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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Cites background from "A model of degassing for Stromboli ..."
...This spectrum of degassing has similarities to that at Stromboli, particularly in the two main groups of gas compositions that are evident (‘‘syn-explosive’’ and ‘‘quiescent’’ plumes; [1]; however, the melt composition and higher viscosity of Erebus phonolite [24] will affect the emitted compositions and types of activity....
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13 citations
13 citations
Cites background from "A model of degassing for Stromboli ..."
...If the pathway made by the propagation of film rupturing continues at a depth of several kilometers, such a pathway may be able to rapidly transport gas from the depths, causing an explosive gas emission [Burton et al., 2007; Aiuppa et al., 2010]....
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...Strombolian eruptions are considered as gas emissions originating at depths and ascending as slugs [Vergniolle and Brandeis, 1996; Seyfried and Freundt, 2000; Burton et al., 2007; James et al., 2009; Aiuppa et al., 2010; Bello et al., 2012; Pioli et al., 2012]....
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13 citations
References
491 citations
"A model of degassing for Stromboli ..." refers background or methods in this paper
...b A note of caution should be spent on the application of the H2O–CO2 model (Papale et al., 2006) on shoshonitic composition....
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...Standard deviations of model binary interaction terms showmaximum values for iron oxides, because they encompass all uncertainties on fO2 conditions within the calibration dataset (Papale et al., 2006)....
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340 citations
"A model of degassing for Stromboli ..." refers background in this paper
...These limitations have long precluded the acquisition of robust and systematic volcanic gas datasets at openvent volcanoes, thus making degassing processes easier to probe by studying volatile contents in silicate melt inclusions (MIs) (Blundy and Cashman, 2008; Métrich and Wallace, 2008)....
[...]
310 citations
"A model of degassing for Stromboli ..." refers background in this paper
...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…...
[...]
...(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....
[...]
301 citations
"A model of degassing for Stromboli ..." refers background in this paper
...These limitations have long precluded the acquisition of robust and systematic volcanic gas datasets at openvent volcanoes, thus making degassing processes easier to probe by studying volatile contents in silicate melt inclusions (MIs) (Blundy and Cashman, 2008; Métrich and Wallace, 2008)....
[...]
294 citations
"A model of degassing for Stromboli ..." refers background or result in this paper
...Our measurements here extend further the conclusions of Burton et al. (2007b): the temporal variability of the composition of the bulk (quiescent) plume requires the existence of a complex degassing regime in which a separate gas ascent plays a key role (Pichavant et al., 2009)....
[...]
...To start with, MI determinations (cfr. 5.1) and gas measurements (Burton et al., 2007a,b, and this study) offer ample evidence for that the shallow Stromboli's plumbing system is fluxed by the ascent of CO2-rich gas bubbles....
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...Transition from closed- to open-system conditions was fixed at 50 MPa (or ∼2 km bsv), the pressure at which vesicularity of the HP magma is thought to become high enough for gas percolation through a network of inter-connected bubbles to occur (Burton et al., 2007a)....
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...Our data support further the earlier conclusions of Burton et al. (2007b), demonstrating that the synexplosive gas phase is significantly richer in CO2 (and poorer in H2O and SO2) than the bulk plume (Fig....
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...%) fraction of CO2-rich gas bubbles at reservoir conditions (Burton et al., 2007a,b)....
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