Volatile Abundances in Basaltic Magmas & Their Degassing
Paths Tracked by Melt Inclusions
Nicole Métrich
Laboratoire Pierre Sue
CNRS-CEA, France
Paul Wallace
Dept. of Geological Sciences
University of Oregon, USA
Volcan Colima, Mexico
Photo by Emily Johnson
Outline
• Formation of melt inclusions & post-entrapment modification
• Application of experimental volatile solubility studies to natural systems
• The record of magma degassing preserved in melt inclusions & the effect
of H2O loss on magma crystallization
• Eruption styles and volatile budgets: information from melt inclusions
• Unresolved questions & directions for future studies
What are melt inclusions & how do they form?
• Primary melt inclusions form in crystals when some process interferes with the
growth of a perfect crystal, causing a small volume of melt to become enclosed.
• Formation mechanisms:
1. Skeletal or other irregular growth forms due to strong undercooling
2. Formation of reentrants (by resorption) followed by additional crystallization
3. Wetting of the crystal by an immiscible phase (e.g. sulfide melt or vapor
bubble) or attachment of another small crystal (e.g. spinel on olivine)
resulting in irregular crystal growth & inclusion entrapment
• Melt inclusions can be affected by post-entrapment processes
Roedder (1984); Lowenstern (1995)
Experimental and natural polyhedral olivine with melt inclusions (slow cooling)
100 mm
Faure & Schiano (2005)
100 mm
Keanakakoi Ash, Kilauea, Hawaii
Jorullo volcano, Mexico
Experimental & natural skeletal (hopper morphology) olivine with melt inclusions (faster cooling)
Keanakakoi Ash
Faure & Schiano (2005)
500 mm
Paricutin, Mexico
Experimental and natural closed dendritic olivine with melt inclusions (very fast cooling)
100 mm
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Blue Lake Maar, Oregon Cascades
Faure & Schiano (2005)
Stromboli Volcano
Effect of Growth Rate on Trapped Melt Compositions
Faure & Schiano (2005)
Experiments in CMAS system.
• Rapid growth morphologies have inclusions that are moderately to strongly enriched in Al2O3.
• This is caused by boundary layer enrichment due to slow diffusion of Al2O3 relative to CaO.
Differences between Experimental & Natural Melt Inclusions
Data from Johnson et al. (2008)
• Most natural melt inclusions show no evidence of anomalous enrichment in slowly diffusing
elements, even in small inclusions and rapid growth forms like skeletal or hopper crystals.
• Volatile components have faster diffusivities than Al2O3 and thus should not generally be
affected by boundary layer enrichment effects.
Post-Entrapment Modification of Melt Inclusions
Cooling
Inclusion entrapment
Crystallization along
melt – crystal interface
Crystal
Fe
Melt
inclusion
Shrinkage vapor
bubble
Diffusive loss of H2 or molecular H2O
• Diffusive loss of H-species
– Should be limited to <1 wt% H2O by redox equilibria & melt FeO
if loss occurs by H2 diffusion (Danyushevsky, 2001).
– Leaves distinct textural features – magnetite dust – from oxidation.
– Possible rapid diffusion of molecular H2O (Almeev et al., 2008).
Review of Experimentally Measured Solubilities for Volatiles
Some key things to remember:
• Volatiles occur as dissolved species in silicate melts & also in a separate vapor
phase if a melt is vapor saturated.
• In laboratory experiments, melts can be saturated with a nearly pure vapor
phase (e.g., H2O saturated).
• In natural systems, however, multiple volatile components are always present
(H2O, CO2, S, Cl, F, plus noble gases, volatile metals, alkalies, etc.).
• When the sum of the partial pressures of all dissolved volatiles in a silicate melt
equals the confining pressure, the melt becomes saturated with a multicomponent
(C-O-H-S-Cl-F-noble gases, etc.) vapor phase.
• Referring to natural magmas as being H2O saturated or CO2 saturated is, strictly
speaking, incorrect because the vapor phase always contains other volatiles.
Solubilities with 2 Volatile Components Present
Solid lines show solubility at
different constant total pressures
Dashed lines show the vapor
composition in equilibrium with
melts of different H2O & CO2
From Dixon & Stolper (1995)
• H2O and CO2 contribute the largest partial pressures, so people often focus
on these when comparing pressure & volatile solubility
Estimating Vapor-Saturation Pressures for Melt Inclusions
Total vapor pressure (PH2O+PCO2) for an inclusion can be calculated assuming:
• Vapor saturation – how do we know melts were vapor saturated?
– Large variations in ratios of bubble volume to inclusion volume
– Presence of dense CO2 liquid in bubbles
– Homogenization not possible in heating experiments
• No post-entrapment loss of CO2 or H2O to bubbles, no leakage, no H2O diffusive loss.
• CO2 lost to bubbles lowers vapor saturation pressure.
Carbonate crystals lining bubble walls
Ca,Mg-bearing
carbonates
CO2 diffuses into a shrinkage bubble during cooling
• CO2 loss demonstrated in heating experiments
on olivine (Fo88) from a Mauna Loa picrite.
• Melt inclusions re-homogenized at 1400°C for
<10 min.
Etna
2001,2002
Etna 3900 BP eruption
Melt inclusions (12-14wt% MgO) in olivine Fo91
(Kamenetsky et al., Geology 2007)
• As much as 80% of the initial CO2 can be
transferred to a shrinkage bubble over a
cooling interval of ~ 100°C.
Cervantes et al., (2002)
Chlorine Solubility in Basaltic Melts
2 kbar
Vapor saturated
H2O (wt%)
Continuous transition from vapor to
hydrosaline melt as Cl concentration
in vapor (% values) rapidly increases
Hydrosaline melt (brine) saturated
From Webster et al., (1995)
Cl (wt%)
• In this simplified experimental system, basaltic melts are either saturated
with H2O-Cl vapor or molten NaCl with dissolved H2O (hydrosaline melt)
• Natural basaltic melts typically have <0.25 wt% Cl.
Sulfur Solubility
• Sulfur solubility depends on temperature, pressure, melt composition & oxygen fugacity.
• Thermodynamic model of Scaillet & Pichavant (2004) relates these variables to fS2.
Jugo et al. (2005)
Basalt
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Sulfide saturated
Trachyandesite
Sulfate saturated
• Changes in fO2 have a strong effect on solubility because S6+ is much more soluble than S2-.
The record of magma degassing preserved in melt inclusions
&
the effect of H2O loss on magma crystallization
Popocatépetl, Mexico
H2O and CO2 Variations in Basaltic Melt Inclusions
Closed-system degassing
Exsolved gas remains entrained
in melt & maintains equilibrium.
Open-system degassing
Exsolved gas is continuously
separated from melt
Closed
Open
Melt inclusions from Keanakakoi
Ash, Kilauea, Hawaii (Hart &
Wallace, unpublished)
• Melt inclusion data from a single volcano or even a single eruptive unit
often show a range of H2O and CO2 values.
• In most cases, this range reflects variable degassing during ascent before
the melts were trapped in growing olivine crystals (i.e., polybaric crystallization)
H2O and CO2 Contents of Basaltic Magmas
Wallace (2005)
• Olivine-hosted melt inclusion pressure estimates rarely exceed ~400 MPa.
• In contrast, CO2-rich fluid inclusions commonly indicate higher pressures (Hansteen & Klügel).
• As much as 90% of the initial dissolved CO2 in melts is lost when they reach crustal depths.
• Melt inclusion CO2 provides information on degassing & crystallization processes.
• H2O, S and Cl are much more soluble than CO2, and give information on degassing
paths and the primary volatile contents of basaltic magmas & their mantle sources.
Open-system degassing
Open-system degassing [Exsolved gas is continuously separated from melt]  Strong decrease
of CO2 and negligible H2O loss until the melt reaches vapor saturation pressure for pure H2O
Mariana Trough samples
 Melt inclusions: CO2 = 875  141 ppm
 Host glasses: CO2 = 18  5 ppm
Comparable H2O concentrations
2.230.07 vs 2.120.10 wt%
Newman et al., 2000 G-cubed1
Closed-system degassing: Exsolved gas remains entrained in melt & maintains equilibrium.
! Studies of melt inclusions from basaltic tephra from explosive volcanic activity (e.g., lava
fountains, strombolian activity) often show significant H2O loss that cannot be strictly explained by
pure open- or closed-system degassing of magmas
Closed-system degassing and gas fluxing
Both major and trace elements of natural inclusions in Fo~82,
match those of the basalt-trachybasalt bulk-rocks
100
Etna 2002
Normalized data/PM
 Etna (Sicily) 2002 flank eruption
10
Not a pure closed-system degassing
 a two-stage (multi-stage) process?
Bulk rocks
Melt inclusions
Lu
Yb
D
y
Eu
f
H
d
N
Pb
La
b
N
Th
Rb
1
Métrich et al., 2007
Photos: P. Allard
Closed system ascent of
magma coexisting with
a CO2-rich gas phase
at 400 MPa
Etna : 2002 Lava fountain activity
CO2 diffusion
in bubble
Spilliaert et al. 2006 JGR
 Volatilecalc (Newman & Lowenstern 2002) computations assuming equilibrium conditions
Closed-system degassing and gas fluxing
CO2-flushed magma
ponding zone
Enhanced magma
dehydration
2.7 wt% H2O
1140 ppm CO2
2.6 wt% H2O
1175 ppm CO2
Spilliaert et al., 2006, JGR
 Combined effect of open-system addition of CO2-rich gas to ascending/ponding magma
Consistency with  high CO2 in primary magmas (e.g. Kilauea, Gerlach et al., 2002; Etna, Allard et al., 1999),
high CO2 flux at basaltic volcanoes (Fisher & Marty 2005; Wallace 2005 for reviews), high CO2/SO2 ratio in
gas emissions with increasing explosivity of eruption (e.g. Burton et al., 2007; Aiuppa et al. 2007)
 if true such a process should be the common case at open-conduit basaltic volcanoes
 Effect of disequilibrium degassing (Gonnermann & Manga 2005) - Need more data on diffusion of CO2
relative to H2O (see Baker et al., 2005)
 Need more data on natural samples combined with experiments on disequilibrium degassing
Closed-system degassing and gas fluxing
500
Irazu-MI-Ol [1]
Irazu-MI-cpx [1]
Arenal-MI-Ol [2]
Ar model parent [2]
CO2 ppm
400
Fo87
CSD 1%
300
CSD 2%
Fo85-87
200
Fo76-79
Wade et al. 2006
100
Fo80-81
Fo76-77

In both cases, the highest CO2 and H2O
contents are preserved in M.I .hosted in Mgrich olivines
Fo79
Fo73
0
0
1
2
3
4
H2O wt%
5
 Volatilecalc computations assuming
equilibrium conditions
Irazù volcano (Costa Rica) - 1763 & 1963-65 eruptions:
Closed-system degassing (CSD 2%), coupled with ascent, crystallization and cooling (10751045°C) (Benjamin et al. 2007, JVGR,168, 68-92)
- Natural M.I. in (1 mm) olivine Fo87-79; with, on average, cp  host scoria (54wt% SiO2; Ba/La = 17-20)
Arenal volcano (Costa Rica) - pre-historic eruptions:
Closed-system degassing (CSD 1%) coupled with fractionation and ascent from 2 to 0.2 kbars (Wade
et al. 2006, JVGR,157, 94-120)
- Natural M.I. in 0.25-1 mm size olivine with Fo79  ol-wr bulk equilibrium
Not a pure closed-system degassing  CO2-rich gas fluxing
Gas fluxing, H2O loss and crystallization
Jorullo (Mexico) monogenic basaltic cinder cone
Central part of the subduction-related Trans-Mexican Volcanic Belt
Phase diagram for early Jorullo melt composition (10.5
wt.% MgO) constructed using MELTS (Ghiorso & Sack,1995;
Asimow & Ghiorso,1998) and pMELTS (Ghiorso et al., 2002).
Johnson et al., 2008 , EPSL 269
High MgO, high H2O M.I. in Fo88-91 Minimum pressure of olivine formation 400 MPa
At 400MPa - H2O-undersaturated melt
Total pressure > 200MPa - Melt interaction with CO2-rich gas
 CO2-rich gas fluxing depletes melt in H2O and thereby causes olivine crystallization
H2O loss and crystallization
Jorullo (Mexico) monogenic basaltic cinder cone
Johnson et al., 2008 , EPSL 269
 Crystallization recorded by melt inclusions mainly driven by H2O loss during
magma ascent
- At 400-200 MPa: Water loss likely due to gas fluxing – olivine crystallization
- At low pressure: CO2-depleted melts lose H2O by its direct exsolution in the vapor phase
H2O loss and crystallization
Melt inclusion studies provide evidence for crystallization driven by H2O loss
(+ cooling) at many volcanoes.
Message can be difficult to decipher because of additional processes such as:
- Mixing involving degassed and undegassed magmas (Popocatépetl & Colima; Atlas et al., 2006)
- Mingling (e.g. Fuego, Roggensack 2001)
- Assimilation (Paricutin, Lurh 2001; Jorullo, Mexico, Johnson et al., 2008)
 A case of efficient control of H2O degassing on magma crystallization is Stromboli an open conduit volcanoe with low magma production rate and high degassing excess - where
magmas share same chemical composition but have contrasting textures, crystal abundances
(<10-50%) and viscosities (Métrich et al., 2001, Landi et al., 2004; Bertagnini et al., 2003, 2008)
Sulfur and halogen degassing
5000
5000
Etna [3]
Irazu-MI-Ol [1]
140 MPa
4000
4000
Arenal-MI-Ol [2]
3000
AR model parent [2]
S ppm
S ppm
~150 MPa
Irazu-MI-Cpx [1]
2000
1000
3000
2000
Sulfide saturation
0
0
1
2
3
4
1000
H2O wt%
500
Pressure (MPa)
(1)
0
P = 58.96x(S/Cl)1.369
400
R2 = 0.92
0
1
2
3
4
5
H2O wt%
300
140 MPa
200
140 MPa
100
0
0.0
0.5
1.0
1.5
S/Cl (wt)
2.0
2.5
Irazù: Benjamin et al. 2007, JVGR,168, 68-92
Arenal: Wade et al. 2006, JVGR,157, 94-120
Etna: Spilliaert et al., 2006, EPSL, 248, 772-786
 80% S is lost between 140 and 10 MPa, whereas Cl starts degassing at low
pressure (Ptot<20-10MPa) and F at Ptot<10MPa ?
 Sulfur starts degassing at pressure (~150 MPa) in oxidized magmas in which
sulfur is dissolved as sulfate > submarine sulfide-saturated basalts (Dixon et al., 1991)
Eruption styles and degassing budget
Information from melt inclusions
What are the recent improvements?
Stromboli - 2006
Volatile budget for basaltic fissure eruptions
Petrologic estimates of the sulfur output
Pre-requisite: no differential transfer of gas
Wallace 2005, JVGR
DS = CS(M.I.) – CS(res)
CS(M.I.): S content in primitive melt (melt
inclusion)
CS(res.) : Residual S content in bulk lava or in
matrix glass corrected for crystallization
 Predicted relationship between SO2 emissions and
eruptive magma volume assuming that SO2 released
during eruption is provided by the sulfur dissolved in
silicate melt
 Compared to sulfur emissions measured by
independent methods as ulraviolet correlation
spectrometer (COSPEC), atmospheric turbidity and
Total Ozone Mapping Spectrometer (TOMS)
Uncertainties in SO2 emission data are generally
considered to be about 30% for the TOMS data and
20–50% for COSPEC.
Basalt: LK: Laki 1783-84 eruption; K: Kilauea, annual
average; ML Mauna Loa; PC Pacaya 1972 eruption;
St: Stromboli annual average
Volatile budget for basaltic fissure eruptions
Fissure eruptions
Magma
Composition
1783-84 Laki eruption (Iceland)
934AD Eldgjà eruption (Iceland)
Rosa Columbia River basalt (USA)
Duration
Qz-norm. Thol. 8 months
Trans. Basalt ~3-8 years
Qz-norm. Thol. ~10 years
Magma
vol. in km3
S conc. M.I.
ppm
Total SO2 output
106 tons
Ref.
15.1
19.6
1300
1675  225
2155  165
1965  110
122
220
12,420
1
2
3
[1,3] Thordarson &Self: (1993) Bull Vocanol 93 and (1996) JVGR 74; [2] Thordarson et al., (2001), JVGR, 108
Melt inclusions
Eldgjà [2]
p-tephra*
s-tephra
lava
Laki [1]
*p-tephra : quenched melts
indicative of magma degassing
during during ascent
 M.I. and W.R. have comparable composition
 >95% of initial sulfur released
Sulfur partly exsolved in gas phase during magma ascent at shallow depth prior to eruption
 75% escaped at vents, lofted by the eruptive column (strong fire fountaining) to 5-15 km altitudes at the
beginning of each eruptive phase and 25% during the lava flowing
 Approach used for assessing the impact of large flood basalts on the atmosphere (Self et al; 2008 Science)
Volatile budget for basaltic fissure eruptions
 The 94 days long flank eruption that occurred in 2002 at Mt Etna:
 Modelling of the pressure related behavior of sulfur at Etna (2002 eruption)  ~80% sulfur released
in the gas phase during magma ascent (between 140 and 10 MPa) in agreement with conclusions drawn
by Self, Thordarson and co-authors
SO2 flux: 6.9108 kg (Petrologic estimates, Spilliaert et al. 2006) / 8.6108 kg (COSPEC, Caltabiano et al.
2006)
Comparable S/Cl molar ratio (~5) in vapor phase derived from melt inclusion data and measured in
gas emissions
 no differential degassing of S (or Cl)
Sulfur partly exsolved in gas phase during magma ascent at shallow depth without
differential transfer of sulfur  Consistency between petrologic estimates of SO2
budget and independent estimates (COSPEC or others)
 Arenal (COSPEC 0.41 Mt of SO2 released since 1968 )
Better agreement with COSPEC when considering the S content
(>2000 ppm) of olivine-hosted melt inclusions representative of the
undegassed basaltic andesitic magma rather than partly degassed
melt trapped in Plag & Cpx
Petrologic estimates even > COSPEC a part of sulfur could be lost?
(Wade et al., 2007)
 Petrologic estimates commonly used for assessing the degassing budget of other
volatiles in particular Cl and F
Differential transfer of gas bubbles – Excessive degassing
 Excessive degassing at persistently active basaltic volcanoes such as:
- Izu-Oshima in Japan (Kazahaya et al 1994)
- Villarica in Chile (Witter et al., 2004),
- Popocatepetl in Mexico (Delgado-Granados et al., 2001; Witter et al., 2005)
- Etna & Stromboli in Italy (Allard., 1997; Burton et al., 2007)
- Masaya in Nicaragua (Delmelle et al., 1999, Stix, 2007)….
 MI data used for assessing the mass (volume) of
unerupted magma when combined with gas flux measurements
Qm = SO2 /2DS
Qm : Mass flux of magma
2DS = SO2 degassed from the magma
SO2 = SO2 flux measured by COSPEC or other techniques
Stromboli
Magma supply rate is assessed to be 0.001 km3 y-1,
154 higher than the magma extrusion rate
Assuming 0.22 wt% S dissolved in magma as derived from M.I.

<10% of magma is extruded
given that quiescent degassing contributes to 95% total SO2
degassing (Allard et al., 2008)
 Differential transfer of gas bubbles
e.g. Jaupart et Vergniolle, 1988, Vergniolle,
1996; Philips and Wood 1998
Unresolved questions and directions for future studies
Benbow (Ambrym, Vanuatu)
 Most suitable melt inclusions for volatile studies  quenched pyroclastites
 Efforts dedicated in the last 15 years  basic data for assessing:
- the SO2 output from syn-eruptive degassing of basaltic magmas ascending in closed system conditions,
with no differential gas transfer (gas loss) prior to eruption
- the volume of non-erupted magma that has degassed in volcanic systems undergoing quiescent
degassing
- the degasssing paths of magmas
- volatiles in arc magma mantle sources
 A new idea  magma fluxed by CO2-rich gas causing magma dehydration
Question: Effect of disequilibrium degassing ?
More data on basaltic melt inclusions in pyroxenes and comparison with data of
olivine-hosted melt inclusions
Critical view of natural and experimental data on melt inclusions
Studies that include both melt inclusion & fluid inclusion analysis from the same
samples
Efforts to improve the modeling of:
 CO2-H2O evolution during decompression:
- experimental and thermodynamic data on the solubility of CO2 in H2O-bearing basaltic
melts
-more data on natural systems during well monitored eruptions allowing the combination of
MI data with gas emission chemistry & seismic records
 Magma ascent in the conduits by combining M.I. data with matrix textures & bubble
distribution
Integrating melt inclusion data with




Accurate studies of their host olivines and the mineralogy of the host magmas
Experimental data on volatile solubility
Degassing models that include both thermodynamic and physical aspects
Field work (gas measurements, acoustic and seismic)
is a necessity and represents a main challenge for the next few years.
VGP special session: Model solubility, diffusive bubble growth, disequilibrium
degassing, conduit processes
Monday 15 December, 16h00, Oral session V14a, MC 3003
Tuesday 16 December, Poster session V21B, MC Hall D