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A model of degassing for Stromboli volcano

15 Jun 2010-Earth and Planetary Science Letters (Elsevier)-Vol. 295, Iss: 1, pp 195-204

AbstractA better understanding of degassing processes at open-vent basaltic volcanoes requires collection of new datasets of H2O–CO2–SO2 volcanic gas plume compositions, which acquisition has long been hampered by technical limitations. Here, we use the MultiGAS technique to provide the best-documented record of gas plume discharges from Stromboli volcano to date. We show that Stromboli'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%). The significant temporal variability in our dataset reflects the dynamic nature of degassing process during Strombolian activity; which we explore by interpreting our gas measurements in tandem with the melt inclusion record of pre-eruptive dissolved volatile abundances, and with the results of an equilibrium saturation model. Comparison between natural (volcanic gas and melt inclusion) and modelled compositions is used to propose a degassing mechanism for Stromboli volcano, which suggests surface gas discharges are mixtures of CO2-rich gas bubbles supplied from the deep (> 4 km) plumbing system, and gases released from degassing of dissolved volatiles in the magma filling the upper conduits. The proposed mixing mechanism offers a viable and general model to account for composition of gas discharges at all volcanoes for which petrologic evidence of CO2 fluxing exists. A combined volcanic gas-melt inclusion-modelling approach, as used in this paper, provides key constraints on degassing processes, and should thus be pursued further.

Topics: Strombolian eruption (57%), Volcanic Gases (56%), Volcano (53%), Magma (51%)

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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A model of degassing for Stromboli volcano
A. Aiuppa
a,b,
, A. Bertagnini
c
, N. Métrich
c,d
, R. Moretti
e
, A. Di Muro
f
, M. Liuzzo
b
, G. Tamburello
a
a
CFTA, Università di Palermo, Italy
b
INGV, Sezione di Palermo, Italy
c
INGV, Sezione di Pisa, Italy
d
LPS, CEA-CNRS, Saclay, France
e
INGV, Sezione di Napoli, Osservatorio Vesuviano, Italy
f
IPGP/UPMR, Paris, France
abstractarticle info
Article history:
Received 8 January 2010
Received in revised form 29 March 2010
Accepted 30 March 2010
Available online 7 May 2010
Editor: R.W. Carlson
Keywords:
volcanic degassing
Stromboli
volcanic gases
CO
2
uxing
A better understanding of degassing processes at open-vent basaltic volcanoes requires collection of new
datasets of H
2
OCO
2
SO
2
volcanic gas plume compositions, which acquisition has long been hampered by
technical limitations. Here, we use the MultiGAS technique to provide the best-documented record of gas
plume discharges from Stromboli volcano to date. We show that Stromboli's gases are dominated by H
2
O
(4898 mol%; mean, 80%), and by CO
2
(250 mol%; mean, 17%) and SO
2
(0.214 mol%; mean, 3%). The
signicant temporal variability in our dataset reects the dynamic nature of degassing process during
Strombolian activity; which we explore by interpreting our gas measurements in tandem with the melt
inclusion record of pre-eruptive dissolved volatile abundances, and with the results of an equilibrium
saturation model. Comparison between natural (volcanic gas and melt inclusion) and modelled compositions
is used to propose a degassing mechanism for Stromboli volcano, which suggests surface gas discharges are
mixtures of CO
2
-rich gas bubbles supplied from the deep (N 4 km) plumbing system, and gases released from
degassing of dissolved volatiles in the magma lling the upper conduits. The proposed mixing mechanism
offers a viable and general model to account for composition of gas discharges at all volcanoes for which
petrologic evidence of CO
2
uxing exists. A combined volcanic gas-melt inclusion-modelling approach, as
used in this paper, provides key constraints on degassing processes, and should thus be pursued further.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The processes driving the endless degassing activity of open-
conduit basaltic volcanoes have attracted the attention of volcanol-
ogists for decades, and have extensively been studied in recent times
thanks to the advent of more and more sophisticated observation
techniques. One of the most important though often overlooked
aspects of basaltic volcanism is its exceptional gas productivity. 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. However, while it is universally
accepted that separate gas transfer exerts a key control on both
quiescent (Burton et al., 2007a) and eruptive (Edmonds and Gerlach,
2007) degassing of basaltic volcanoes, the mechanisms (structural vs.
uid-dynamic control) and depths (shallow vs. deep) of such gas
separation are still not entirely understood (Edmonds, 2008).
Volcanic gas investigations have long been hampered by mea-
surement of the most abundant volcanic volatile, water vapour (H
2
O):
because of the large H
2
O concentrations in the background atmo-
sphere, volcanic H
2
O 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. These limitations have long precluded the
acquisition of robust and systematic volcanic gas datasets at open-
vent 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). 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 (H
2
O, CO
2
and SO
2
) in volcanic
gas plumes (Aiuppa et al., 2007). This, combined with recent
developments in H
2
OCO
2
micro-analysis in silicate materials and
the renement of thermodynamic saturation codes, now opens the
way to more detailed inspection of degassing processes.
Here, we report on the rst MultiGAS measurements including
H
2
O of the volcanic gas plume of Stromboli, an active basaltic volcano
in Southern Italy (Fig. 1). Stromboli, world-known for its mild and
uninterrupted Strombolian activity (Rosi et al., 2000), is an ideal
Earth and Planetary Science Letters 295 (2010) 195204
Corresponding author. CFTA, Università di Palermo, Italy. Tel.: +39 091 23861624;
fax: +39 091 6168376.
E-mail address: aiuppa@unipa.it (A. Aiuppa).
0012-821X/$ see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2010.03.040
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
journal homepage: www.elsevier.com/locate/epsl

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 persistent
Strombolian activity of the volcano and the related seismicity are well
characterised (Ripepe et al., 2008), (iii) the petrology of the magmas is
intensively studied (Bertagnini et al., 2008), and (iv) clear experi-
mental evidence exists for an efcient gas-melt separation in the
plumbing system (Burton et al., 2007b). In spite of this existing
knowledge, the structure of the deep and shallow plumbing system is
still a matter of debate (Métrich et al., 2010; Pichavant et al., 2009),
and information of volcanic gas compositions is still fragmentary,
particularly for H
2
O. In the attempt to provide a comprehensive model
of degassing, we integrate here our volcanic gas observations with
recent determinations of volatile contents in melt inclusions (Métrich
et al., 2010); and we compare the natural (volcanic gas and MI) data
with result s from the Moretti and Papale (2004) equili brium
saturation model, which we use to numerically reproduc e the
degassing trends of Stromboli's magmas upon their ascent and
decompression. This combined volcanic gas-melt inclusion-thermo-
dynamic approa ch nally leads to thorough characterization of
degassing processes at Stromboli volcano, with general implications
for all basaltic volcanism. Our focus is on the routine Strombolian
activity, making our study complementary to recent work (Métrich
et al., 2010; Allard, 2010) on t he genetic mechanisms of t he
Stromboli's large scale explosions.
2. Stromboli volcano
The persistent Strombolian activity, for which the volcano is
famous, began after the 3rd7th centuries AD, and since then has
continued without signicant breaks or variations (Rosi et al., 2000).
The current activity takes place at three main craters located in a NE
SW elongated area (the crater terrace) at about 750 m a.s.l. within the
Sciara del Fuoco, a deep horse-shoe depression resulting from several
lateral collapses (Fig. 1). A variable number of vents (515) sustain
the typical activity, consisting of intermittent mild explosions lasting
few seconds (430 s), and with a typical frequency of 13 events/
h(Ripepe et al., 2008). The activity is highly variable over timescales
of hours and days, and ranges from ash-dominated eruptions to bursts
throwing incandescent scoriae and bombs. Emitted products attain
heights of a few tens up to hundreds of meters and usually fall in the
vicinity of the crat ers. Explosive activity is associated with a
continuous passive streaming of gas from the crater area and with
active degassing (pufng) originating from discrete small gas
bursts, every 12s.
This routine activity is sporadically interrupted by more energetic
explosive events (paroxysms) in which the ejecta fallout reaches the
volcano slopes and settled areas along the coast in the largest
eruptions. Paroxysms are impulsive events consisting of several
explosions from different craters, associated with the ejection of large
ballistic blocks and the emission of vertical jets of gas and pyroclasts
evolving in short-lived convective columns. A peculiar feature of
paroxysms is the co-emission of a nearly aphyric basaltic pumices
along with the usual crystal-rich scoria (Bertagnini et al., 2003).
Effusive phases also occur on Stromboli, on average every 4 years
since 1888. Lava ows are usually related to overows from the
craters or vent opening inside the Sciara del Fuoco. The last effusive
episode occurred from 27 February to 2 April 2007, and emitted
10
7
m
3
of lava. During the lava effusion, a paroxysmal eruption also
occurred (on 15 March), which erupted a signicant amount of
basaltic pumice (Landi et al., 2009).
During JulyDecember 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
Fig. 1. A map of Stromboli showing the location of the permanent MultiGAS on Pizzo Sopra la Fossa.
196 A. Aiuppa et al. / Earth and Planetary Science Letters 295 (2010) 195204

an average frequency of 1015 events/h (see open-le 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 Geosica and
Vulcanologia (Sezione di Palermo). This fully-automated instrument
has been used for routine measurements of CO
2
and SO
2
concentra-
tions in Stromboli's plume since 2006 (Aiuppa et al., 2009 report on
principles of operation, and on CO
2
/SO
2
plume ratios in the period
from May 2006 to December 2007). In order to measure H
2
O, and
improve further the quality of CO
2
and SO
2
detection, we used in this
study an updated MultiGAS conguration, and more specically a LI-
840 NDIR closed-path spectrometer for both CO
2
(measurement
range, 03000 ppm; accuracy, ±1.5%) and H
2
O (measurement range,
080 ppt; accuracy, ±1.5%) (see Shinohara et al., 2008 for details);
and a sensitive electrochemical sensor (model 3ST/F, Cod.TD2D-1A,
City Technology Ltd., calibration range, 030 ppmv; repeatability 1%)
for SO
2
. Signals from both sensors were captured every 9 s from a
data-logger board, which also enabled data logging and storage. After
a cycle of 200 measurements, lasting 1800 s in total, a radio link
operated automatic data transfer from the remote MultiGAS to the
base station in Palermo, where data were elaborated.
Four measurement cycles were operated daily. However, because
the instrument is located 150 m SSE 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 Mult iGAS consistently detected the typical H
2
O ( 13,000
18,000 ppm), CO
2
( 380 ppm), and SO
2
(b 0.1 ppm) concentrations
in background air, and the cycle was considered null (e.g., no ratio was
calculated from the data). In addition, volcanic H
2
O detection was
limited to relatively dry and cloud-free conditions on Stromboli's
summit (when the plume was not condensing), and in situations
when the plume was dense e nough for volc anic H
2
Otobe
distinguished from background variations (Fig. 2). In summary, whilst
a record of CO
2
/SO
2
ratios was achieved on a nearly daily basis,
simultaneous detection of the 3 species was only attained 124 times
during the record period (Table 1). Accuracy and precision on the
Fig. 2. An example of a 1800 s MultiGAS acquisition at Stromboli (acquisition frequency,
9 s). Whilst small erratic variations of H
2
O concentrations are typically measured when
the plume is condensing (curve a), more systematic variations (curve b) are observed in
dry weather conditions and when the plume fumigates the Pizzo Sopra la Fossa area.
These are correlated with variations of CO
2
(curve c) and SO
2
(curve d) concentrations.
In such circumstances, volcanic H
2
O was derived from the raw data (b) by subtracting
background air H
2
O content; this required tting a polynomial function (shown as a
dotted line) to H
2
O measurements for which a SO
2
content of nearly 0 was consistently
detected.
Table 1
Compositions of Stromboli's volcanic gas plume (in mol%). We derive compositions for
both the bulk plume (essentially contributed by persistent passive degassing) and the
syn-explosive plume (the gas jet of a Strombolian explosion, reaching the MultiGAS a
few seconds after the burst, and before being diluted in the main plume).
Date H
2
OCO
2
SO
2
Date H
2
OCO
2
SO
2
Bulk plume
24/7/08 0.91 0.06 0.03 17/9/08 0.81 0.16 0.02
25/7/08 0.91 0.07 0.02 21/9/08 0.56 0.38 0.06
27/7/08 0.95 0.03 0.02 22/9/08 0.93 0.07 0.01
28/7/08 0.89 0.08 0.03 25/9/08 0.72 0.26 0.02
29/7/08 0.94 0.04 0.02 25/9/08 0.78 0.19 0.04
30/7/08 0.95 0.04 0.02 26/9/08 0.56 0.39 0.05
31/7/08 0.97 0.02 0.01 26/9/08 0.82 0.17 0.01
1/8/08 0.97 0.02 0.01 29/9/08 0.59 0.39 0.02
3/8/08 0.97 0.02 0.01 29/9/08 0.79 0.19 0.02
4/8/08 0.83 0.07 0.10 3/10/08 0.48 0.47 0.05
5/8/08 0.97 0.02 0.01 4/10/08 0.75 0.19 0.05
6/8/08 0.81 0.13 0.06 4/10/08 0.96 0.04 0.01
7/8/08 0.87 0.12 0.01 5/10/08 0.70 0.27 0.03
8/8/08 0.97 0.03 0.01 5/10/08 0.80 0.17 0.03
9/8/08 0.98 0.02 0.01 5/10/08 0.82 0.15 0.03
10/8/08 0.85 0.11 0.04 8/10/08 0.89 0.10 0.01
11/8/08 0.97 0.02 0.01 12/10/08 0.93 0.06 0.02
13/8/08 0.98 0.02 0.01 12/10/08 0.95 0.04 0.01
14/8/08 0.96 0.03 0.01 13/10/08 0.89 0.08 0.02
17/8/08 0.91 0.07 0.03 14/10/08 0.90 0.09 0.02
18/8/08 0.94 0.04 0.01 15/10/08 0.83 0.16 0.01
20/8/08 0.72 0.25 0.03 17/10/08 0.77 0.20 0.03
23/8/08 0.98 0.02 0.01 17/10/08 0.86 0.13 0.02
25/8/08 0.98 0.02 0.01 25/10/08 0.75 0.22 0.03
29/8/08 0.90 0.09 0.01 31/10/08 0.79 0.19 0.02
31/8/08 0.86 0.10 0.05 4/11/08 0.80 0.17 0.03
1/9/08 0.95 0.02 0.03 7/11/08 0.83 0.15 0.02
2/9/08 0.96 0.03 0.01 8/11/08 0.68 0.27 0.06
5/9/08 0.93 0.05 0.02 11/11/08 0.55 0.31 0.14
6/9/08 0.95 0.05 0.00 11/11/08 0.80 0.17 0.03
7/9/08 0.78 0.15 0.07 11/11/08 0.54 0.38 0.08
7/9/08 0.96 0.04 0.01 18/11/08 0.89 0.10 0.01
7/9/08 0.91 0.05 0.04 19/11/08 0.91 0.07 0.02
7/9/08 0.97 0.03 0.00 19/11/08 0.77 0.19 0.03
7/9/08 0.88 0.10 0.02 22/11/08 0.90 0.07 0.03
8/9/08 0.73 0.24 0.03 22/11/08 0.65 0.30 0.04
9/9/08 0.89 0.10 0.01 23/11/08 0.49 0.41 0.10
9/9/08 0.86 0.13 0.01 26/11/08 0.78 0.19 0.03
10/9/08 0.75 0.22 0.03 27/11/08 0.64 0.28 0.08
11/9/08 0.93 0.05 0.02 1/12/08 0.71 0.26 0.03
11/9/08 0.74 0.16 0.10 6/12/08 0.54 0.39 0.07
12/9/08 0.64 0.34 0.02 7/12/08 0.72 0.23 0.05
12/9/08 0.91 0.08 0.01 7/12/08 0.65 0.28 0.07
13/9/08 0.95 0.04 0.01 7/12/08 0.90 0.07 0.03
13/9/08 0.97 0.03 0.00 7/12/08 0.60 0.34 0.06
15/9/08 0.90 0.09 0.01 9/12/08 0.63 0.34 0.04
15/9/08 0.77 0.21 0.02 10/12/08 0.58 0.35 0.07
16/9/08 0.81 0.17 0.02 12/12/08 0.59 0.34 0.07
16/9/08 0.88 0.11 0.01 17/12/08 0.78 0.20 0.02
16/9/08 0.60 0.31 0.09 24/12/08 0.80 0.16 0.04
17/9/08 0.89 0.06 0.04 24/12/08 0.85 0.13 0.02
Syn-explosive gas
27/7/08 0.66 0.31 0.03 4/10/08 0.83 0.16 0.01
2/8/08 0.88 0.12 0.00 6/10/08 0.59 0.39 0.02
7/9/08 0.88 0.11 0.01 12/10/08 0.87 0.12 0.01
13/9/08 0.86 0.14 0.00 14/10/08 0.78 0.21 0.01
15/9/08 0.72 0.27 0.01 15/10/08 0.85 0.15 0.01
15/9/08 0.77 0.23 0.01 27/10/08 0.72 0.28 0.01
16/9/08 0.72 0.25 0.02 11/11/08 0.60 0.36 0.04
17/9/08 0.83 0.16 0.01 19/11/08 0.70 0.27 0.02
17/9/08 0.58 0.41 0.01 4/12/08 0.71 0.28 0.01
23/9/08 0.71 0.28 0.01 7/12/08 0.67 0.31 0.02
29/9/08 0.49 0.50 0.01 12/12/08 0.65 0.34 0.01
197A. Aiuppa et al. / Earth and Planetary Science Letters 295 (2010) 195204

sensors was periodically checked (every 2 moths) using standard gas
mixtures and a dew point generator (for H
2
O).
4. Results
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. When the plume was condensing, H
2
O
concentrations varied smoothly and randomly during the 1800 s
acquisition period (curve a), precluding any retrieval. In contrast,
larger variations of H
2
O concentrations (curve b) were captured in the
optimal conditions (dry weather conditions, plume fumigating the
area of Pizzo Sopra la Fossa, see Fig. 1), which were broadly correlated
with time variations of CO
2
(curve c) and SO
2
(curve d). In these
circumstances, the temporal changes of concentrations reected
variable extents of dilution of volcanic gases in the background
atmosphere upon plume dispersal (due to changes in plume travelling
speed and direction, or changes in source strength).
From the raw plume concentration data (in ppm), the volcanic gas
plume H
2
O/SO
2
and H
2
O/CO
2
ratios were derived by calculating the
gradients of the best-t regression lines in H
2
O vs. SO
2
and H
2
O vs. CO
2
scatter plots (Fig. 3), as previously reported for Etna (Shinohara et al.,
2008). Then, the (air-free) composition of volcanic gases (in mol%;
Table 1) was nally calculated by combing together each suit of gas
concentration ratios, and normalizing to 100%. This assumes that
contribut ions from undetect ed species (e.g., H
2
,H
2
S, HCl) are
relatively minor.
While plume ratios were generally relatively constant within each
measurement cycle (e.g., R
2
of best-t regression lines were normally
N 0.7, and standard deviations of the derived ratios 25%), brief but
signicant variations of the ratios were sometimes observed (Fig. 4).
Visual observations and cross correlations of our dataset with seismic
and thermal signals (available at http://www.ct.ingv.it) indicated that
such short-term variations (ge nerally lasting less than 2 min)
systematically occurred soon after individual Strombolian bursts.
We therefore suggest they reect our measurements capturing of the
composition of the syn-explosive gas phase (e.g., the gas jet released
during the short-lived Strombolian explosions). 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 homo-
genised) within the bulk plume. Our data support further the earlier
conclusions of Burton et al. (2007b), demonstrating that the syn-
explosive gas phase is signicantly richer in CO
2
(and poorer in H
2
O
and SO
2
) than the bulk plume (Fig. 4 and Table 1). The latter is mainly
contributed by (quiescent) passive degassing in between the explo-
sions, and by pufng activity at the open vents (Ripepe et al., 2008).
4.2. The H
2
OCO
2
SO
2
composition of Stromboli's plume
Ignoring minor components, Stromboli 's gas composition is
dominated by H
2
O (4898 mol%; mean, 80%), CO
2
(250 mol%;
mean, 17%) and SO
2
(0.214 mol%; mean, 3%) (Table 1). 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 CO
2
-enrichment (most volcanic gases from
arc basaltic volcanoes have N 90% H
2
O; Shinohara, 2008).
Stromboli's syn-explosive gas phase is richer in CO
2
(1150%;
mean 26%) and poorer in H
2
O (4888%; mean, 73%) than the bulk
plume passively released by the volcano's open vents in between the
explosions (mean CO
2
and H
2
O, 15 and 82%, respectively) (Table 1).
Our measurements of the syn-explosive gas phase are in qualitative
agreement with previous determinations (CO
2
1933%; H
2
O6479%;
Burton et al., 2007b), and thus conrm further the bimodal nature of
the emission chemistry at Stromboli.
The most striking feature of the dataset is the large spread of
plume compositions observed in only 6 months of observations. This
is clearly shown in Fig. 5, where H
2
O/CO
2
and CO
2
/SO
2
ratios show a
distinct antithetic behaviour: the syn-explosive gas phase is charac-
terised by high CO
2
/SO
2
ratios (N 10, and as high as 47) and low H
2
O/
CO
2
ratios (b 6, but typically between 1 and 3); while the bulk plume is
Fig. 3. Scatter diagram of H
2
O vs. SO
2
and H
2
O vs. CO
2
concentrations acquired during 1
measurement cycle. The H
2
O/SO
2
and H
2
O/CO
2
plume ratios are calculated from the
gradient of the best-t regression lines.
Fig. 4. High-resolution (9 s) record of (a) plume ratios and (b) CO
2
concentrations,
showing the contrasting compositions of the passive and syn-explosive gas plume
emissions. In the most favourable conditions (strong winds blowing from the N), a
Strombolian explosion (grey arrow labelled EXP) is followed (with a time-lag of a few
seconds) by a brief ( lasting a fe w minutes ) but signi can t increase of CO
2
concentrations and CO
2
/SO
2
ratios detected by the MultiGAS. The syn-explosive gas
phase is typically H
2
O-poorer (and CO
2
-richer) than the passive plume released in
between explosions (this contribution by far dominating Stromboli's bulk plume
emissions in the long-term).
Fig. 5. In a H
2
O/CO
2
vs. CO
2
/SO
2
scatter plot, Stromboli's plume gas emissions are shown
to range from CO
2
-rich to H
2
O-rich. The syn-explosive (black circles) and quiescent
(open circles) plumes have distinct compositions, with some overlap. Grey circles are
FTIR-sensed gas compositions for Strombolian explosions (Burton et al., 2007b). Curves
labelled Mixing lines are calculated as described in the caption of Fig. 8, and in
Section 5.2.
198 A. Aiuppa et al. / Earth and Planetary Science Letters 295 (2010) 195204

generally characterised by lower (b 15) CO
2
/SO
2
ratios and higher
H
2
O/CO
2
ratios (1.565). Note that virtually all H
2
O-rich (H
2
O/CO
2
ratios N 30) bulk plume compositions have low (b 6) CO
2
/SO
2
ratios.
The same diagram highlights however that syn-explosive and bulk
plume compositions are somewhat overlapping, and that the bulk
plume can be substantially CO
2
-richer (but also more H
2
O rich) than
previously measured (representative CO
2
/SO
2
and H
2
O/CO
2
ratios of
8 and 6 were previously quoted for the bulk plume, respectively;
Burton et al., 2007b). The results are in agreement with the large
variation of the bulk plume CO
2
/SO
2
ratio (range, 0.926) observed in
a 19 months period encompassing the recent FebruaryApril 2007
effusive eruption of Stromboli (Aiuppa et al., 2009); including the
detection of an exceptionally CO
2
-rich plume (CO
2
/SO
2
up to 26)
before the onset of the eruption, and prior to the paroxysm on March
15, 2007.
5. Discussion
The striking range of volcanic gas compositions at Stromboli
suggest dynamic magma degassing processes at this open-vent
volcano. Indeed, whilst some persistently degassing volcanoes display
an appare nt stability in b oth 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 time-
changing nature of both volcanic activity state and volcanic gas
composition.
Remarkable short-period (seconds) variations in volcanic gas
compositions at Stromboli were rst documented based on high-
frequency FTIR measurements (Burton et al., 2007b); these demon-
strated that the volcanic gas phase released during the short-lived
Strombolian explosions are richer in CO
2
(and poorer in Cl) than the
bulk (quiescent) plume. Since CO
2
is signicantly less soluble in
basaltic melts than H
2
O, S, and Cl, and thus deeply exsolved, it was
concluded that the gas slugs feeding Strombolian explosions have a
relatively deep provenance (0.82.7 km below the summit vents).
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). 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). Visual observations suggest that the bulk
Stromboli's plume is essentially c ontributed by both quiescent
(passive) 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). It follows then
that the most obvious source for the bulk gas emissions would be
degassing of volatiles dissolved in the magma lling the upper
conduits, and ultimately the high porphyricity (HP) magma ponding
at the op en vents, and erupted as scoriae during Strombolia n
explosions. However, the variable composition of the bulk plume,
and its recurrent CO
2
-rich signature (see Fig. 5) are not consistent
with this hypothesis: the HP magma is volatile-poor (see Sections 5.1
and 5.2 below), and its degassing upon decompression (followed by
near-surface gas separation) cannot produce a gas phase with a CO
2
/
SO
2
ratio greater than 0.51 (see Section 5.3 below), which is
substantially lower than observed (Fig. 5).
In order to model the source processes controlling the chemistry of
Stromboli's volcanic gases, we combine in the sections below the
record of pre-eruptive volatile contents in Stromboli's magma, as
derived from MI analysis (Section 5.1), with the results of numerical
simulations carried out using the Moretti and Papale (2004)
saturation model (Section 5.2). These calculations allow quantitative
reproduction of the evolving composition of the gas phase released by
Stromboli's magmas upon their storage and ascent within the crust.
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 emission of
nearly aphyric highly vesicular pumice during paroxysmal eruptions
highlights the existence of a low porphyritic (LP), volatile-rich HK
basalt magma residing in the deep volcano plumbing system (Fig. 6).
Dissolved CO
2
and H
2
O contents (0.150.2 and 2.53.5 wt.%, respec-
tively; Fig. 7a) in olivine-hosted basaltic melt inclusions (MIs) were
used (Métrich et al., 2010) to show that the LP magma is stored in a 7
10 km deep reservoir (equivalent to 190260 MPa pressure) (all
depths are below the summit vents, bsv). The LP magma is thought to
coexist with a substantial (2 wt.%) fraction of CO
2
-rich gas bubbles
at reservoir conditions (Burton et al., 2007a,b). Observations on
erupted pumices strongly suggest that, prior to a paroxysm, the LP
magma is rapidly decompressed, maintaining virtually unchanged his
deep petrological (Métrich et al., 2010) and textural (Polacci et al.,
2009) properties.
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). However, since the LP magma is only erupted during
high energy explosive activity, while a volatile-poor shoshonitic
basalt (the HP magma) feeds the normal Strombolian activity, a
mechanism leading to LP to HP magma transition must normally
take place somewhere in the plumbing system. According to melt
inclusion record (Métrich et al., 2010), ascending LP magmas
undertake an extensive water loss in the 24 km bsv depth range
(equivalent to 5 0100 MPa pressure), with H
2
Odecreasingto
b 1.5 wt.%. This has three main implications and consequences:
(i) rst, de-hydration of a magma can be caused by uxing with
deep-rising CO
2
-rich gas (Spilliaert et al., 2006), a fact which is
suggestive of the presence of a magma ponding zone at 24km
bsv, where CO
2
-rich gas bubbles accumulate to contents N 5 wt.
%(Métrich et al., 2010). An intermediate magma ponding zone
at Stromboli is also supported by geodetic data (Bonaccorso et
al., 2008);
(ii) secondly, for the magma to become extensively de-hydrated, it
is required that gas bubbles escape from this ponding zone, a
fact which might be favoured by the presence of a geological
discontinuity (the interface between volcanic rocks and the
basement lies at about 2.43.5 km depth; Di Roberto et al.,
2008), and/or promoted by vesicularity of the magma reaching
a critical threshold for gas percolation (and permeable gas
ow) (Burton et al., 2007a). Whatever the cause, magma de-
hydrated thus implies gas-melt separation (and thus transition
to open-system degassing regime) at 24 km bsv depth;
(iii) nally, de-hydration of the stored magmas raises their liquidus
temperatures, hence promoting extensive cr ystallization
(Métrich et al., 2001, 2010; Di Carlo et al., 2006), and ultimately
leading to transition from the LP to the H
2
O poor (b 1.5 wt.%;
Fig. 7a) and crystal-rich (3050%) HP magma (Fig. 6). The
highest dissolved volatiles contents in MIs from the plagio-
clase-bearing HP magma (Fig. 7a) indicate entrapment pres-
sures of 50 100 MPa pressure (Métrich et al., 2010),
conrming that a change f rom closed- to open-system
degassing regime (with the consequent water de pletion
being the trigger for transition from LP to HP magma) occurs
in the 24 km bsv depth range.
199A. Aiuppa et al. / Earth and Planetary Science Letters 295 (2010) 195204

Figures (10)
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Abstract: Over long periods of time (~Ma), we may consider the oceans, atmosphere and biosphere as a single exospheric reservoir for CO2. The geological carbon cycle describes the inputs to this exosphere from mantle degassing, metamorphism of subducted carbonates and outputs from weathering of aluminosilicate rocks (Walker et al. 1981). A feedback mechanism relates the weathering rate with the amount of CO2 in the atmosphere via the greenhouse effect (e.g., Wang et al. 1976). An increase in atmospheric CO2 concentrations induces higher temperatures, leading to higher rates of weathering, which draw down atmospheric CO2 concentrations (Berner 1991). Atmospheric CO2 concentrations are therefore stabilized over long timescales by this feedback mechanism (Zeebe and Caldeira 2008). This process may have played a role (Feulner et al. 2012) in stabilizing temperatures on Earth while solar radiation steadily increased due to stellar evolution (Bahcall et al. 2001). In this context the role of CO2 degassing from the Earth is clearly fundamental to the stability of the climate, and therefore to life on Earth. Notwithstanding this importance, the flux of CO2 from the Earth is poorly constrained. The uncertainty in our knowledge of this critical input into the geological carbon cycle led Berner and Lagasa (1989) to state that it is the most vexing problem facing us in understanding that cycle. Notwithstanding the uncertainties in our understanding of CO2 degassing from Earth, it is clear that these natural emissions were recently dwarfed by anthropogenic emissions, which have rapidly increased since industrialization began on a large scale in the 18th century, leading to a rapid increase in atmospheric CO2 concentrations. While atmospheric CO2 concentrations have varied between 190–280 ppm for the last 400,000 years (Zeebe and Caldeira 2008), human activity has produced a remarkable increase …

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  • ...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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Abstract: Despite its relatively minor abundance in magmas (compared with H2O and CO2), sulfur degassing from volcanoes is of tremendous significance. It can exert substantial influence on magmatic evolution (potentially capable of triggering eruptions); represents one of the most convenient opportunities for volcano monitoring and hazard assessment; and can result in major impacts on the atmosphere, climate and terrestrial ecosystems at a range of spatial and temporal scales. The complex behavior of sulfur in magmas owes much to its multiple valence states (−II, 0, IV, VI), speciation (e.g., S2, H2S, SO2, OCS and SO3 in the gas phase; S2−, SO42− and SO32− in the melt; and non-volatile solid phases such as pyrrhotite and anhydrite), and variation in stable isotopic composition (32S, 33S, 34S and 36S; e.g., Metrich and Mandeville 2010). Sulfur chemistry in the atmosphere is similarly rich involving gaseous and condensed phases and invoking complex homogeneous and heterogeneous chemical reactions. Sulfur degassing from volcanoes and geothermal areas is also important since a variety of microorganisms thrive based on the redox chemistry of sulfur: by reducing sulfur, thiosulfate, sulfite and sulfate to H2S, or oxidizing sulfur and H2S to sulfate (e.g., Takano et al. 1997; Amend and Shock 2001; Shock et al. 2010). Understanding volcanic sulfur degassing thus provides vital insights into magmatic, volcanic and hydrothermal processes; the impacts of volcanism on the Earth system; and biogeochemical cycles. Here, we review the causes of variability in sulfur abundance and speciation in different geodynamic contexts; the measurement of sulfur emissions from volcanoes; links between subsurface processes and surface observations; sulfur chemistry in volcanic plumes; and the consequences of sulfur degassing for climate and the environment. ### Geodynamics and the geochemical behavior of sulfur The …

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  • ...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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Journal ArticleDOI
Abstract: Magma degassing processes are commonly elucidated by studies of melt inclusions in erupted phenocrysts and measurements of gas discharge at volcanic vents, allied to experimentally constrained models of volatile solubility. Here we develop an alternative experimental approach aimed at directly simulating decompression-driven, closed-system degassing of basaltic magma in equilibrium with an H^C^O^S^Cl fluid under oxidized conditions (fO2 of 1·0^2· 4l og units above the Ni^NiO buffer). Synthetic experimental starting materials were based on basaltic magmas erupted at the persistently degassing volcanoes of Stromboli (Italy) and Masaya (Nicaragua) with an initial volatile inventory matched to the most undegassed melt inclusions from each volcano. Experiments were run at 25^400 MPa under super-liquidus conditions (11508C). Run product glasses and starting materials were analysed by electron microprobe, secondary ion mass spectrometry, Fourier transform infrared spectroscopy, Karl-Fischer titration, Fe 2þ /Fe 3þ colorimetry and CS analyser. The composition of the exsolved vapour in each run was determined by mass balance. Our results show that H2O/ CO2 ratios increase systematically with decreasing pressure, whereas CO2/S ratios attain a maximum at pressures of 100^300 MPa. S is preferentially released over Cl at low pressures, leading to a sharp increase in vapour S/Cl ratios and a sharp drop in the S/Cl ratios of glasses. This accords with published measurements of volatile concentrations in melt inclusion and groundmass glasses at Stromboli (and Etna). Experiments with different S abundances show that the H2O and CO2 contents of the melt at fluid saturation are not affected. The CO2 solubility in experiments using both sets of starting materials is well matched to calculated solubilities using published models. Models consistently overestimate H2O solubilities for the Stromboli-like composition, leading to calculated vapour compositions that are more CO2-rich and calculated degassing trajectories that are more strongly curved than observed in experiments. The difference is less acute for the Masaya-like composition, emphasizing the important compositional dependence of solubility and melt^ vapour partitioning. Our novel experimental method can be readily extended to other bulk compositions.

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  • ...…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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Journal ArticleDOI
Abstract: Continental intraplate volcanoes, such as Erebus volcano, Antarctica, are associated with extensional tectonics, mantle upwelling and high heat flow. Typically, erupted magmas are alkaline and rich in volatiles (especially CO2), inherited from low degrees of partial melting of mantle sources. We examine the degassing of the magmatic system at Erebus volcano using melt inclusion data and high temporal resolution open-path Fourier transform infrared (FTIR) spectroscopic measurements of gas emissions from the active lava lake. Remarkably different gas signatures are associated with passive and explosive gas emissions, representative of volatile contents and redox conditions that reveal contrasting shallow and deep degassing sources. We show that this unexpected degassing signature provides a unique probe for magma differentiation and transfer of CO2-rich oxidised fluids from the mantle to the surface, and evaluate how these processes operate in time and space. Extensive crystallisation driven by CO2 fluxing is responsible for isobaric fractionation of parental basanite magmas close to their source depth. Magma deeper than 4 kbar equilibrates under vapour-buffered conditions. At shallower depths, CO2-rich fluids accumulate and are then released either via convection-driven, open-system gas loss or as closed-system slugs that ascend and result in Strombolian eruptions in the lava lake. The open-system gases have a reduced state (below the QFM buffer) whereas the closed-system gases preserve their deep oxidised signatures (close to the NNO buffer).

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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…...

    [...]


Journal ArticleDOI
Abstract: Some 300–600 Tg of volatiles are globally vented each year by arc volcanism. Such arc gas emissions have contributed to past and present-day evolution of the Earth atmosphere and climate by recycling mineral-bound volatiles subducted along active slabs. Carbon dioxide (CO 2 ) and total sulphur (S T ) are, after water, the major components of volcanic arc gases. Understanding their relative abundances (e.g., the CO 2 /S T ratio) in arc volcanic gases is important to constrain origin and recycling efficiency of these volatiles along the subduction factory, and to better constrain the global arc volcanic CO 2 flux. Here, we review currently available information on global variations of volcanic arc CO 2 /S T gas ratios. We analyse a dataset of > 2000 published volcanic arc gas measurements that comprise (i) low-temperature hydrothermal gas emissions, in which S T is dominated by hydrothermal hydrogen sulphide (H 2 S), and (ii) high temperature “magmatic” gases rich in sulphur dioxide (SO 2 ). We show that the global CO 2 /S T population of hydrothermal gases is mainly controlled by S loss to hydrothermal fluids/rocks. We then select a subset of high-temperature (≥ 450 °C) arc gases which, being less affected by S hydrothermal loss, can be used to infer the “deep” source of volatiles. Using a subset of time-averaged high-T gas compositions for 56 arc volcanoes, we identify sizeable along-arc and inter-arc variations in the “magmatic” arc gas CO 2 /S T ratio, which we ascribe to distinct volatile origins in the magma generation/storage zone. In the attempt to resolve the slab vs. crustal contributions to arc gas budgets, we explore the global association between volcanic gas CO 2 /S T ratios and non-volatile (trace elements) tracers in arc magmas. For the first time in a global study, we find evidence for higher carbon output (CO 2 /S T ) in arcs where carbonate sediment subducts on the seafloor. Indeed, most arc volcanoes exhibit gas vs. trace element relationships that are explained by addition of slab-sediment melts ± fluids to the mantle wedge. We also identify a subset of CO 2 -rich arc volcanoes with unusually high CO 2 /S T ratios (Etna, Stromboli, Vulcano Island, Popocatepetl, Soufriere of St Vincent, Bromo and Merapi), which we interpret as the product of magma-limestone interactions in the upper crust. Evidence for this process comes from carbonate xenoliths and/or carbonate basement that characterise these volcanic systems. Although the mean global CO 2 /S T ratio of arc gas (~ 2.5) reflects a predominant source from subducted sediment, limestone-assimilation-derived C may account for a substantial (~ 19–32%) fraction of the present-day global arc budget, and may have contributed to elevated atmospheric CO 2 levels and warmer climate in Earth's past. Our global CO 2 /S T vs. trace element association paves the way to identifying the gas signature of volcanoes (or arc segments) for which gas information is currently missing, and so improve our current global volcanic arc CO 2 flux inventory.

86 citations


References
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Journal ArticleDOI
Abstract: The volatile saturation surface in H 2 O–CO 2 –silicate melt systems is modeled by applying thermodynamic equilibrium between gaseous and liquid volatile components. The whole database of existing saturation data in the C–O–H–silicate liquid systems has allowed us to re-calibrate a previously developed fully multicomponent H 2 O–CO 2 saturation model [Papale, P., 1999. Modeling of the solubility of a two-component H 2 O + CO 2 fluid in silicate liquid. Am. Mineral., 84, 477–492]. The new database nearly doubles the previous one, greatly improving the performances of the whole model, which now adopts a significantly lower number of model parameters with respect to the previous calibration. The multicomponent H 2 O + CO 2 saturation model is fully non-ideal, the only assumption being that the excess Gibbs free energy of the silicate mixture can be represented by an expansion of first-order symmetric interaction terms. No a-priori assumption is made on the P – T dependence of the volatile–oxide interaction terms, meaning that no assumption is made on the partial molar volume and enthalpy of the dissolved volatiles. The whole treatment is evaluated by restrictive statistical algorithms, which confirm the model validity on an extended database. The model allows to investigate extensively the dependence of the complex volatile saturation surface on composition. In order to explore the non-linear behaviors implicit in the physics of the dissolution process, the model is employed in a series of calculations aimed at illustrating some of the compositional features of the volatile saturation surface in both one-component and two-component volatile conditions. The results show compositional-dependent minima and maxima, some of which are known from the experiments. Non-ideal behavior is enhanced in two-component fluid phase conditions and pressures above a few hundreds MPa, where calculated isobaric H 2 O–CO 2 saturation curves reveal the possible existence of a maximum in CO 2 saturation at non-zero H 2 O contents. Due to the compositional dependence of the volatile saturation surface, it is outlined the important role played by redox conditions, especially in iron-rich melt systems like basalts.

432 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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Journal ArticleDOI
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 …

302 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)....

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Journal ArticleDOI
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.

280 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…...

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  • ...(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....

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Journal ArticleDOI
13 Jul 2007-Science
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.

271 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)....

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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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Journal ArticleDOI
Abstract: Explosive volcanic eruptions constitute a major class of natural hazard with potentially profound economic and societal consequences. Although such eruptions cannot be prevented and only rarely may be anticipated with any degree of accuracy, better understanding of how explosive volcanoes work will lead to improved volcano monitoring and disaster mitigation. A major goal of modern volcanology is linking of surface-monitored signals from active volcanoes, such as seismicity, ground deformation and gas chemistry, to the subterranean processes that generate them. Because sub-volcanic systems cannot be accessed directly, most of what we know about these systems comes from studies of erupted products. Such studies shed light on what happens underground prior to and during eruptions, thereby providing an interpretative framework for post hoc evaluation of monitoring data. The aim of this review is to present some of the current petrological techniques that can be used for studying eruptive products and for constraining key magmatic variables such as pressure, temperature, and volatile content. We first review analytical techniques, paying particular attention to pitfalls and strategies for analyzing volcanic samples. We then examine commonly used geothermometry schemes, evaluating each by comparison with experimental data not used in the original geothermometer calibrations. As there are few mineral-based geobarometers applicable to magma storage regions, we review other methods used to determine pre-eruptive magma equilibration pressures. We then demonstrate how petrologically-constrained parameters can be compared to the contemporaneous monitoring record. These examples are drawn largely from Mount St. Helens volcano, for which there are abundant petrological and monitoring data. However, we emphasize that our approaches can be applied to any number of active volcanoes worldwide. Finally, we illustrate the application of these techniques to two different types of magmatic systems—large silicic magma chambers and small intermediate-composition magma storage regions—with particular focus on the combined evolution of melt …

265 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)....

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