JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 9
PAGES 1737^1762
2011
doi:10.1093/petrology/egr027
Experimental Simulation of Closed-System
Degassing in the System Basalt^H2O^CO2^S^Cl
PRISCILLE LESNE1*, SIMON C. KOHN1, JON BLUNDY1,
FRED WITHAM1, ROMAN E. BOTCHARNIKOV2 AND
HARALD BEHRENS2
1
SCHOOL OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING, QUEENS ROAD,
BRISTOL BS8 1RJ, UK
2
INSTITUT FU«R MINERALOGIE, LEIBNIZ UNIVERSITA«T HANNOVER, CALLINSTRASSE 3,
D-30167 HANNOVER, GERMANY
RECEIVED JULY 7, 2010; ACCEPTED MAY 10, 2011
ADVANCE ACCESS PUBLICATION AUGUST 4, 2011
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 1·0^2·4 log
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, Fe2þ/Fe3þ 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
Arc magmas are characterized by high concentrations of
the volatile species H2O, CO2, SO2, H2S and HCl (e.g.
Symonds et al., 1994). To understand the role played by
these volatiles in volcanic processes, such as magma degassing and eruption, their solubilities in silicate melts and
their partitioning between coexisting melt and vapour
phases need to be known. Of particular importance is a
better understanding of how the concentrations and ratios
of H2O, CO2, sulphur species and chlorine dissolved in
magmas and volcanic gases can be used to both reconstruct and forecast the evolution of magmatic systems.
*Corresponding author. E-mail: [email protected]
ß The Author 2011. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
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.
KEY WORDS:
experiments; solubility; degassing; basalt; Stromboli;
Masaya
I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 52
Previous experimental studies on volatile contents in
vapour-saturated basaltic melts and vapour/melt partitioning have focused on the solubilities of single volatile species
or binary mixtures such as H2O^CO2 (e.g. Dixon et al.,
1995; Dixon, 1997; Papale, 1999; Newman & Lowenstern,
2002; Behrens et al., 2004; Botcharnikov et al., 2005a, 2006;
Papale et al., 2006; Behrens et al., 2009; Shishkina et al.,
2010; Lesne et al., 2011a, 2011b), H2O^S (Carroll &
Rutherford, 1985, 1988; Luhr, 1990; Carroll & Webster,
1994; Mavrogenes & O’Neill, 1999; Clemente et al., 2004;
Moune et al., 2009) and H2O^Cl (e.g. Webster et al., 1999,
2009; Signorelli & Carroll, 2000; Stelling et al., 2008).
Recently, experimental studies have been extended to silicate melts containing C^O^H^Cl (Botcharnikov et al.,
2007; Alletti et al., 2009) and H^O^S^Cl species
(Botcharnikov et al., 2004; Webster et al., 2009). The behaviour of H2O and CO2 in basaltic systems has been well studied (e.g. Dixon et al., 1995; Dixon, 1997; Papale, 1999;
Newman & Lowenstern, 2002; Behrens et al., 2004;
Botcharnikov et al., 2005a, 2006; Papale et al., 2006;
Behrens et al., 2009; Shishkina et al., 2010; Lesne et al.,
2011a, 2011b). All these studies have shown that CO2
degasses predominantly from the basaltic melt at high
pressures, and H2O starts to degas to any significant
degree only at lower pressures. The addition of S to H2Oand Cl-bearing fluid-saturated melts tends to decrease the
Cl concentrations in the melt (Botcharnikov et al., 2004;
Webster et al., 2009), with corresponding increases in the
Cl content of the fluids; that is, it tends to increase the
fluid/melt partition coefficient of Cl (Dfl=melt
).
Cl
The motivation of this study is to provide a dataset
for volatiles dissolved in the melt and coexisting fluid
phase as a function of pressure for a fixed bulk composition, providing for the first time experimental data
for the case of closed-system, equilibrium degassing in
the system basalt^H2O^CO2^S^Cl. We focused on two
synthetic basaltic compositions based on tephra erupted
from two active, passively degassing, subductionrelated volcanoes: Stromboli (Italy) and Masaya
(Nicaragua). Our experimental results are compared with
natural melt inclusions and gas emissions from both volcanoes to provide a link between high-P evidence from melt
inclusions (MI) and the geochemistry of surface gas
discharges.
Stromboli is the northernmost volcano of the Aeolian
archipelago, renowned for its persistent activity (Rosi
et al., 2000), erupting shoshonitic to high-K basalts
(Francalanci et al., 2004). It is an exceptionally well-studied
volcano for which there exists an extensive database of
whole-rock and mineral analyses as well as volatile element
analyses of olivine-hosted melt inclusions (e.g. Me¤trich
et al., 2001; Bertagnini et al., 2003; Francalanci et al., 2004)
and gas emission data (Allard et al., 1994; Burton et al.,
2007a, 2007b; Aiuppa et al., 2009, 2010).
NUMBER 9
SEPTEMBER 2011
Masaya, on the Central American volcanic front
(Burton et al., 2000) is one of the few basaltic volcanoes
known for its Plinian activity (Williams, 1983). Gas emission crises have recurred in the historical period at
25 year intervals (Stoiber et al., 1986). The geochemical
and mineralogical database for Masaya is much less complete than that for Stromboli. However, a few papers (e.g.
Sadofsky et al., 2008) give the composition of bulk-rocks,
and the gas composition measured on Masaya through different activity periods (e.g. Duffell et al., 2003). Melt inclusion data are available from Horrocks (2001) and Atlas &
Dixon (2006). The passive, but persistent, degassing activity of these two volcanoes provides an excellent example
of a volcanic system where magmatic fluids are thought to
have equilibrated with silicate melts and rocks in the
magma chamber and conduit. Thus, the concentrations of
volatiles measured in melt inclusions and volcanic gases
from these volcanoes can provide quantitative reference
data that can be directly interpreted by experimental
simulations of equilibrated systems.
E X P E R I M E N TA L T E C H N I Q U E S
Two synthetic basalt compositions were chosen for the
experiments. The first (St8.1) is based on a crystal-poor,
high-K basaltic ‘golden pumice’ from Stromboli [sample
St8.1 of Bertagnini et al. (2003)], analogous to pumices
emitted during both paroxysms and major explosions. The
second (MAS.1) is a based on a basaltic lapilli sample
from Masaya [sample P2-47 of Sadofsky et al. (2008)]. The
two starting compositions are considered to be broadly
representative of the parental magma at depth beneath
each volcano.
The starting material consisted of a mechanical mixture
of synthetic oxides (SiO2, TiO2, Al2O3, Fe2O3, MgO) and
carbonates (CaCO3, K2CO3, Na2CO3). Oxides and carbonates were stored in a drying oven at 1208C prior to
mixing in appropriate proportions, and homogenized by
grinding in an agate mortar. The mixture was decarbonated at 700^10008C for 6 h in alumina crucibles. Pressed
pellets of the starting materials were reduced in a
gas-mixing furnace, at 10008C for 2 h at an fO2 close to
the intrinsic fO2 of the internally heated pressure vessel
(IHPV) of Leibniz University of Hannover. This step is designed to minimize gradients in hydrogen fugacity (fH2)
during the experiments and consequent H2O loss or gain
by redox reactions of iron-bearing components.
Structurally bound volatiles were added to the oxide
mixture after the decarbonation step as follows: H2O was
added as brucite [Mg(OH)2], CO2 as CaCO3, sulphur as
gypsum (CaSO4.2H2O) and chlorine as NaCl. Volatile
addition was performed in such a way as to achieve the
desired volatile-free bulk composition. Initial volatile contents in both mixtures are representative of the highest
volatile contents measured in melt inclusions from basalts
1738
LESNE et al.
CLOSED-SYSTEM DEGASSING
Table 1: Experimental starting compositions (in wt %, except for CO2, S and Cl, expressed in ppm)
Stromboli
Masaya
St8.1.A
Start. mat.
St8.1.B
Av. glass
s
Start. mat.
n¼9
MAS.1.A
Av. glass
s
Start. mat.
n¼9
MAS.1.B
Av. glass
s
Start. mat.
n ¼ 11
Av. glass
s
n¼8
SiO2
50·02
51·64
0·31
50·07
51·86
0·28
49·39
50·86
0·30
49·42
50·84
TiO2
0·86
0·81
0·05
0·86
0·84
0·05
1·24
1·17
0·06
1·24
1·18
0·06
Al2O3
19·06
18·57
0·15
19·13
18·63
0·16
19·16
18·91
0·28
19·18
18·76
0·20
Fe2O3
8·37
8·15
0·12
8·37
8·12
0·14
13·26
12·42
0·17
13·27
12·44
0·19
MgO
6·70
6·45
0·11
6·24
6·02
0·12
3·58
3·41
0·08
3·34
3·17
0·07
CaO
10·50
10·90
0·12
10·85
11·11
0·12
8·53
9·23
0·09
8·70
9·41
0·11
Na2O
2·71
2·05
0·07
2·67
1·98
0·09
3·31
2·77
0·09
3·34
2·90
0·11
K2O
1·80
1·44
0·08
1·80
1·44
0·05
1·51
1·23
0·04
1·51
1·29
0·05
Fe2þ/Fetot1
0·022
H2O2
2·70
0·022
0·025
2·92
1·54
0·025
1·66
ppm CO23
4600
4890
ppm S3
1930
3560
595
1400
ppm Cl4
2040
1570
1700
1420
7410
0·24
6850
All analyses are normalized to 100%. Start. mat., composition of starting material; major elements from mass of reagents,
volatiles as described in footnotes 1–4. Av. glass, average composition of experimental run product glasses. s, standard
deviation. n, number of analyses averaged.
1
Fe2þ/Fetot measured by colorimetric wet-chemistry on initial powder using the technique of Schuessler et al. (2008).
2
Initial H2O determined by KFT on initial powder.
3
Determined by CS analyzer on initial powder.
4
Determined by EMPA on fused glass in piston-cylinder apparatus.
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). 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. These mixes are
labelled A (low-sulphur) and B (high-sulphur) and contain 1930 and 3560 ppm for St8.1 and 590 and 1400 ppm
for MAS.1, respectively. The sulphur and carbon contents
of the starting materials were measured using an ELTRA
CS 800 analyzer at Leibniz University, Hannover; their
H2O contents were measured by Karl-Fischer titration
(see below). Initial chlorine contents were determined by
melting the starting materials in a g inch piston-cylinder
at the University of Bristol at 1.2 GPa, 13758C for 1·5 h,
with subsequent measurements by electron microprobe
analysis (EMPA) on the resultant glasses. These conditions
were chosen to ensure that the entire volatile budget remained in solution. All the above measurements of volatile
concentrations are preferred to the gravimetric estimates
for the starting proportions of reagents, although agreement is generally good. Major elements and initial volatiles
compositions of all starting material are reported in
Table 1.
Between 30 and 50 mg of the starting material were
loaded into Au80Pd20 capsules of 15 mm length, 2·5 mm
inner diameter and 2·9 mm outer diameter, and welded
shut. Prior to loading, the capsules were annealed, cleaned
in HCl in a heated bath for 1h, rinsed with distilled
water, and then cleaned again in heated distilled H2O for
1h. The capsules were weighed before and after welding
and then placed for 2 h in an oven at 1208C to check for
leakage.
All equilibrium experiments were performed at 11508C;
that is, at a temperature corresponding to super-liquidus
conditions in the investigated systems from 25 to
400 MPa, a pressure range consistent with the entrapment
pressures for melt inclusions from Stromboli (Me¤trich
et al., 2001; Bertagnini et al., 2003; Di Carlo et al., 2006;
Pichavant et al., 2009). Experiments were performed in a
vertically run IHPV at Leibniz University, Hannover,
using pure argon as the pressure medium. Four to six capsules were run simultaneously, hanging from a Pt wire
1739
JOURNAL OF PETROLOGY
VOLUME 52
Table 2: Experimental run conditions
Experiment no.
P (MPa)
T (8C)
Time (h)
1
200
1150
2
100
1150
6
3
250
1150
12
4
300
1150
12
5
150
1150
6
6
200
1150
12
7
50
1150
6
8
50
1150
6
9
25
1150
15
10
400
1150
20
6
All four starting materials were run at each condition,
hence St8.1.A10 denotes starting composition St8.1.A
run in experiment 10.
near the top of a double-wound molybdenum wire furnace.
Temperature was measured by four S-type thermocouples,
placed along the 3 cm hot zone. The temperature difference between the top and bottom of the capsules was
always less than 108C. Pressure was monitored by a digital
pressure transducer with an uncertainty of about 1MPa.
The variation of pressure during the experiments was
5 MPa. Experiments lasted between 6 and 12 h, and
were ended by drop-quenching, with a cooling rate of
108C s1 (Berndt et al., 2002). These run times were
chosen to ensure attainment of equilibrium in the system,
but minimize possible S and Fe loss to the capsule walls.
After quenching, the capsules were weighed to verify that
they had remained sealed during the experiment. The
vapour-saturated conditions of the experimental charges
were confirmed by the presence of water hissing out from
the capsules during opening, together with the wet appearance of the recovered glass chips. Quenched glasses were
mounted in epoxy resin and polished with diamond paste
for subsequent electron microprobe analysis. The pressure,
temperature and duration of all experiments performed in
this study are reported in Table 2.
A N A LY T I C A L T E C H N I Q U E S
Scanning electron microscope (SEM)
All charges were examined using a Hitachi S-3500N SEM
at the University of Bristol in back-scattered electron
mode to check for the occurrence of quench crystallization
or bubble formation.
Electron microprobe analysis (EMPA)
Experimental run products were analyzed by EMPA at
Bristol University using a Cameca SX100 electron microprobe. Major elements were analysed separately from
NUMBER 9
SEPTEMBER 2011
sulphur and chlorine. Analytical conditions applied were
15 kV accelerating voltage, 6 nA sample current, beam
diameter of 10 mm and peak counting time of 10 s. For Cl,
an acceleration voltage of 15 kV, a 10 nA beam current, a
beam diameter of 10 mm and counting time on peak of
60 s were applied. NaCl was used as a standard for chlorine
and pyrite was used as a standard for sulphur. To minimize
alkali migration during the analysis of hydrous, alkali-rich
glasses, the analytical conditions were 15 kV, 4 nA and 5 s
total counting time for Na and K, which were analyzed
first. A ZAF correction procedure was applied. Major
element calibration utilized wollastonite (Ca), hematite
(Fe), albite (Na, Si), corundum (Al), olivine (Mg) and
orthoclase (K) standards. Multiple measurements were
made for each sample to check for homogeneity.
Some samples were also analysed for their sulphur oxidation state. Measurements of l(SKa) were performed on
two spectrometers of the Cameca SX-100 microprobe at
the University of Bristol following the method reported by
Carroll & Rutherford (1988), Wallace & Carmichael
(1994), Me¤trich & Clocchiatti (1996) and Jugo et al. (2005).
PET crystals (2d ¼ 8·742 —) were used on each spectrometer to simultaneously obtain two independent measurements of the SKa wavelength shift. Standards used were
FeS (sulphide, S2) and BaSO4 (sulphate, S6þ). EMPA
operating conditions were 20 kV and 25 nA with a spot
diameter of 15 mm. Each spectrometer was moved 0·00004
sin units for 100 steps over the range of 0·61198^0·61594
sin during a single spot analysis. For each step, counting
time varied between 100 and 1600 ms, which results in a
maximum beam exposure time of 10^160 s. Sixteen wavescan spectra were collected for counting times of 100 ms
for each step, eight spectra for 200 ms, four for 400 ms,
two for 800 ms and one for 1600 ms. Each summed spectrum was fitted with a Gaussian function to obtain the
peak position. The resulting SKa wavelength shifts for
each spectrometer are calculated as the difference of the
FeS standard value (reported as — 103):
lðSKa Þsample ¼ lðSKa Þsample lðSKa Þsulphide ð1Þ
lðSKa Þstandard ¼ lðSKa Þanhydrite lðSKa Þsulphide:
ð2Þ
The proportion of sulphur that is sulphate (S6þ/S) in a
sample can be calculated relative to the peak shift of the
barite standard:
X
S6þ =
S ¼ lðSKa Þsample=lðSKa Þstandard: ð3Þ
Analyses of samples and standards showed different results for the two crystal spectrometers used; a better
Gaussian fit was obtained with spectrometer 1 than spectrometer 2, hence the former was used for all analyses reported here.
1740
LESNE et al.
CLOSED-SYSTEM DEGASSING
Fourier transform infra-red
spectroscopy (FTIR)
IR spectra were recorded with a Bruker IFS 88 FTIR
spectrometer coupled with an IR-scopeII microscope at
Leibniz University, Hannover, with the following operating conditions: MCT narrow range detector for both
mid-infrared (MIR) and near-infrared (NIR); globar
light source and KBr beamsplitter for MIR; tungsten
lamp and CaF2 beamsplitter for NIR. Spectral resolution
was 1cm1 in MIR and 4 cm1 in NIR. To minimize the effects of varying atmospheric CO2, the sample stage of the
IR microscope was shielded and purged with dry air.
Fifty and 100 scans, respectively, were used to obtain MIR
and NIR spectra with good signal/noise ratio.
Water dissolved in glasses was analyzed by using the
bands at 5200 and 4500 cm1. The spectra were obtained
on doubly polished glass chips (thickness 200 mm); 2^4
spots were analyzed from each sample. To calculate H2O
dissolved in the melt, both as molecular H2O and hydroxyl
groups, we referred to the work of Shishkina et al. (2010),
who calibrated molar absorption coefficients (e) for
tholeiitic basaltic melt (eH2O ¼ 0·69 0·07 and
eOH ¼ 0·69 0·07 L cm1mol1). A tangential baseline correction was drawn to measure the height of the peaks. The
accuracy of our FTIR analysis is estimated to be better
than 20% for H2O. Behrens et al. (2009) stated that
FTIR reproduces Karl-Fischer titration (KFT) data
within 6% relative.
The concentrations of CO2 dissolved in quenched glasses
were determined by measuring the heights of peaks of
1
(to avoid overlap of the H2O peak at
CO2
3 at 1430 cm
1
1630 cm
with the CO2
peak at 1520 cm1; see
3
Botcharnikov et al., 2006) after subtracting a carbonate-free
spectrum obtained from a volatile-free basaltic sample, adjusted to the same sample thickness. For the molar absorption coefficient at 1430 cm1, we used the value of Fine &
Stolper (1986): 375 20 L cm1 mol1, for consistency with
previous FTIR studies on similar melt compositions
(Pichavant et al., 2009; Lesne et al., 2011b).
Density measurements, required for determination of
volatile concentrations in glasses, are reliable only if measurements are made using a single piece of glass more than
15 mg in mass. However, owing to the low initial amount
of powder loaded (30^50 mg), and breakage during capsule
opening, no glass fragments 45 mg could be recovered.
Hence, density was estimated using the linear relationship
given by Ohlhorst et al. (2001) for a basaltic glass, assuming
a partial molar volume of H2O in the glass of
12·0 0·5 cm3 mol1 (Richet et al., 2000):
r ¼ ð 20 8 6 6ÞcH2 O þ 2819 13 5
3
ð4Þ
where r is the density of the sample in kg m and cH2O is
the concentration of water in the glass in wt %. The density for each quenched glass was calculated by iteration.
Sample thicknesses were determined by micrometer to a
precision of 3 mm. The run-product glasses are not vesicular (1 vol. % bubbles) and the presence of so few bubbles does not influence the FTIR analyses.
Karl-Fischer titration (KFT)
The initial H2O contents of the starting materials were
measured by Karl-Fischer titration at the Leibniz
University, Hannover, using a procedure similar to that detailed by Behrens (1995). Starting powders were loaded in
a Pt crucible and heated to 13008C with an induction furnace to extract dissolved water. All water released was conducted by an Ar flux to a CuO furnace to convert any
H-bearing species present into H2O, which was analyzed
in the titration cell by a coulometric method (Behrens,
1995). It is known that about 0·10 wt % of unextracted
H2O is found in samples containing initially more than
1·5 wt % H2O (Behrens, 1995). However, the compositions
for which unextracted water was observed are more silicic
than our basaltic melts (Behrens et al., 2004), hence we did
not make any correction for unextracted water in our
KFT analyses. Instead, an additional uncertainty of
0·10 wt %, originating from possible incomplete degassing, has been incorporated into the error propagation calculations. Although KFT results are highly reproducible
(Behrens, 1995), KFT is a destructive technique that requires a large sample, and therefore could not be used to
determine total water dissolved in experimental quenched
glasses because of insufficient material.
Secondary ion mass spectrometry (SIMS)
SIMS was applied to experimental glasses whose H2O and
CO2 concentrations were below detection by FTIR. In
addition we analysed several glasses by SIMS that had
also been analysed by FTIR to evaluate the consistency of
the two techniques. SIMS analyses were carried out on
Au-coated grain mounts using a Cameca IMS-4f ion
microprobe at the University of Edinburgh. Single glass
fragments were polished and multiply mounted in indium
blocks to minimize the carbon background from epoxy.
Background was further minimized by pumping down to
a vacuum of 109 Torr in a custom-built airlock attached
to the sample chamber. Operating conditions were 10 kV
O primary-beam with 5 nA current at the sample surface. Positive secondary ions were extracted at 4·5 kV with
an offset of 50 V (for C) and 75 V (for H) to reduce transmission of molecular ions. A mass resolving power of 1500
was used to eliminate interferences of 24Mg2þ on 12Cþ. To
calibrate H2O and CO2 we followed the method of
Blundy & Cashman (2008), by analyzing basaltic glass
standards with known H2O and CO2 contents, and by
building working curves of 1H/30Si vs H2O and 12C/30Si vs
CO2. A total of eight basaltic glass standards were run,
covering a range of 0·15^4·6 wt % H2O and 0^1000 ppm
CO2. Calibration was performed afresh on each analysis
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JOURNAL OF PETROLOGY
VOLUME 52
NUMBER 9
SEPTEMBER 2011
4.50
H2O (wt%) measured by FTIR
4.00
3.50
y = 1.1854x
R2 = 0.8845
3.00
2.50
2.00
1.50
St8.1.A
St8.1.B
MAS.1.A
MAS.1.B
1.00
0.50
0.00
0.00
1.00
2.00
3.00
4.00
H2O (wt%) measured by SIMS
Fig. 1. Comparison of H2O contents of experimental glasses measured by both FTIR and SIMS. Deviation from a 1:1 line is attributed to uncertainty in the extinction coefficient used for processing the FTIR data. A correction factor of 1·1854 has been applied to all of the FTIR data to
bring them into line with SIMS data. No such correction was required for CO2 data.
iron can produce water via the equilibrium
day and uncertainties on the working curve were propagated through to calculate uncertainty on dissolved H2O
and CO2.
For glasses analysed by both SIMS and FTIR we
observed a consistent offset in H2O contents, with FTIR
giving values 18·5% relative higher than SIMS (Fig. 1). We
attribute this difference to uncertainty in the FTIR absorption coefficients, which are very sensitive to bulk composition. This affects accuracy rather than precision. As
SIMS analysis of H2O is less compositionally sensitive
than FTIR, and given that SIMS analyses of the most
H2O-rich glasses give better overall agreement with the
initial H2O content as determined by KFT, we have
reduced all FTIR H2O data by a factor of 1·185. Both the
original and corrected H2O data are reported in Table 3,
but only the latter are used in the figures and discussion.
For CO2 agreement between SIMS and FTIR showed no
such offset, suggesting that the absorption coefficients of
Fine & Stolper (1986) are well suited to our glass compositions. Consequently no correction was made to the TIR
CO2 data.
Where the hydrogen fugacity during the experimental
run differs from that of the starting material synthesis, H2
diffusion will significantly modify the original H2O content of the capsule. As we need to know precisely the total
volatile content of the system (melt þ fluid) to calculate
the fluid composition by mass balance (see below) we measured the Fe2þ/Fetot ratios in the starting material and the
experimental charges. By comparing the Fe2þ/Fetot of the
run products with that of the starting materials it is possible to calculate any H2O added to (or lost from) the
starting material via reaction (5). The Fe2þ/Fetot ratio of
our quenched melts was determined by colorimetric
wet-chemical analysis, following the method of Schuessler
et al. (2008). They estimated the uncertainty on Fe2þ/Fetot
ratios to be 0·03. Fe2þ/Fetot ratios of our starting materials, measured using the same technique, are reported in
Table 1.
Iron redox state and implications for
water gain and loss
R E S U LT S A N D D I S C U S S I O N
Run products
In an iron-bearing experimental charge open to hydrogen
exchange through the capsule walls, reduction of ferric
All quenched glasses from the St8.1 starting composition
are brown and free of crystals. A few bubbles are present
Fe2 O3 ðmeltÞ þ H2 ðvapÞ ¼ 2FeOðmeltÞ þ H2 OðvapÞ:
ð5Þ
1742
LESNE et al.
CLOSED-SYSTEM DEGASSING
Table 3: Experimental results
P (MPa) Run
NNO Fe2þ/Fetot H2O
Volatiles dissolved in the melt
Volatiles dissolved in the fluid (mol %)
added
by iron
H2O1
reduction (wt %)
s
H2O2
S3
s
CO2
(wt %) (ppm)
(ppm)
s
Cl3
s
H2O
s
CO2
s
S
s
Cl
s
Min. of
S6þ/Stot
(ppm)
St8.1.A
26
St8.1.A9
n.d.
0·744
1·73
0·15 1·47
455
4
2·44
0·15 2·08
n.d.
115
41 2026
82 87·2
751
57 1994
60
925
66 2430
3001
30 1719
0·22 2·70
5151
43 1733
3·18
0·15 2·70
4851
3·37
0·15 2·86
52
St8·1.A8a
n.d.
0·74
52
St8·1.A8b
n.d.
0·744
2·53
0·15 2·15
555
150
St8·1.A5
2·4
0·60
0·69
3·03
0·15 2·57
200
St8·1.A1
2·0
0·64
0·73
3·18
200
St8·1.A6
1·8
0·744
250
St8·1.A3
1·6
0·69
0·77
300
St8·1.A4
1·7
0·67
0·76
406
St8·1.A10
n.d.
26
St8·1.B9
n.d.
50
St8·1.B7
1·7
52
St8·1.B8
n.d.
100
St8·1.B2
1·8
150
St8·1.B5
2·0
200
St8·1.B6
n.d.
250
St8·1.B3
1·8
300
St8·1.B4
1·6
406
St8·1.B10
n.d.
25
MAS.1.A9
n.d.
50
MAS.1.A7
1·0
52
MAS.1.A8
52
15
4
16·2
n.d.
8·25
0·92 4·53 0·53 0·03 0·35
n.d.
n.d.
n.d.
179 84·1
23·8 12·2
2·0
3·71 0·68 n.c.
155 1979
134 81·0
34·4 17·5
4·0
1·19 0·95 0·31 0·95 0·84
13 1730
138 78·9
42·4 18·1
5·1
1·21 0·45 1·71 1·16 0·76
48 1782
142 2071
142 80·7
37·8 18·4
4·6
0·92 0·95 n.c.
5751
57 1595
156 1963
162 76·4
42·4 20·7
5·8
2·38 1·33 0·50 1·34 0·82
2·08 1·97 0·71 1·74 0·76
3·59
0·15 3·05
9601
96 1721
179 1961
142 71·1
57·3 26·1
9·6
4
n.d.
3·17
15405
48 1564
142 2141
79 64·9
77·8 30·1
14·8
0·84
0·68
0·74
0·754
1·21
0·15 1·03
405
2
36
41 1485
50 86·8
7·1
6·5
0·3
6·52 0·36 0·14 0·25
0·68
0·77
2·20
0·16 1·87
605
5 1607
314 1476
136 85·4
10·7
9·3
0·7
5·14 0·94 0·23 0·47
0·754
2·40
0·15 2·04
655
6 1189
73 1533
61 83·0
10·7 10·1
0·7
6·81 0·59 0·10 0·40
0·66
0·75
2·83
0·15 2·40
601
6 3043
201 1537
337 84·7
14·2 13·2
1·3
1·95 0·84 0·12 1·24
0·63
0·73
3·12
0·15 2·65
2301
23 3170
197 1570
133 82·4
17·1 15·8
1·8
1·81 1·01 0·01 0·82
0·754
3·24
0·15 2·76
4151
41 3230
187 1582
148 81·9
18·5 16·4
2·1
1·65 1·0
n.c.
0·66
0·75
3·54
0·15 3·01
6351
64 3240
203 1985
167 77·4
24·4 20·5
3·4
2·13 1·5
n.c.
0·68
0·77
3·66
0·15 3·11
9201
92 3140
187 1560
173 75·7
25·9 21·2
3·7
3·07 1·6
0·07 1·48 0·79
0·754
3·98
0·15 3·38
14901 149 3090
67 1635
63 63·5
41·7 30·7
8·1
5·81 2·0
n.c.
2·14 0·27 1·30 1·58
4·98 3·19 n.c.
0·82
St8·1.B
0·88
MAS.1.A
1·174
1·86
0·15 1·58
355
9
30
20 1320
360 76·2
18·2 20·3
2·5
1·26
2·39
0·12 2·03
905
4
270
118 1260
159 69·4
21·3 26·9
3·7
1·63 0·64 2·03 1·52
n.d.
1·174
2·35
0·15 1·99
855
13
205
32 1265
67 67·6
24·2 28·2
4·3
2·07 0·37 2·10 1·44
MAS.1.A8b
n.d.
1·174
2·39
0·15 2·03
705
12
170
48 1170
55 66·0
24·8 29·1
4·5
2·32 0·45 2·61 1·49
100
MAS.1.A2
1·7
0·67
1·13
2·51
0·15 2·13
1201
12
395
127 1425
213 62·3
29·2 34·7
6·2
1·31 0·87 1·65 2·13
150
MAS.1.A5
1·7
0·66
1·13
2·61
0·15 2·22
2251
22
410
137 1645
145 59·6
32·7 38·6
7·6
1·36 1·05 0·38 2·14
200
MAS.1.A1
1·5
0·68
1·14
3·03
0·15 2·58
4901
49
480
125 1150
125 24·5
55·6 67·3
18·3
1·52 1·73 6·65 4·17
200
MAS1.A6
n.d.
1·174
2·81
0·15 2·39
3451
34
375
104 1640
178 51·6
38·6 45·9
10·2
1·97 1·04 0·49 2·72
250
MAS.1.A3
1·4
0·70
1·17
3·09
0·15 2·62
6101
61
370
111 1480
153 22·0
60·2 71·8
20·8
3·30 1·88 2·90 4·32
300
MAS.1.A4
1·2
0·72
1·20
2·99
0·15 2·54
7501
75
310
108 1330
244 39·3
46·9 53·8
13·6
406
MAS.1.A10a n.d.
1·174
3·37
0·15 2·86
14451 145
165
36 1315
79
n.d.
n.d.
n.d.
n.d.
406
MAS.1.A10b n.d.
1·174
3·23
0·09 2·75
13901 139
40
21 1590
61
n.d.
n.d.
n.d.
n.d.
0·75
3·17 1·46 3·76 3·89
MAS.1.B
26
MAS.1.B9
n.d.
1·204
1·93
0·03 1·64
355
6
60
25 1220
63 77·0
6·7 17·6
0·8
4·76 0·82 0·65 0·50
50
MAS.1.B7
n.d.
1·204
2·15
0·14 1·83
755
13
505
126 1225
143 75·3
14·0 20·3
1·9
3·68 1·10 0·73 0·75 0·67
3·37 0·95 n.c.
52
MAS.1.B8
n.d.
1·204
2·03
0·15 1·73
905
13
525
73 1510
179 77·7
13·9 18·9
1·8
100
MAS.1.B2
1·4
0·69
1·204
2·59
0·15 2·20
651
6
870
119 1470
217 68·2
19·9 28·8
3·7
3·09 1·51 n.c.
150
MAS.1.B5
1·5
0·68
1·16
2·63
0·15 2·24
1951
20
935
192 1245
117 65·5
21·3 30·5
4·1
2·92 1·89 1·01 1·05 0·62
200
MAS.1.B6
n.d.
1·15
2·84
0·15 2·41
n.d.
950
112 1400
163
250
MAS.1.B3
1·5
0·68
1·14
2·87
0·15 2·44
4501
45
835
243 1440
140 55·4
27·9 39·9
6·7
4·8
2·9
n.c.
300
MAS.1.B4
0·1
0·82
1·36
3·00
0·15 2·55
7051
71
605
179 1220
112 60·4
24·0 32·5
4·9
5·8
2·2
1·31 1·20 0·65
1
n.d.
n.d.
n.d.
n.d.
0·70
Determined by FTIR.
2
H2O recalculated from SIMS analyses.
3
Determined by EMPA.
4
Fe2þ/Fetot not measured: average value is taken.
5
Determined by SIMS.
Min. of S6þ/Stot is a minimum of oxidation state
of sulphur was measured by EMPA (see text for explanation). n.d., not
determined. n.c., not calculable. Volatiles dissolved in the fluid are expressed in mol %.
1743
JOURNAL OF PETROLOGY
VOLUME 52
(less than 1 vol. %), mostly at the rim of the sample.
Quenched glasses from Masaya are dark, owing to their
higher iron contents, again with a few bubbles at the rims
of the samples, but occasionally within the glass. In most
MAS.1 glasses we noticed the presence of few tiny iron
oxide crystals (around 1 vol. %). Melt compositions for
volatile elements in each run are reported in Table 3.
Major element analyses are given as Supplementary Data
(available for downloading at http://www.petrology.
oxfordjournals.org/). Based on comparison of starting materials and experimental glasses (Table 1), iron loss to the
capsule is calculated to be 6% relative for MAS.1 and
3% relative for St8.1, which is at the low end of the
range reported by Di Carlo et al. (2006) for similar starting
materials and run conditions.
Fe2þ/Fetot: iron reduction, H2O production, redox state
Fe2þ/Fetot analyses are reported in Table 1 for the starting
materials and in Table 3 for Fe2þ/Fetot measured in the experimental glasses. Results obtained from the starting materials showed that the Fe2þ/Fetot ratios are 0·022 for St8.1
and 0·025 for MAS.1. Experimental charges show significantly higher values of Fe2þ/Fetot as a result of H2 ingress
through the capsule walls: from 0·596 for St8.1.A5 to
0·681 for St8.1.B7 and St8.1.B4; and from 0·662 for
MAS.1.A5 to 0·818 for MAS.1.B4. This is a consequence of
synthesizing our starting materials at an fO2 higher than
that prevailing in the IHPV apparatus during the
experiments.
The change of Fe2þ/Fetot ratios between the starting materials and the experimental charges reveals significant
water production through iron reduction, via reaction (5).
This additional water must be added to that measured by
KFT in the starting materials to constrain the proportions
of volatile components via mass balance. Thus in the St8.1
samples, an average of 0·75 wt % of H2O is produced
during the experiments, whereas in the MAS.1 samples,
an average of 1·17 wt % H2O is produced in MAS.1.A
and an average of 1·2 wt % H2O in MAS.1.B. The difference in water production between St8.1 and the MAS.1
starting materials may be attributable to the higher iron
content of the latter (Table 1). The amount of H2O added
by reaction (5) in each run where Fe2þ/Fetot was measured
is given inTable 3. Where no Fe2þ/Fetot value was measured
we took an average of the Fe2þ/Fetot measured for the different compositions.
The measured Fe2þ/Fetot ratios can be used to calculate
the experimental redox conditions following empirical relationships (Sack et al., 1980; Kilinc et al., 1983; Kress &
Carmichael, 1988, 1991) of the form
X
X i di
ð6Þ
ln½XFe2 O3 =XFeO ¼ a ln fO2 þ b=T þ c þ
where a, b, c and d are constants found by regression of a
large number and variety of silicate melts equilibrated
NUMBER 9
SEPTEMBER 2011
from air to almost the iron^wu«stite oxygen buffer, over a
range of temperatures at 1bar. Although this equation
was calibrated for dry systems, the effect of dissolved
water on the Fe2þ/Fetot ratio at a given fO2 is negligible
(Botcharnikov et al., 2005b). Calculated fO2 varies between
NNO þ1·5 and NNO þ 2·4 for the St8.1 experiments and
NNO þ1·0 and NNO þ1·7 for MAS.1 (where NNO is the
nickel^nickel oxide buffer). Kress & Carmichael (1991)
gave a standard error of 0·21 for the ln[XFe2O3/XFeO] calculated, giving uncertainties in measured fO2 of 0·86 log
units. Calculated fO2 values are given in Table 3. Not all
of the experimental charges could be analyzed, because of
the paucity of product, but results show that the St8.1.A
samples are always more oxidized than St8.1.B, with
the exception of the experiments performed at 250 MPa.
The same observation is made for the MAS.1 A and B
samples.
Volatiles dissolved in the melt
The amount of H2O dissolved in the melt is pressure dependent, as shown in Figs 2a and 3a. No systematic differences in water content were measured between
sulphur-rich (St8.1.B and MAS.1.B) and sulphur-poor
(St8.1.A and MAS.1.A) starting materials, indicating that
S has a negligible effect on H2O solubility. At pressures
above 100 MPa, H2O contents in St8.1 are 20% higher
than in MAS.1, whereas at 50 MPa H2O contents are similar in both compositions (around 2·6 wt %). At 25 MPa
MAS.1 dissolves 15^38% relative more H2O than St8.1.
We attribute these differences to melt compositional factors. A noteworthy feature is the good agreement between
H2O measured in the highest pressure (400 MPa) glasses
by SIMS and the bulk H2O content as determined by a
combination of KFT in the starting material and H2O
from iron reduction. This vindicates our decision to use
the SIMS values of H2O to correct the FTIR data (see
above).
CO2 dissolved in experimental glasses also shows a
strong pressure dependence (Figs 2b and 3b). For both
bulk compositions, dissolved CO2 concentrations are similar, again irrespective of the initial sulphur contents. CO2
contents are very similar for both St8.1 and MAS.1 over
the entire pressure range, with the exception of the experiment at 300 MPa, where St8.1 contains about 20% more
CO2.
Sulphur concentration (reported as S total) measured in
the St8.1 experiments remains constant (or increases very
slightly) from 400 MPa to 100 MPa (Fig. 2d) and then decreases sharply from 100 MPa to 25 MPa for both initial
sulphur concentrations. The maximum sulphur contents
dissolved in the melt were measured in samples synthesized at 200 MPa for both the S-rich (St8.1.B6 and
St8.1.B3, 3200 ppm) and the S-poor (St8.1.A1 and
St8.1.A6, 1800 ppm) starting materials. The maximum
measured sulphur matched the initial sulphur added to
1744
LESNE et al.
CLOSED-SYSTEM DEGASSING
4.00
3.50
(a)
2.50
2.00
S (ppm)
H2O (wt%)
3.00
STROMBOLI
1.50
1.00
St8.1.A
St8.1.B
0.50
0.00
0
100
200
300
400
4000
3500
3000
2500
2000
1500
1000
500
0
500
(d)
0
100
2000
1800
1600
1400
1200
1000
800
600
400
200
0
500
(e)
2500
2000
1500
1000
500
0
0
100
200
300
400
500
0
100
Ptot (MPa)
2000
1800 (c)
1600
1400
1200
1000
800
600
400
200
0
0.00
200
300
400
500
Ptot (MPa)
2.50
2.00
(f)
S/Clmelt
CO2 (ppm)
400
3000
(b)
Cl (ppm)
CO2 (ppm)
Ptot (MPa)
200
300
Ptot (MPa)
1.50
1.00
0.50
0.00
1.00
2.00
3.00
4.00
0
H2O (ppm)
100
200
300
400
500
Ptot (MPa)
Fig. 2. Volatiles dissolved in quenched glasses from experiments on Stromboli basalts St8.1.A (filled squares) and in St8.1.B (open squares) as a
function of pressure. The data are from Table 3. (a) H2O, measured by FTIR and corrected by SIMS measurements (see Fig. 1); (b) CO2, measured by FTIR or SIMS; (c) CO2 vs H2O, defining a degassing trend; (d) S, determined by EMPA; (e) Cl determined by EMPA; (f) S/Cl
ratios in melts. Continuous (St8.1.A) and dashed (St8.1.B) horizontal lines represent the initial volatiles added, with their uncertainties
(Table 1). Shaded grey fields bracket the range of initial S, Cl and S/Cl.
the system, suggesting that the S loss to the capsule is negligible. Results for MAS.1 (Fig. 3d) show a more pronounced
increase in dissolved S from 400 to 150 MPa, although the
data are less precise. Dissolved S decreases sharply below
100 MPa. As for St8.1, the highest sulphur contents dissolved were measured in experiments performed at
200 MPa. The single MAS.1 run at 400 MPa shows the
same sulphur content as at the lowest pressure (25 MPa).
1745
JOURNAL OF PETROLOGY
VOLUME 52
3.50
(d)
1200
2.50
2.00
S (ppm)
1000
MASAYA
1.50
800
600
MAS.1.A
MAS.1.B
1.00
0.50
400
200
0.00
0
0
100
200
300
400
500
0
100
2000
1800
1600
1400
1200
1000
800
600
400
200
0
(b)
0
100
200
300
200
300
400
500
Ptot (MPa)
Cl (ppm)
CO2 (ppm)
Ptot (MPa)
400
2000
1800
1600
1400
1200
1000
800
600
400
200
0
500
(e)
0
100
Ptot (MPa)
200
300
400
500
Ptot (MPa)
2000
1800 (c)
1600
1400
1200
1000
800
600
400
200
0
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
S/Clmelt
CO2 (ppm)
SEPTEMBER 2011
1400
(a)
3.00
H2O (wt%)
NUMBER 9
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
(f)
0
100
200
300
400
500
Ptot (MPa)
H2O (wt%)
Fig. 3. Volatiles dissolved in quenched glasses from experiments on Masaya basalts MAS.1.A (filled diamonds) and in MAS.1.B (open diamonds) as a function of pressure. Data from Table 3. (a) H2O, measured with FTIR and corrected by SIMS measurements (see Fig. 1);
(b) CO2, measured with FTIR or SIMS; (c) CO2 vs H2O, defining a degassing trend: (d) S, determined by EMPA; (e) Cl determined by
EMPA; (f) S/Cl ratio in melts. Continuous (MAS-1.A) and dashed (MAS.1.B) lines represent the initial volatiles added, with uncertainties;
shaded grey field bracket the range.
Sulphur concentration does not appear to show any systematic variation with redox conditions (Fig. 4), partly because of the large analytical error on the calculated fO2.
That aside, the highest values of sulphur dissolved in the
melt were measured in the more oxidized experimental
charges.
In terms of sulphur speciation, expressed as S6þ/S, our
results
show
that
under
oxidized conditions
(fO24NNO þ 2) 80% of the total sulphur occurs as sulphate (compare Carroll & Rutherford, 1988; Jugo et al.,
2010), whereas at slightly lower fO2, (NNO þ1·5) 65%
of the sulphur is sulphate (Table 3). Figure 5 shows the
1746
LESNE et al.
CLOSED-SYSTEM DEGASSING
4000
: error on ΔNNO
3500
Smelt (ppm)
3000
2500
2000
St8.1.A
St8.1.B
MAS.1.A
MAS.1.B
1500
1000
500
0
0.00
0.50
1.00
1.50
2.00
2.50
ΔNNO
Fig. 4. Variation of the sulphur dissolved in experimental glasses with fO2 (log units relative to NNO buffer) calculated from the measured
Fe2þ/Fe ratio. Data from Table 3.
speciation obtained for some samples of St8.1.A and MAS.1
B. It appears that the shorter the analytical duration, the
better the results are. Longer duration analyses result in
beam damage leading to large discrepancies in S6þ/S.
Therefore, the data obtained with 16 wavescans of 100 ms
are the most reliable. None the less, even for reduced duration analyses it is likely that S6þ/S is underestimated,
and the values given in Table 3 should be considered as
minima.
For both St8.1 and MAS.1, nearly all the initial chlorine
added to the charges remains in the melt from 400 MPa to
25 MPa (Figs 2e and 3e). In a single St8.1 experiment at
50 MPa (St8.1.A8b) Cl in the glass is slightly higher than
the bulk Cl, as determined by EMPA. We attribute this to
either analytical error or a small amount of accidental
NaCl contamination of the starting material. The consistent values of dissolved Cl indicate minimal loss to the
vapour over a wide pressure range. Consequently, S/Cl
ratios in the glass decrease markedly at low pressure
(Figs 2f and 3f).
F LU I D - P H A S E COM POSI T IONS
A N D VO L AT I L E PA RT I T I O N I N G
B E T W E E N M E LT A N D F L U I D
The fluid phase composition could not be analyzed directly, so mass-balance calculations were used instead. This requires accurate knowledge of the initial amounts of
volatiles loaded, plus the additional H2O content introduced via reaction (5), and of the concentrations of volatiles dissolved in the melt. As detailed above, the most
reliable estimates were used to constrain initial volatile
contents, including the effects of H2-mediated ferric iron
reduction. We assume that there is no volatile loss during
the experiments, and that there is no reaction between the
volatiles and the capsule. The principal uncertainty relates
to sulphur, because it is known that S easily reacts with
the noble metals of the capsules, and in particular with
Pd. No specific measurement on the S content of the capsules was made. As it is impossible to calculate the
amount of S dissolved in the capsule, we considered the
loss of S to be minimal, as our experiment duration was
relatively short and the experiments were run at relatively
oxidized conditions (Table 2). At such conditions most S is
present in the melt as sulphate, limiting the possible reactions with the metal capsules. Confirmation of the minimal S loss to the capsules comes from the observation
that for the St8.1 experiments, the sulphur dissolved in the
melt remains almost constant between 100 and 400 MPa,
whatever the run duration and fO2. Nevertheless, we emphasize that our calculated S concentrations in the fluid
phase provide maximum values.
The concentration of each volatile in the fluid phase can
be calculated via mass balance as described by the following equation, assuming that no SiO2 or other major oxide
is dissolved in the fluid phase (as no change in major
1747
S6+/ΣS
JOURNAL OF PETROLOGY
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
VOLUME 52
NUMBER 9
SEPTEMBER 2011
(a)
A1
A3
A4
A5
A6
A10
0
500
1000
1500
measurement time (ms)
2000
0.8
(b)
0.7
S6+/ΣS
0.6
0.5
0.4
0.3
B3
B4
B5
B6
B7
0.2
0.1
0
0
500
1000
1500
measurement time (ms)
2000
Fig. 5. Sulphur speciation, as S6þ/S, measured for some (a) St8.1.A and (b) MAS.1.B samples obtained for different measurement times using
spectrometer 1 of the Cameca-SX 100 electron microprobe. Numbers denote different experimental runs, as listed in Table 2. The convergence
at low measurement times should be noted. These values of S6þ/S represent minimum estimates of the true S6þ/S.
oxide concentrations appears between each experimental
run):
Xifluid ¼
ðminitial
mmelt
Þ
100
i
i
P ðminitial
mmelt
Þ
Mi
i
i
ð7Þ
Mi
where Xi is the mole per cent of species, minitial
and mmelt
i
i
are the weight per cent of volatile species added initially
and measured in the melt, respectively, Mi is the molar
mass of the volatile species considered and denotes the
sum over Cl, S, H2O and CO2.
H2O^CO2
There are several potential sources of uncertainty on the
initial and final water contents, and the fraction of water
in the fluid phase. (1) Errors on the total amount of water
measured in the melt are propagated through the uncertainty on the water measured by FTIR (10% relative)
1748
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8
7
6
5
4
3
2
1
St8.1.A St8.1.B 0
X Cl
X Cl
100 200 300 400 500
0
XS
XS
P (MPa)
X CO2
X CO2
X H2O X H2O 9
8
7
(c)
6
5
4
3
2
1
0
0
100 200 300 400 500
P (MPa)
100
90
80
70
60
50
40
30
20
10
0
0
(b)
Xi fluid (mol%)
(a)
0
Xi fluid (mol%)
CLOSED-SYSTEM DEGASSING
100
200 300
P (MPa)
400
500
(d)
Xi fluid (mol%)
Xi fluid (mol%)
LESNE et al.
100
200 300
P (MPa)
400
500
Fig. 6. Composition of the fluid phase (mol %), calculated from mass balance, as a function of pressure in experiments on Stromboli basalt:
St8.1.A [(a) H2O and CO2; (b) S and Cl]; St8.1.B [(c) H2O and CO2; (d) S and Cl]. Pressures have been slightly offset for clarity. Error bars
have been omitted; they are probably smaller than those given in Table 3 because of the systematic offset to all data points arising from uncertainty in the initial volatile content of the starting materials.
and/or SIMS (58%), and the SIMS^FTIR correction
factor (2·6%). (2) Additional water could have been adsorbed onto powdered starting materials. However, a
good agreement between H2O in the highest pressure
runs and calculated initial water (KFT analysis plus iron
reduction) suggests that the effect of this process is negligible. (3) H2O could be produced by H2-mediated sulphate
reduction within the charge. The lack of a correlation between dissolved S and fO2, and the relatively oxidized
nature of our runs suggest that such reduction process is
also not significant. Uncertainties on the calculated fluid
composition increase as pressure increases, owing to the
greater solubility and smaller volume of exsolved vapour.
Calculated fluid phase compositions for the St8.1 and
MAS.1 experiments, with fully propagated calculated
uncertainties, are shown in Fig. 6 and Fig. 7, respectively.
Calculated fluids in all samples are dominantly H2O^
CO2 mixtures with 510 mol % S and Cl. XH2O decreases
with increasing experimental pressure from 25 to
400 MPa, consistent with greater solubility of H2O in the
melt relative to CO2. XH2O in the fluid phase from
both the St8.1 and MAS.1 experiments increases from
60 mol % at 300^400 MPa to 80 mol % at the lowest
pressures. There is no difference (within error) in H2O/
CO2 ratios in the S-rich and S-poor compositions.
We could not identify the presence of any phase containing S as a major element (e.g. sulphate mineral in the
melt) in the experimental products using SEM analyses.
The low Cl concentrations in the fluid (see below) suggest
that the basaltic melts coexisted with a single fluid phase
in all our systems. These observations imply that the
mass-balance approach was not compromised by partitioning of volatiles into any additional phase.
Sulphur
The sulphur content of the fluid is systematically higher for
the S-rich starting compositions, consistent with observed
differences in the S content of the glasses (Figs 2d and
3d). For St8.1, the sulphur concentration in the fluid phase
(XS) in both starting materials decreases strongly from
400 MPa to 250 MPa, remains approximately constant
until 100 MPa, then increases sharply from 100 MPa to
25 MPa (Fig. 6b and d). A similar minimum in XS around
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(b)
X i fluid (mol%)
Xi fluid (mol%)
100
200
300
P (MPa)
0
400
(d)
Xi fluid (mol%)
100
90
80
70
60
50
40
30
20
10
0
NUMBER 9
8
7
6
5
4
3
2
1
MAS.1.A MAS.1.B
0
X Cl 0
X Cl
100
200
300 400
XS
XS
P (MPa)
X CO2
X CO2
X H2O
X H2O
8
7
(c)
6
5
4
3
2
1
0
100
200
300
400
0
P (MPa)
(a)
0
Xi fluid (mol%)
VOLUME 52
100
200
300
P (MPa)
400
Fig. 7. Composition of the fluid phase (mol %), calculated from mass balance, as a function of pressure in experiments on Masaya basalt:
MAS1.A [(a) H2O and CO2; (b) S and Cl]; MAS1.B [(c) H2O and CO2; (d) S and Cl]. Pressures have been slightly offset for clarity. Error
bars have been omitted; they are probably smaller than those given inTable 3 because of the systematic offset to all data points arising from uncertainty in the initial volatile content of the starting materials. The cause of the discrepant results for two experiments on MAS.1.A at
200 MPa is not known. Comparison with data at higher and lower pressures suggests that the true 200 MPa value probably lies between these
two extremes.
100 MPa is observed for MAS.1 (Fig. 7b and d), although
the data are more scattered. The behaviour of sulphur in
the fluid contrasts with that of sulphur in the melt, which
remains constant from 400 MPa to 100 MPa for St8.1
(Fig. 2d) and which exhibits a bell-shaped curve for
MAS.1 (Fig. 3d).
Our data allow us to calculate DSfl=melt (Fig. 8). Variations
in DSfl=melt may result from changes in P, melt or fluid composition. Fluid composition varies with pressure in all sets
of experiments, evolving from CO2-rich to H2O-rich with
decreasing pressure. In contrast, the melt composition is
different between the MAS and St8.1 experiments, but is
unaffected by pressure. Figure 8 shows that Dfl=melt
is deS
pendent on pressure, with a pronounced minimum at
150 Ma, for both St8.1 and MAS.1 compositions, irrespective of whether the starting material is S-poor or
S-rich (Fig. 8). This is indicative of strong fluid compositional and pressure controls on DSfl=melt. Conversely,
Dfl=melt
is higher for MAS.1 than St8.1 at any given pressure
S
(Fig. 8), consistent with the well-known compositional
dependence of S activity coefficients in basaltic melts
(Scaillet & Pichavant, 2005).
Chlorine
Calculated chlorine concentrations in fluids from the St8.1
and MAS.1 experiments are compromised by the small
amount of initial added chlorine and the fact that most of
the initial chlorine (99%) remains dissolved in the melt.
Consequently, the calculated chlorine concentrations in
the fluid phase are very low with large errors (Figs 6 and
7). What is clear, however, is that chlorine does not show
any sudden change in fluid^melt partitioning at low pressure and is therefore strongly fractionated from sulphur
during degassing at pressures below 100 MPa.
Comparison with calculated solubility and
degassing trends
Our experiments reproduce the degassing of an ascending
basaltic magma under conditions of a closed system (fixed
initial composition) and are therefore amenable to
1750
LESNE et al.
CLOSED-SYSTEM DEGASSING
(a) 1000
St8.1.A
St8.1.B
DSfl/melt
100
10
1
0.1
0
100
200
300
400
500
P (MPa)
(b) 1000
MAS.1.A
MAS.1.B
DSfl/melt
100
10
1
0.1
0
100
200
300
400
500
P (MPa)
Fig. 8. Sulphur partition coefficients between fluid and melt (Dfl=melt
) calculated for Stromboli (a) and Masaya (b), with fully propagated
S
uncertainties from both melt and fluid compositions in Table 3. The minima in both panels at 150 MPa should be noted.
comparison with solubility models, the conventional
means of calculating (closed- or open-system) degassing
paths. Here we make the comparison with the widely used
VolatileCalc programme of Newman & Lowenstern
(2002) and the model of Papale et al. (2006). Although
these models consider CO2 and H2O as the only volatile
species, our experimental measurements show that the
presence of sulphur and chlorine in the system does not significantly influence H2O and CO2 partitioning between
melts and fluids at the investigated conditions.
Calculations were performed at 11508C for basalt with
49 wt % SiO2 (the only compositional variable in
VolatileCalc) having the initial H2O and CO2 contents of
St8.1 and MAS.1. In calculations using the Papale et al.
(2006) model we input the actual composition of the
quenched glass, as this model takes greater account of compositional sensitivity. We considered a closed system with
no initially exsolved vapour. At these conditions, the
system is vapour saturated at pressures below 800 MPa (as
calculated from VolatileCalc).
In Fig. 9a we compare our measured H2O and CO2 contents of glasses from the St8.1 (A and B) set of experiments
1751
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(a)
3.50
2.50
2.00
1.50
1.00
0.50
0.00
0
100
200
300
400
CO2 (ppm)
H2 O (wt%)
3.00
2000
1800
1600
1400
1200
1000
800
600
400
200
0
500
P (MPa)
H2O
St8.1.A - exp.
St8.1.B - exp.
St8.1.A - Papale
St8.1.A - VolatileCalc
CO2
St8.1.A - exp.
St8.1.B - exp.
St8.1.A - Papale
St8.1.A - VolatileCalc
Fig. 9. H2O and CO2 dissolved in experimental charges (symbols) and calculated (lines) H2O and CO2 contents in basaltic melts using
VolatileCalc (Newman & Lowenstern, 2002) and the model of Papale et al. (2006) at the pressure and temperature of the experiment using the
starting compositions in Table 1. (a) Stromboli; (b) Masaya.
with those calculated using VolatileCalc and Papale
(Papale et al., 2006). The CO2 concentrations are very
well described by VolatileCalc over the entire pressure
range. Conversely, Papale’s model reproduces very well
the dissolved CO2 contents at pressures below 200 MPa,
but deviates at higher pressures: dissolved CO2 is overestimated by 450% relative at 400 MPa. Both models predict
similar dissolved H2O contents: at the highest (400 MPa)
and lowest (100 MPa) ends of the pressure range the
measured and calculated H2O values agree well. At intermediate pressures VolatileCalc overestimates dissolved
H2O by 0·5^0·7 wt %. As a consequence, the modelled
vapour compositions are significantly depleted in H2O
compared with those that we have calculated from
experimental data. This is apparent from Fig. 10a, where
we plot our experimental data on a conventional CO2^
H2O plot, contoured for pressure and vapour composition
using VolatileCalc at 11508C. Experimental data at different pressures are shown with different symbols and have
been colour-coded for calculated vapour composition for
ease of comparison. At the lowest pressures, calculated
solubility and vapour composition are well matched to the
experiments. There is progressively greater deviation as
pressure increases, with modelled volatile compositions
consistently more CO2-rich than we have calculated from
our experiments. This in part reflects the greater uncertainty on the calculated vapour compositions at higher
pressures, where there is less exsolved vapour present.
1752
LESNE et al.
3.50
(b)
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0
100
200
300
400
2000
1800
1600
1400
1200
1000
800
600
400
200
0
500
CO2 (ppm)
H2O (wt%)
CLOSED-SYSTEM DEGASSING
P (MPa)
H 2O
CO2
MAS.1.A - exp.
MAS.1.B - exp.
MAS.1.A - Papale
MAS.1.A - VolatileCalc
MAS.1.A - exp.
MAS.1.B - exp.
MAS.1.A - Papale
MAS.1.A - VolatileCalc
Fig. 9. Continued.
However, the degassing trend itself (which is based on precise measured dissolved volatile contents) is also at odds
with the modelled trend, in having less curvature
(Fig. 10a). The curvature is a function of the relative partition coefficients of H2O and CO2 between melt and
vapour; in an extreme case where both H2O and CO2
had the same partition coefficient, the degassing trend
would be a straight line. The less curved experimental
trend is consistent with a smaller experimental melt^
vapour partition coefficient for H2O than is calculated
from VolatileCalc, as previously deduced from Fig. 9a.
A mismatch is also observed between VolatileCalc and
the experimental data of Pichavant et al. (2009) for another
Stromboli bulk composition (PST-9). These data, which include both measured dissolved H2O and CO2 and calculated (mass-balance) vapour compositions, are plotted
alongside our data in Fig. 10a. Again, we see a displacement to more H2O-rich vapours than would be calculated
from VolatileCalc. Moreover, for the Pichavant et al.
(2009) data there is a large pressure discrepancy, most
marked at 400 MPa. This can be attributed to the more
calcic, less aluminous composition of PST-9, compared
with St8.1, which may enhance CO2 solubility, as calculated by Papale et al. (2006).
Finally, we have calculated the vapour compositions in
equilibrium with glasses having our measured H2O and
CO2 at the experimental conditions using both
VolatileCalc and the Papale et al. (2006) model (Fig. 11a
and c, respectively). For both models we see that calculated
XH2O underestimates our experimental values by up to
40 mol % (VolatileCalc) or 20 mol % (Papale et al., 2006)
at the highest pressures, but is in good agreement at the
lowest pressures.
In the case of the Masaya compositions (MAS.1.A and
MAS.1.B) the agreement between experiments and calculations is better (Figs 9b and 10b) and the vapour compositions are much closer to the experimental values for both
solubility models (Fig. 11b and d). As both the calculated
(from VolatileCalc) isopleths and the isobars closely reproduce our MAS.1 experiments over the entire pressure
range, the calculated degassing trend has a curvature that
closely matches the experiments (Fig. 10b).
We conclude, in agreement with Papale et al. (2006), and
references therein, that there is significant compositional
control on both volatile solubility and melt^vapour partitioning that is not fully captured by VolatileCalc. In the
most extreme case (here exemplified by St8.1),
VolatileCalc predicts vapour that is too CO2-rich, leading
1753
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(a) 3000
XH2Ov=0.4
XH2Ov=0.2
2500
400 MPa
CO2 (ppm)
2000
1500
XH2Ov=0.5
300 MPa
XH2Ov=0.6
1000 200 MPa
XH2Ov=0.7
500
100 MPa
XH2Ov=0.8
50 MPa
25 MPa
0
0.00
1.00
2.00
St8.1.A
3.00
H2O (wt%)
St8.1.B
400 MPa
300 MPa
250 MPa
200 MPa
150 MPa
100 MPa
50 MPa
25 MPa
4.00
5.00
6.00
XH2Ov : 0.9 - 0.8
0.8 - 0.7
0.7 - 0.6
< 0.6
Pichavant et al., 2009
Melt inclusions from:
Bertagnini et al., 2003
Metrich et al., 2001
Fig. 10. Experimental glass CO2 and H2O contents for (a) Stromboli and (b) Masaya, compared with calculated isobars (fine continuous
lines), vapour isopleths (dashed lines) and degassing trends (bold continuous line) calculated using VolatileCalc at 11508C and for the experimental starting compositions in Table 1. Experimental data are colour coded to denote the range in calculated vapour composition. In (a) we
also show the experimental data of Pichavant et al. (2009) for a ‘golden pumice’ basalt (PST-9) from Stromboli at pressures of 401, 208 and
88 MPa. The offset of these data from our own at similar pressures can be ascribed to the compositional difference between their starting material and ours and its influence on CO2 solubility. The shaded region encompasses data from Stromboli melt inclusions measured by Me¤trich
et al. (2001) and Bertagnini et al. (2003).
to much more sharply curved degassing trends than are
observed experimentally. This discrepancy can lead to
problems when attempting to interpret melt inclusion
volatile contents in terms of open- or closed-system
degassing.
Comparison with melt inclusions
Melt inclusion data for volatile elements (H2O, CO2, Cl,
S) are available for samples from Stromboli in the studies
by Bertagnini et al. (2003) and Me¤trich et al. (2001, 2010).
These data can be usefully compared with our
1754
LESNE et al.
CLOSED-SYSTEM DEGASSING
(b) 2500
XH2Ov=0.2
400 MPa
CO2 (ppm)
2000
300 MPa
1500
XH2Ov=0.4
1000 200 MPa
XH2Ov=0.5
XH2Ov=0.6
100 MPa
500
XH2Ov=0.7
50 MPa
25 MPa
XH2Ov=0.8
0
0
0.5
1
1.5
2
2.5
3
3.5
4
H2O (wt%)
XH2Ov : 0.9 - 0.8
0.8 - 0.7
0.7 - 0.6
< 0.6
MAS.1.A MAS.1.B
400 MPa
300 MPa
250 MPa
200 MPa
150 MPa
100 MPa
50 MPa
25 MPa
: fluid phase not determined.
Fig. 10. Continued.
experimental isobars, isopleths and degassing trends
(Fig. 10a). In terms of our experimental CO2^H2O systematics, melt inclusions are located between the 250 and
400 MPa isobars at vapour compositions of XH2O between 0·6 and 0·7. Using the more sparse experimental
data of Pichavant et al. (2009) would yield a slightly
lower pressure range (180^300 MPa) and slightly more
carbonic vapour. In contrast, VolatileCalc gives a pressure range of 200^400 MPa, but XH2O of between 0·2
and 0·4. The trend described by the melt inclusions
shows too little curvature compared with either our
experimental degassing trends or those calculated by
VolatileCalc.
Deviation of melt inclusion data arrays from calculated
degassing paths is not uncommon (e.g. Blundy et al., 2010).
For example, at Mount Etna, Spilliaert et al. (2006) and
Me¤trich & Wallace (2008) identified a group of
H2O-depleted melt inclusions that they interpreted as
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St8.1.B
80
60
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1:1
1:1
(a)
(c)
0
0
20
40
60
80
0
100
20
40
60
80
100
Exp. XH2O
Exp. XH2O
100
100
90
80
70
60
50
40
30
20
10
0
MAS.1.A
MAS.1.B
MAS.1.A
MAS.1.B
80
Papale XH2O
VolatileCalc XH2O
SEPTEMBER 2011
100
St8.1.A
St8.1.B
Papale XH2O
VolatileCalc XH2O
JOURNAL OF PETROLOGY
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40
20
1:1
1
1:
(b)
(d)
0
0
20
40
60
80
0
100
Exp. XH2O
20
40
60
Exp. XH2O
80
100
Fig. 11. Experimentally determined fluid phase compositions (in mol % H2O) for St8.1 and MAS.1 compositions compared with those calculated using (a, b) VolatileCalc (Newman & Lowenstern, 2002) and (c, d) the model of Papale et al. (2006) using the measured experimental
glass CO2 and H2O contents, pressures and temperatures. A 1:1 line is shown for reference. Errors on experimental fluid compositions are
fully propagated from uncertainties on starting compositions and glass compositions. Because of some cross-correlation of these uncertainties,
the error bars are maxima.
re-equilibration of the magma with a deeper CO2-rich gas
phase during ponding and crystallization at 200 MPa.
Also, melt inclusions from Jorullo and Colima volcanoes
in central Mexico have been reported to deviate from the
calculated degassing trend, for open or closed systems,
and have been interpreted as influenced by a fluxing
CO2-rich gas (Johnson et al., 2009; Blundy et al., 2010).
Me¤trich & Wallace (2008) proposed that gas-fluxing by
deeply derived magmatic CO2 may be a common process
at basaltic volcanoes. Such an interpretation is consistent
with our experimental data. An issue is whether the
vapour composition responsible for fluxing at Stromboli
has XH2O in the range 0·2^0·4 (VolatileCalc) or 0·6^0·7
(our experimental data).
It is also instructive to compare our St8.1 experimental
data for S and Cl with the melt inclusions from Stromboli
(Me¤trich et al., 2001, 2010; Bertagnini et al., 2003).
In Fig. 12a and b, respectively, we plot S and Cl in melt inclusions against the calculated H2O^CO2 saturation pressure for the same melt inclusion using VolatileCalc [it
would make relatively little difference if we calculated
pressure from our experimental data or used Papale et al.
(2006)]. Melt inclusions show a good match to the
low-sulphur series of experiments (St8.1.A), showing little
change in dissolved S and Cl from 400 to 200 MPa, followed by a sharp decrease in S, but not Cl, at
P5150 MPa. The matrix glass analyses of Me¤trich et al.
(2001) plot at the low-pressure extremity of this trend.
These glasses have negligible H2O and CO2 (indicative of
degassing to low pressure), very low S (500 ppm) but Cl
contents (1200 ppm) only slightly below those of the melt
inclusions. This strong fractionation of S from Cl at low
pressures is entirely consistent with our experimental data
(Figs 2f and 13c).
1756
LESNE et al.
CLOSED-SYSTEM DEGASSING
4000
4000
(a)
3500
3000
Smelt (ppm)
Smelt (ppm)
3000
2500
2000
1500
2500
2000
1500
1000
1000
500
500
0
0
0
100
200
300
Ptot (MPa)
400
0
500
3000
100
200
300
Ptot (MPa)
400
500
3000
(d)
(c)
2500
2500
2000
2000
Clmelt (ppm)
Clmelt (ppm)
(b)
3500
1500
1500
1000
1000
500
500
0
0
0
100
Experimental data:
St8.1.A
St8.1.B
200
300
Ptot (MPa)
400
500
0
100
Stromboli data:
200
300
Ptot (MPa)
400
500
Etna data:
Bertagnini et al., 2003 - MI
Métrich et al., 2010 - Large scale eruption - MI
Spilliaert et al., 2006 - MI
Métrich et al., 2010 - Small scale eruption - MI
Métrich et al., 2001 - Groundmass
Fig. 12. Sulphur (a, b) and chlorine (c, d) dissolved in experimental glasses from St8.1 compared with analyses of melt inclusions and matrix
glasses from Stromboli (a, c) (Me¤trich et al., 2001, 2010; Bertagnini et al., 2003) and Etna (b, d) (Spilliaert et al., 2006). The consistent behaviour
of S and Cl in experimental and natural glasses over a wide range in pressure should be noted.
Further support for low-pressure S^Cl fractionation is
provided by basaltic melt inclusion data from Etna
(Spillaert et al., 2006). Although Etna basalts are compositionally different from those of Stromboli, the melt inclusions are a reasonable match to the high-sulphur
experiments (St8.1.B). Moreover, the Etna melt inclusions
cover a very wide range of calculated H2O^CO2 saturation pressures. Once again we see near-constant S and Cl
from 400 to 150 MPa, followed by a rapid decline in S
with little change in Cl (Fig. 12). In fact, the melt inclusion
Cl contents increase slightly at pressures below 50 MPa,
probably because of further enrichment in the residual
melt owing to microlite crystallization. Melt inclusion
data from Stromboli and Etna strongly suggest that fractionation of S from Cl is diagnostic of degassing at pressures below 100 MPa.
There are rather fewer melt inclusion data for Masaya
with which to make comparisons. Sadofsky et al. (2008)
measured S, Cl and H2O (but not CO2) dissolved in melt
inclusions from Masaya. H2O is low, between 1·4 and
1·7 wt %. On the basis of the water content only, we calculated saturation pressures with VolatileCalc, for a basalt of
49 wt % SiO2, at 11508C, of between 20 and 29 MPa.
However, Atlas & Dixon (2006) measured 6000 ppm of
CO2 in melt inclusions from Masaya, suggesting that
these pressures are serious underestimates. In the absence
of a Masaya melt inclusion and groundmass dataset with
H2O, CO2, S and Cl measurements, it does not seem instructive to make comparisons with our experimental
data. It is, however, noteworthy that of the six melt inclusion analyses from Masaya given by Sadofsky et al. (2008)
the overall variation is S is much greater (1482 to
241ppm) than that in Cl (599 to 264 ppm), with the
lowest S contents corresponding to the lowest H2O (most
degassed) melt inclusions. Again, this supports our conclusion that S/Cl fractionation is a low-pressure phenomenon.
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St8.1.A
St8.1.B
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H2O/CO2
VOLUME 52
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6
4
2
0
0
100
200
300
400
500
P (MPa)
30
(b)
25
Typical exp
explosions
CO2/S
20
15
10
Quiescent activity
5
0
0
100
200
300
400
500
P (MPa)
1000.0
(c)
S/Cl
100.0
10.0
1.0
0.1
0
100
200
300
P (MPa)
400
500
Fig. 13. Molar ratios in experimental fluids from Stromboli (St8.1.A
and St8.1.B), as functions of pressure, showing the sensitivity of some
ratios to the pressure of last equilibrium between fluid and melt.
(a) H2O/CO2; (b) CO2/S; (c) S/Cl. [Note the logarithmic scale in
(c).] In (b) the range is indicated in CO2/S for gases measured
during typical explosions and quiescent activity by Burton et al.
(2007a).
Comparison with gas chemistry measured
at the vent
Gases released from the volcanic vents at Stromboli and
Masaya are the complement to the melt inclusion data. It
is therefore instructive to compare gas chemistry with our
calculated volatile compositions. Magmatic gases offer
richer potential to interpret and anticipate subvolcanic
NUMBER 9
SEPTEMBER 2011
processes as they can be measured in real time, rather
than a posteriori as is the case with melt inclusions. At
Stromboli, petrological studies show that a basaltic
magma in equilibrium with a fluid phase in a deep-seated
reservoir at 400 MPa (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
400 MPa, to progressively more H2O-rich until 150 MPa,
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. (2010) have measured the compositions of emitted gases at Stromboli
during ‘quiescent periods’,‘typical explosions’and ‘small explosions’. By making measurements within the crater they
were able to minimize atmospheric contamination, such
that the measured H2O/CO2 ratios are thought to reflect
those of the magmatic gas. The highest observed values of
molar H2O/CO2 ratios (6·1) were measured during quiescent degassing by Burton et al. (2007a). Aiuppa et al.
(2010) obtained values up to 50 for the same period.
Burton’s value is reproduced experimentally for the
St8.1 basalt in equilibrium with a fluid phase at a pressure of less than 100 MPa. Lower ratios have been measured during smaller explosions (4·5) and typical
explosions (2·3). According to our experimental results,
these ratios indicate pressures of 200^300 MPa (Fig. 13a).
Also, H2O/CO2 ratios reported by Aiuppa et al. (2010)
have been measured to be the smallest during strombolian
explosions.
Burton et al. (2007a) measured high molar CO2/S
(20·7 2·1) ratios during typical explosions. When activity
intensity decreases, these ratios decrease, reaching 7·8
during quiescent periods. According to our experimental
data, elevated CO2/S ratios are characteristic of intermediate pressures, between 100 and 300 MPa (Fig. 13b), when
S is at its lowest level in the vapour phase (Fig. 6). The
CO2/S ratio associated with explosions (20) is matched
experimentally in the range 150^300 MPa, whereas the
lower values typical of quiescence would require lower or
higher pressures than this. There would, then, appear to
be a congruency between the gas data and the experimental data, in that explosions involve degassing at pressures
of 200 MPa, whereas quiescent degassing occurs at
lower pressures 100 MPa. Aiuppa et al. (2009, 2010) proposed a model of degassing at Stromboli in which they suggested that the compositional characteristics of the gas
emissions during quiescent and syn-explosive activity at
Stromboli result from the mixing of gases sourced by (1)
the degassing of dissolved volatiles in the shallow part
and (2) CO2-rich gas bubbles coming from depth
1758
LESNE et al.
6
CLOSED-SYSTEM DEGASSING
Papale (2004) calculated fluid phase compositions in equilibrium with a shoshonitic melt and showed that CO2 con5
tents in the fluid phase are weakly dependent on the redox
conditions, with sulphur exsolution occurring later in
4
more oxidized systems. Redox conditions in our experimental charges are NNO þ1·5, and we observed that
3
CO2/S ratios start to decrease below 200 MPa. These re2
sults are in good agreement with Moretti & Papale’s
(2004) calculations. CO2 partitions strongly into the
1
vapour phase at all pressures, whereas sulphur largely remains in the melt until relatively low pressures. CO2/S is
0
therefore a powerful indicator of magma ascent and degas0
100
200
300
400
sing below 200 MPa (Fig. 13b).
P (MPa)
The pressure dependence of sulphur degassing makes
S/Cl ratios a sensitive indicator of degassing pressures.
60
The sharp decrease in S/Cl of low-pressure melt inclusions
(b)
and groundmass glasses has already been shown (Fig. 12).
50
The corollary is elevated S/Cl ratios in vapours derived
40
from shallow levels. For this reason S/Cl ratios are often
used to monitor volcanoes. S and Cl are much less abun30
dant in the atmosphere than H2O and CO2, and therefore
measurements give much higher signal to noise ratios
20
than for volcanogenic H2O and CO2. Burton et al. (2007a)
measured S/Cl molar ratios in the gas phase ranging from
10
1·0^1·5 during quiescent periods to 4·5 0·8 during typical
0
explosions. This behaviour appears to disagree with our
0
100
200
300
400 previous interpretations, in that the higher S/Cl ratios
P (MPa)
during explosions would imply lower degassing pressures
than the lower S/Cl ratios during quiescent periods
100
(Fig. 13c). However, our S/Cl ratios should be viewed with
(c)
caution, as Cl in the fluid phase is very small, leading to
large uncertainties in fluid S/Cl ratios. Additional factors
10
that may influence the behaviour of Cl include enrichment
owing to low-pressure crystallization (as shown above for
1
Etna), or separation of a brine phase upon intersection of
the low-pressure vapour solvus. At Stromboli, Burton et al.
(2007a) described a second nucleation event involving
0.1
Cl-rich bubbles. Alternatively, this may reflect ascent
from depth of fresh, undegassed magma with a higher
S content.
0.01
For Masaya, the recent work of Martin et al. (2010) pro0
100
200
300
400
vides data on all major volatile components, as measured
P (MPa)
by open-path FTIR across the active vent between 1998
Fig. 14. Molar ratios in experimental fluids from Masaya (MAS.1.A
and 2009. The molar SO2/Cl ranges from 1·6 to 4·6. These
and MAS.1.B), as functions of pressure, showing the sensitivity of
values are consistent with our experiments on both
some ratios to the pressure of last equilibrium between fluid and
melt. (a) H2O/CO2; (b) CO2/S; (c) S/Cl. [Note the logarithmic scale MAS.1.A and MAS.1.B (Fig. 14), although they are not
in (c).]
diagnostic of any particular pressure. The observed H2O/
CO2 and CO2/S value are in the ranges 10^41 and 1·5^3·5,
respectively. These are substantially different from our ex(P4100 MPa). Such a model is consistent with our experi- perimental ratios even at 25 MPa. Although the observed
values could be related to degassing at very low pressures
mental data.
It is striking that the trends in gas chemistry observed by (525 MPa), a more likely explanation is that the initial
Burton et al. (2007a) and defined by our experimental magma at Masaya is much poorer in CO2 than our experidata show a similar pressure dependence. Moretti & mental starting materials (7000 ppm), which were
S/Cl
CO2/S
H2O/CO2
MAS.1.A (a)
MAS.1.B
1759
JOURNAL OF PETROLOGY
VOLUME 52
based on the melt inclusion data reported by Atlas
& Dixon (2006). Involvement of meteoric water may also
play a role, although that would have little effect on CO2/
SO2 ratios, which also vary by a factor of two at Masaya.
It is interesting to note that the observed H2O/CO2 and
CO2/S ratios for the five campaigns (1998^2009) reported
by Martin et al. (2010) are inversely correlated [see also
the data of Aiuppa et al. (2009) for Stromboli]. This is entirely consistent with our experimental data at pressures
below 100 MPa (Fig. 14). This relationship would characterize fluctuation in degassing pressures at low pressure even
if the initial CO2 contents were lower than used in our
experiments.
NUMBER 9
SEPTEMBER 2011
of intermediate pressure (100^300 MPa). The Cl content of vapour may be complicated if low-pressure
phase separation occurs.
(7) H2O/CO2 and CO2/S ratios measured at Stromboli
are in reasonable agreement with our St8.1 experiments, suggesting that the initial volatile budget of
our starting materials provides a good match to that
of relatively undegassed magma at depth. At Masaya
vent gases have much higher H2O/CO2 and lower
CO2/S ratios than in our experiments, suggesting
that relatively undegassed magma at depth beneath
Masaya has considerably less than the 7000 ppm in
our MAS.1 starting material.
CONC LUSIONS
Experiments have been performed to simulate
closed-system equilibrium degassing of two different basaltic magmas containing the volatile H2O, CO2, S and Cl
abundances under oxidized, super-liquidus conditions
over the pressure range 400^25 MPa. Volatiles dissolved in
the melt were measured by different techniques and fluid
phase composition was calculated through mass-balance
calculations. This is the first time that equilibrium experiments have been performed with such a complex fluid
phase. Our principal findings are as follows.
(1) Exsolved fluids are predominantly H2O^CO2 mixtures, with the H2O/CO2 ratio increasing with
decreasing pressure.
(2) Adding different initial amounts of S to the starting
composition does not affect the behaviour of H2O
and CO2 (for the amounts of volatiles considered in
this study).
(3) S starts to degas significantly at 150 MPa in basaltic
systems, under oxidized conditions (NNO þ1·5),
whereas Cl remains in the melt. This leads to strong
fractionation of S and Cl at low pressures. This behaviour is consistent with observations on melt inclusions
and matrix glasses from Etna and Stromboli.
(4) Sulphur partitioning between fluid and melt is sensitive to pressure, fluid composition and melt composition. The variation in DSfl=melt with pressure is
strongly non-linear, with a pronounced minimum in
the vicinity of 150 MPa.
(5) Experimental results are broadly consistent with
available melt solubility models, for S- and Cl-free systems. However, the models fail to capture some details
of the experiments, notably the composition of the
fluid.
(6) Changing the pressure at which vapour segregates
from its parent magma has a profound influence on
gas chemistry. Low-pressure (100 MPa) gas loss is
characterized by elevated H2O/CO2 and S/Cl ratios,
whereas elevated CO2/S ratios seem to be diagnostic
AC K N O W L E D G E M E N T S
We acknowledge O. Diedrich for the preparation of samples for analysis, T. Shishkina for H2O^CO2-bearing basaltic glass samples used to calibrate SIMS, and U. Bauer
and A. Wegorzewski at Hannover who helped with KFT,
carbon analysis and Fe determination. We acknowledge
C. J. De Hoog, R. Hinton and J. Craven for SIMS analyses
at the University of Edinburgh, and S. Kearns for help
with the Bristol electron microprobe. We acknowledge
helpful discussions with M. Burton, J. Phillips, M. Polacci
and H. Mader. The paper was much improved following
the helpful reviews of C. Martel, R. Moretti and J. Webster.
FU NDI NG
This research was supported by NERC standard grant NE/
F004222/1.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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