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Journal ArticleDOI

A model of degassing for Stromboli volcano

TL;DR: In this article, the authors used the MultiGAS technique to provide the best documented record of gas plume discharges from Stromboli volcano to date, and showed that Strombolian's gases are dominated by H2O (48−98−mol); mean, 80%), and by CO2 (2−50−mol%; mean, 17%) and SO2 (0.2−14−mol; mean, 3%).
About: This article is published in Earth and Planetary Science Letters.The article was published on 2010-06-15 and is currently open access. It has received 144 citations till now. The article focuses on the topics: Strombolian eruption & Volcanic Gases.

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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  • ...This depressurizationinduced CO2 degassing event was thus precursory to fast ascent and eruption of the5 LP magma during the 15 March 2007 paroxysm (Aiuppa et al., 2010a; Métrich et al., 2010)....

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  • ...Our systematic determinations in January–July 2010 (using the technique detailed in Aiuppa et al., 2010b) actually demonstrate an increasing CO2/SO2 ratio (>40) also during single Strombolian out-15 bursts prior to the strong 25 June major explosion (Fig....

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  • ...It is noteworthy that Aiuppa et al. (2010a) already detected large increases in Strom-5 boli’s CO2 plume flux (>10 times larger than normal) during the 1–2 weeks preceding the 15 March 2007 paroxysmal explosion (Fig....

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  • ...Since the CO2 flux is a proxy for magma supply from the deep LP magma storage zone (Aiuppa et al., 2010b), and SO2 flux is a proxy for magma supply from 2–4 km depth, we attribute this CO2-specific deceleration to a phase of partial CO2-rich bubble retention (and accumulation) somewhere at depth in…...

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TL;DR: Papale et al. as mentioned in this paper applied thermodynamic equilibrium between gaseous and liquid volatile components to model the volatile saturation surface in H 2 O−CO 2 -silicate melt systems.

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  • ...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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TL;DR: The abundances of CO2, H2O, S and halogens dissolved in basaltic magmas are strongly variable because their solubilities and ability to be fractionated in the vapor phase depend on several parameters such as pressure, temperature, melt composition and redox state as mentioned in this paper.
Abstract: The abundances of CO2, H2O, S and halogens dissolved in basaltic magmas are strongly variable because their solubilities and ability to be fractionated in the vapor phase depend on several parameters such as pressure, temperature, melt composition and redox state. Experimental and analytical studies show that CO2 is much less soluble in silicate melts compared to H2O (e.g., Javoy and Pineau 1991; Dixon et al. 1995). As much as 90% of the initial CO2 dissolved in basaltic melts may be already degassed at crustal depths, whereas H2O remains dissolved because of its higher solubility such that H2O contents of basaltic magmas at crustal depths may reach a few percents. Most subduction-related basaltic magmas are rich in H2O (up to 6–8 wt%; Sisson and Grove 1993; Roggensack et al. 1997; Newman et al. 2000; Pichavant et al. 2002; Grove et al. 2005) compared to mid-ocean ridge basalts (<1 wt%; Sobolev and Chaussidon 1996; Fischer and Marty 2005; Wallace 2005). During magma movement towards the surface, exsolution of major volatile constituents (CO2, H2O) causes gas bubble nucleation, growth, and possible coalescence that exert a strong control on the dynamics of magma ascent and eruption (Anderson 1975; Sparks 1978; Tait et al. 1989). Gas bubbles have the ability to move faster than magma (Sparks 1978), particularly in low viscosity basaltic magmas. Bubble accumulation, coalescence and foam collapse give rise to differential transfer of gas slugs and periodic gas bursting (Strombolian activity; Jaupart and Vergniolle 1988, 1989) or periodic lava fountains (Vergniolle and Jaupart 1990; Philips and Wood 2001) depending on magma physical properties and ascent rate. It is also thought that strombolian and lava …

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  • ...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)....

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Abstract: [1] Two unusual, highly explosive flank eruptions succeeded on Mount Etna in July August 2001 and in October 2002 to January 2003, raising the possibility of changing magmatic conditions. Here we decipher the origin and mechanisms of the second eruption from the composition and volatile (H2O, CO2, S, Cl) content of olivine-hosted melt inclusions in explosive products from its south flank vents. Our results demonstrate that powerful lava fountains and ash columns at the eruption onset were sustained by closed system ascent of a batch of primitive, volatile-rich (≥4 wt %) basaltic magma that rose from ≥10 km depth below sea level (bsl) and suddenly extruded through 2001 fractures maintained opened by eastward flank spreading. This magma, the most primitive for 240 years, probably represents the alkali-rich parental end-member responsible for Etna lavas' evolution since the early 1970s. Few of it was directly extruded at the eruption onset, but its input likely pressurized the shallow plumbing system several weeks before the eruption. This latter was subsequently fed by the extrusion and degassing of larger amounts of the same, but slightly more evolved, magma that were ponding at 6–4 km bsl, in agreement with seismic data and with the lack of preeruptive SO2 accumulation above the initial depth of sulphur exsolution (∼3 km bsl). We find that while ponding, this magma was flushed and dehydrated by a CO2-rich gas phase of deeper derivation, a process that may commonly affect the plumbing system of Etna and other alkali basaltic volcanoes.

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  • ...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…...

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"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)....

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TL;DR: Spectroscopic measurements performed during both quiescent degassing and explosions on Stromboli volcano are used to demonstrate that gas slugs originate from as deep as the volcano-crust interface (∼3 kilometers), where both structural discontinuities and differential bubble-rise speed can promote slug coalescence.
Abstract: Strombolian-type eruptive activity, common at many volcanoes, consists of regular explosions driven by the bursting of gas slugs that rise faster than surrounding magma. Explosion quakes associated with this activity are usually localized at shallow depth; however, where and how slugs actually form remain poorly constrained. We used spectroscopic measurements performed during both quiescent degassing and explosions on Stromboli volcano (Italy) to demonstrate that gas slugs originate from as deep as the volcano-crust interface (∼3 kilometers), where both structural discontinuities and differential bubble-rise speed can promote slug coalescence. The observed decoupling between deep slug genesis and shallow (∼250-meter) explosion quakes may be a common feature of strombolian activity, determined by the geometry of plumbing systems.

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"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)....

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  • ...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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