A model of degassing for Stromboli volcano
Summary (4 min read)
1. Introduction
- This, combined with recent developments in H2O–CO2 micro-analysis in silicate materials and the refinement of thermodynamic saturation codes, now opens the way to more detailed inspection of degassing processes.
- Here, the authors report on the first MultiGAS measurements including H2O of the volcanic gas plume of Stromboli, an active basaltic volcano in Southern Italy (Fig. 1).
- This combined volcanic gas-melt inclusion-thermodynamic approach finally leads to thorough characterization of degassing processes at Stromboli volcano, with general implications for all basaltic volcanism.
2. Stromboli volcano
- The persistent Strombolian activity, for which the volcano is famous, began after the 3rd–7th centuries AD, and since then has continued without significant breaks or variations (Rosi et al., 2000).
- Explosive activity is associated with a continuous “passive” streaming of gas from the crater area and with active degassing (“puffing”) originating from discrete small gas bursts, every 1–2 s. During the lava effusion, a paroxysmal eruption also occurred (on 15 March), which erupted a significant amount of basaltic pumice (Landi et al., 2009).
- During July–December 2008 (the period over which the volcanic gas measurements are reported here), the volcano showed its typical activity, with rhythmic Strombolian explosions of variable energy at an average frequency of 10–15 events/h (see open-file reports at www.ct.ingv.it).
- On September 7, December 6 and 17, three slightly more energetic events occurred.
3. Technique
- The volcanic gas measurements reported here were carried out from July to December 2008, using the permanent MultiGAS installed on the summit of Stromboli by Istituto Nazionale di Geofisica and Vulcanologia (Sezione di Palermo).
- Signals from both sensors were captured every 9 s from a data-logger board, which also enabled data logging and storage.
- Because the instrument is located ∼150 m S–SE of the crater terrace (Fig. 1), plume gas sensing was only possible when moderate to strong winds from the northern quadrants blew on the island.
- In contrast when the plume was gently lofting, rising vertically, or being dispersed north, the MultiGAS consistently detected the typical H2O (13,000– 18,000 ppm), CO2 (∼380 ppm), and SO2 (b0.1 ppm) concentrations in background air, and the cycle was considered null (e.g., no ratio was calculated from the data).
4.1. Raw data and calculation of volcanic gas composition
- Fig. 2 shows an example of 1-cycle acquisition from the permanent MultiGAS at Stromboli.
- From the raw plume concentration data (in ppm), the volcanic gas plume H2O/SO2 and H2O/CO2 ratios were derived by calculating the gradients of the best-fit regression lines in H2O vs. SO2 and H2O vs. CO2 scatter plots (Fig. 3), as previously reported for Etna (Shinohara et al., 2008).
- This assumes that contributions from undetected species (e.g., H2, H2S, HCl) are relatively minor.
- Visual observations and cross correlations of their dataset with seismic and thermal signals (available at http://www.ct.ingv.it) indicated that such short-term variations (generally lasting less than 2 min) systematically occurred soon after individual Strombolian bursts.
- When the wind was particularly strong and explosive activity high, this syn-explosive gas phase, known to be compositionally distinct from the quiescent plume (Burton et al., 2007b), eventually reached the instrument (a few seconds after the explosion) before being diluted (and homogenised) within the bulk plume.
4.2. The H2O–CO2–SO2 composition of Stromboli's plume
- As such, they resemble quite closely the typical composition of volcanic gases from arc-settings, though sharing with nearby Etna (Shinohara et al., 2008) a characteristic of CO2-enrichment (most volcanic gases from arc basaltic volcanoes have N90% H2O; Shinohara, 2008).
- The most striking feature of the dataset is the large spread of plume compositions observed in only 6 months of observations.
5. Discussion
- The striking range of volcanic gas compositions at Stromboli suggest dynamic magma degassing processes at this open-vent volcano.
- This deep source area also supported the idea of a separate ascent of gas and melt in the shallow (less than 2.7 km) plumbing system, as also proposed for other basaltic systems (Edmonds and Gerlach, 2007).
- The authors measurements here extend further the conclusions of Burton et al. (2007b): the temporal variability of the composition of the bulk plume requires the existence of a complex degassing regime in which a separate gas ascent plays a key role (Pichavant et al., 2009).
- Visual observations suggest that the bulk Stromboli's plume is essentially contributed by both quiescent gas release from the magma ponding at the crater terrace' open vents, and by small bursts of over-pressurised gas pockets at the magma-free atmosphere (Harris and Ripepe, 2007).
- Finally, comparison between modelled and observed volcanic gas compositions (Section 5.3) offers new clues on volcanic degassing processes, and on the structure of the magmatic plumbing system of Stromboli.
5.1. Melt inclusion record of magma ascent and degassing
- There is consensus (Bertagnini et al., 2008) that two magma types are involved in the present-day Stromboli's activity.
- The persistent behaviour of the volcano implies that a supply of deeply derived magmas must occur not only prior to/during a paroxysm, but also during the normal Strombolian activity (yet at a slower rate).
- This has three main implications and consequences: (i) first, de-hydration of a magma can be caused by fluxing with deep-rising CO2-rich gas (Spilliaert et al., 2006), a fact which is suggestive of the presence of a magma ponding zone at 2–4 km bsv, where CO2-rich gas bubbles accumulate to contents N5 wt. % (Métrich et al., 2010).
- The contrasting compositions, volatile contents, and depth of storage of LP and HP magmas (Table 2) imply that the magmatic gas phases in equilibrium with (and separated from) these two magma types are inevitably different, as calculated below.
5.2. Numerical modelling
- Volatile contents in MIs (Table 2) are used here to initialize model calculations of volatile partitioning between the magmatic gas phase and the melt, which the authors performed using the code described in Moretti and Papale (2004).
- The authors utilised the code to perform two sets of complementary calculations.
- LP runs were initialised with the input parameters summarised in Table 2.
- Themodel results are critically dependent on the choice of the total (exsolved+dissolved) magma CO2 content: four sets of LP runs were thus carried out at different CO2 contents (0.2, 2, 5 or 20%, respectively), to account for the presence of a non-negligible (but poorly constrained) fraction of CO2-rich gas bubbles at reservoir conditions.
- The highest entrapment pressure (∼100 MPa) derived from volatile contents in MIs (Table 2) was taken as the starting pressure of their simulations, followed by step-wise pressure decrease in first closed-system to then opensystem conditions.
5.2.1. Model results, and comparison with natural data
- The outputs of model calculations are, for each run and at each pressure, the equilibrium volatile compositions of coexisting melt and vapour phases.
- The authors model results are qualitatively similar to the pressure-related model degassing trends presented by Allard (2010) (see his Fig. 3), which were yet based on the use of different saturation model and assumptions.
- As such, the volatile compositions of glass embayments may reflect gas-melt interactions within the CO2-rich intermediate (2–4 km deep) magma ponding zone (cfr. 5.1).
- Modelled dissolved sulphur contents (Fig. 7b) are also consistent with MI record, and again support a mechanism of progressive increase of the CO2TOT/H2OTOT ratio from trends 1 to 4.
- The authors note however that some of the richest CO2 volcanic gas data are consistent with model gas compositions calculated at P=100–120 MPa in the LP model run 1 (CO2TOT=0.2 wt.%; Fig. 8).
5.3. A model of degassing for Stromboli volcano
- The authors model calculations above provide a quantitative background for interpreting the source processes controlling the time-changing composition of Stromboli's volcanic gases.
- In the most extreme conditions, the CO2-rich gas bubbles may be thought to be sourced by the deep (7–11 km deep) LP magma storage zone; though partial gas-melt reequilibration at shallower depths (and particularly upon gas bubble accumulation within — before leakage from — the intermediate 2– 4 km deep magma ponding zone) cannot be ruled out.
- Secondly, there is supporting evidence at Stromboli for that continuous magma convection takes place within the shallow (b1 km) dyke system (Harris and Stevenson, 1997).
- The shallow convective overturning of the HPmagma obviously gives rise to a second source of volatiles: degassing of dissolved volatiles in the ascending HP magma will produce gas bubbles which pressure-dependent compositional evolution is best described by curves 5 and 6 in Fig.
6. Conclusions
- The MultiGAS volcanic gas observations presented here show that, in spite of the relatively uniform activity and petrology of erupted solid materials, Stromboli shares with other basaltic volcanoes an exceptional variability in gas compositions.
- The mechanisms controlling such time-changing nature of Stromboli's gas emissions have been explored by combining gas measurements with the MI record of volatile abundance in magmas, and by contrasting natural compositions with model results derived with an equilibrium saturation code.
- From this, the authors propose that the compositional features of Stromboli's quiescent and syn-explosive gas emissions result from themixing of gases persistently sourced by (i) degassing of dissolved volatiles in the porphyric magma filling the upper (b1 km) dyke-conduit system; and (ii) CO2-rich gas bubbles, originated at depth (at depths N4 km, or PN100 MPa) in the plumbing system.
- The proposed mixing mechanism is constrained by independent petrologic and model data, and it is geologically straightforward since it only requires a persistent but time-modulated source of deep gas bubbles; this however does not exclude that additional control mechanisms on volcanic gas composition might be at work.
- The authors conclude however that, since magma fluxing by a free CO2-rich vapour phase is a recurrent process, the proposed degassing mechanism is probably a key to interpret volcanic gas observations at many basaltic volcanoes.
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Citations
309 citations
Cites background from "A model of degassing for Stromboli ..."
...Gas can stream through magma from depth to the surface (Wallace et al. 2005), as surmised to occur at Soufrière Hills volcano, Montserrat (Edmonds et al. 2010) and Stromboli volcano (Aiuppa et al. 2010)....
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172 citations
Cites background from "A model of degassing for Stromboli ..."
...An important development is that long-term installations (using Wi-Fi or cell-phone networks, or satellite telemetry) are beginning to provide valuable and near-real time insights into the relationships between surface emissions and magmatic processes (e.g., Aiuppa et al. 2007b, 2010)....
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...2003, 2008), and volcanic gas emission data (Allard et al. 1994 ; Burton et al. 2007a ; Aiuppa et al. 2010; Allard 2010)....
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...…whose plumbing system is constrained by both phase equilibria (Di Carlo et al. 2006; Pichavant et al. 2009), detailed melt inclusion work (Métrich et al. 2001; Bertagnini et al. 2003, 2008), and volcanic gas emission data (Allard et al. 1994; Burton et al. 2007a; Aiuppa et al. 2010; Allard 2010)....
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148 citations
Cites background or methods or result from "A model of degassing for Stromboli ..."
...…from https://academic.oup.com/petrology/article-abstract/52/9/1737/1437269/Experimental-Simulation-of-Closed-System-Degassing by guest on 16 September 2017 from the two volcanoes: Stromboli data are from Bertagnini et al. (2003) and Me¤ trich et al. (2010); Masaya data from Sadofsky et al. (2008)....
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...At Stromboli, petrological studies show that a basaltic magma in equilibrium with a fluid phase in a deep-seated reservoir at 400MPa (Me¤ trich et al., 2001; Bertagnini et al., 2003; Pichavant et al., 2009) starts to degas during ascent. Under these conditions, according to our experimental results, fluids evolve from dominantly CO2-rich at 400MPa, to progressively more H2O-rich until 150MPa, and then become dramatically H2O-enriched at lower pressures. These results are consistent with experimental results obtained for golden pumices from Stromboli (Landi et al., 2004) equilibrated with an H2O^CO2 fluid phase (Pichavant et al., 2009). Burton et al. (2007a) and Aiuppa et al....
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...1 experimental data for S and Cl with the melt inclusions from Stromboli (Me¤ trich et al., 2001, 2010; Bertagnini et al., 2003)....
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...(2003) and Me¤ trich et al. (2010); Masaya data from Sadofsky et al. (2008). As no CO2 data for Masaya were presented by Sadofsky et al. (2008), we used the highest values ( 7000 ppm) reported by Atlas & Dixon (2006). For each volcano two mixtures were prepared with different initial sulphur contents to better investigate the behaviour of sulphur and its potential influence on the behaviour of other volatiles....
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...1 experimental data for S and Cl with the melt inclusions from Stromboli (Me¤ trich et al., 2001, 2010; Bertagnini et al., 2003). In Fig. 12a and b, respectively, we plot S and Cl in melt inclusions against the calculated H2O^CO2 saturation pressure for the same melt inclusion using VolatileCalc [it would make relatively little difference if we calculated pressure from our experimental data or used Papale et al. (2006)]. Melt inclusions show a good match to the low-sulphur series of experiments (St8.1.A), showing little change in dissolved S and Cl from 400 to 200MPa, followed by a sharp decrease in S, but not Cl, at P5150MPa. The matrix glass analyses of Me¤ trich et al. (2001) plot at the low-pressure extremity of this trend....
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126 citations
Additional excerpts
...The first scenario is reminiscent of trends observed at open-conduit volcanoes such as Stromboli and Etna ( [Aiuppa et al., 2007], [Aiuppa et al., 2010] and [Shinohara et al., 2008]); in the Erebus case, the explosive gas composition can be manufactured with ~ 50 wt.% of the deep, almost CO2-pure,…...
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...…P. Papale, H. Shinohara, M. Valenza Forecasting Etna eruptions by real-time observation of volcanic gas composition Geology, 35 (2007), pp. 1115–1118 Aiuppa et al., 2010 A. Aiuppa, A. Bertagnini, N. Métrich, R. Moretti, A. Di Muro, M. Liuzzo, G. Tamburello A degassing model for Stromboli volcano…...
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113 citations
References
287 citations
"A model of degassing for Stromboli ..." refers background or methods in this paper
...As such, they resemble quite closely the typical composition of volcanic gases from arc-settings, though sharing with nearby Etna (Shinohara et al., 2008) a characteristic of CO2-enrichment (most volcanic gases from arc basaltic volcanoes have N90% H2O; Shinohara, 2008)....
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...Indeed, whilst some persistently degassing volcanoes display an apparent stability in both activity state and volcanic gas composition for years (e.g., Nyiragongo, Sawyer et al., 2008), Stromboli shares with nearby Etna (Aiuppa et al., 2007) a timechanging nature of both volcanic activity state and volcanic gas composition....
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...…degassing volcanoes display an apparent stability in both activity state and volcanic gas composition for years (e.g., Nyiragongo, Sawyer et al., 2008), Stromboli shares with nearby Etna (Aiuppa et al., 2007) a timechanging nature of both volcanic activity state and volcanic gas composition....
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...3), as previously reported for Etna (Shinohara et al., 2008)....
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...Recently, however, the MultiGAS technique (Shinohara et al., 2008) has been established as a cheap and powerful tool for in-situ simultaneous sensing of the three major volcanogenic components (H2O, CO2 and SO2) in volcanic gas plumes (Aiuppa et al., 2007)....
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264 citations
"A model of degassing for Stromboli ..." refers background in this paper
...…degassing regime) at 2–4 km bsv depth; (iii) finally, de-hydration of the stored magmas raises their liquidus temperatures, hence promoting extensive crystallization (Métrich et al., 2001, 2010; Di Carlo et al., 2006), and ultimately leading to transition from the LP to the H2O poor (b1....
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260 citations
"A model of degassing for Stromboli ..." refers background in this paper
..., 2008) a characteristic of CO2-enrichment (most volcanic gases from arc basaltic volcanoes have N90% H2O; Shinohara, 2008)....
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...The so-called “excess degassing” (Shinohara, 2008), the fact that basaltic volcanoes no doubt emit more gas than potentially contributed by erupted magma, implies an effective gas bubble-melt separation at some point during the ascent....
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...As such, they resemble quite closely the typical composition of volcanic gases from arc-settings, though sharing with nearby Etna (Shinohara et al., 2008) a characteristic of CO2-enrichment (most volcanic gases from arc basaltic volcanoes have N90% H2O; Shinohara, 2008)....
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...This degassingdriven process (Shinohara, 2008) occurs in response to the sinking of the degassed (non-erupted) HP magma back into the conduit, and its replacement with ascending vesicular (and thus less-dense) magma blobs....
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241 citations
"A model of degassing for Stromboli ..." refers background in this paper
..., 2000) or passive (using the magma as the source of radiation; Allard et al., 2004)measurements....
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...…(H2O): because of the large H2O concentrations in the background atmosphere, volcanic H2O detection using FTIR and solar oscultation is currently impossible, thus demanding either active (Burton et al., 2000) or passive (using the magma as the source of radiation; Allard et al., 2004)measurements....
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239 citations
"A model of degassing for Stromboli ..." refers background in this paper
...The persistent Strombolian activity, for which the volcano is famous, began after the 3rd–7th centuries AD, and since then has continued without significant breaks or variations (Rosi et al., 2000)....
[...]
...Stromboli, world-known for its mild and uninterrupted Strombolian activity (Rosi et al., 2000), is an ideal...
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...Stromboli, world-known for its mild and uninterrupted Strombolian activity (Rosi et al., 2000), is an ideal target for the modelling of degassing processes, since (i) the persistent open-vent gas emissions are relatively easy to measure (Allard et al., 2008), (ii) the mechanisms driving the…...
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