be found in refs. 56,57,111,112,113. The model input conditions are summarised in Supplementary Table 4. Model runs are initialised using representative parental melt compositions (major and volatile elements) determined from analysis of the Vulcano Island melt inclusion record (Supplementary Table 3). Runs are performed by simulating magma decompression from an initial pressure (see table) down to atmospheric conditions (0.1 MPa), under isothermal conditions, and in either closed-system (i.e., gas and melt continuously re-equilibrating during the decompression path) or open-system (gas-melt separation at each decompression step) conditions.
Data availability
All data produced in this work are listed in Supplementary Tables 1–4. The volcanic SO2 flux dataset and the volcanic gas composition dataset are publicly available in the EarthChem data repository (https://www.earthchem.org/ecl/) as Supplementary Data 1, 2 (ref. 114). Major elements and volatile composition of melt inclusions from Vulcano Island are publicly available in the EarthChem data repository (https://www.earthchem.org/ecl/) as Supplementary Data 3 (ref. 115).
Code availability
The CO2-H2O-SO2-H2S-melt saturation and degassing code is publicly available for download at https://github.com/charlesll//chosetto (https://doi.org/10.5281/zenodo.5554941) (ref. 116). The CHOSETTO.exe program (for Windows®) is available together with instructions to run it and relevant information on licensing and credits to be given.
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Acknowledgements
This work received funding from the Italian Ministero Istruzione Università e Ricerca (Miur, Grant N. 2017LMNLAW) and from the NSFGEO-NERC grant Sulfur cycling in subduction zones (subcontract between Lamont-Doherty Earth Observatory and DiSTeM, co-Pi A.A.). Financial support from BHP is acknowledged for help with the acquisition and processing of gas composition and flux data collected in June 2021 only. The authors wish to thank A. Mazot, J. Stix and Y. Taran for useful comments on an earlier version of the manuscript.
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A.A. study concept, data collection and processing, interpretation and modelling, drafting of the original version of the manuscript. M.B., S.C., F.P.L.M., A.C. data collection and processing. D.D.D., J.L. data collection and processing, interpretation and modelling. G.C. refinement of the initial concept, interpretation and modelling, contributed to writing. G.T., P.F., A.G., E.J.L., M.P. data collection and processing, interpretation and modelling, contributed to writing. R.M. data modelling.
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The authors declare no competing interests. E.J.L. is an Editorial Board Member for Communications Earth & Environment, but was not involved in the editorial review of, nor the decision to publish this article.
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Communications Earth & Environment thanks Agnes Mazot, John Stix and Yuri Taran for their contribution to the peer review of this work. Primary Handling Editor: Joe Aslin. Peer reviewer reports are available.
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Aiuppa, A., Bitetto, M., Calabrese, S. et al. Mafic magma feeds degassing unrest at Vulcano Island, Italy. Commun Earth Environ 3, 255 (2022). https://doi.org/10.1038/s43247-022-00589-1
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DOI: https://doi.org/10.1038/s43247-022-00589-1
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