Volatile Abundances in Basaltic Magmas

1
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
2
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
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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)
4
Experimental and natural polyhedral olivine with
melt inclusions (slow cooling)
100 mm
100 mm
Keanakakoi Ash, Kilauea, Hawaii
Jorullo volcano, Mexico
Faure Schiano (2005)
5
Experimental and natural closed dendritic olivine
with melt inclusions (very fast cooling)
100 mm
Blue Lake Maar, Oregon Cascades
Faure Schiano (2005)
Stromboli Volcano
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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.
7
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.
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Post-Entrapment Modification of Melt Inclusions
Diffusive loss of H2 or molecular H2O

• Diffusive loss of H-species
• Should be limited to lt1 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).

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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.
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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
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Estimating Vapor-Saturation Pressures for Melt
Inclusions
Total vapor pressure (PH2OPCO2) 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.
CO2 diffuses into a shrinkage bubble during
cooling
Carbonate crystals lining bubble walls

• CO2 loss demonstrated in heating experiments
• on olivine (Fo88) from a Mauna Loa picrite.
• Melt inclusions re-homogenized at 1400C for
• lt10 min.
• As much as 80 of the initial CO2 can be
• transferred to a shrinkage bubble over a
• cooling interval of 100C.

Etna 2001,2002
Etna 3900 BP eruption Melt inclusions (12-14wt
MgO) in olivine Fo91 (Kamenetsky et al.,
Geology 2007)
Cervantes et al., (2002)
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Chlorine Solubility in Basaltic Melts
2 kbar
Vapor saturated
Continuous transition from vapor to hydrosaline
melt as Cl concentration in vapor ( values)
rapidly increases
H2O (wt)
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 lt0.25 wt Cl.
13
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
Trachyandesite
Sulfide saturated
Sulfate saturated
Changes in fO2 have a strong effect on
solubility because S6 is much more soluble than
S2-.
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The record of magma degassing preserved in melt
inclusions the effect of H2O loss on magma
crystallization
Popocatépetl, Mexico
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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)
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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.
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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
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Closed-system degassing and gas fluxing
? Etna (Sicily) 2002 flank eruption
Etna 2002
Both major and trace elements of natural
inclusions in Fo82, match those of the
basalt-trachybasalt bulk-rocks
Bulk rocks Melt inclusions
Not a pure closed-system degassing ? a two-stage
(multi-stage) process?
Métrich et al., 2007
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Closed-system degassing and gas fluxing
CO2-flushed magma ponding zone Enhanced magma
dehydration
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

20
Closed-system degassing and gas fluxing
Wade et al. 2006
? In both cases, the highest CO2 and H2O
contents are preserved in M.I .hosted in Mg-rich
olivines
? Volatilecalc computations assuming equilibrium
conditions
?Irazù volcano (Costa Rica) - 1763 1963-65
eruptions Closed-system degassing (CSD 2),
coupled with ascent, crystallization and cooling
(1075-1045C) (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
21
Gas fluxing, H2O loss and crystallization
Jorullo (Mexico) monogenic basaltic cinder
cone Central part of the subduction-related
Trans-Mexican Volcanic Belt
Johnson et al., 2008 , EPSL 269
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).

• High MgO, high H2O M.I. in Fo88-91? Minimum
  pressure of olivine formation 400 MPa
• At 400MPa - H2O-undersaturated melt
• Total pressure gt 200MPa - Melt interaction
  with CO2-rich gas
• ? CO2-rich gas fluxing depletes melt in H2O and
  thereby causes olivine crystallization

22
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

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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 (lt10-50) and
  viscosities (Métrich et al., 2001, Landi et al.,
  2004 Bertagnini et al., 2003, 2008)

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Sulfur and halogen degassing
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 (Ptotlt20-10MPa)
  and F at Ptotlt10MPa ?
• Sulfur starts degassing at pressure (150 MPa)
  in oxidized magmas in which sulfur is dissolved
  as sulfate gt submarine sulfide-saturated basalts
  (Dixon et al., 1991)

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Eruption styles and degassing budget Information
from melt inclusions What are the recent
improvements?
Stromboli - 2006
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Volatile budget for basaltic fissure eruptions
DS CS(M.I.) CS(res)
Petrologic estimates of the sulfur output
Pre-requisite no differential transfer of gas
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
Wallace 2005, JVGR

• 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
  ?2050 for COSPEC.

Basalt LK Laki 1783-84 eruption K Kilauea,
annual average ML Mauna Loa PC Pacaya 1972
eruption St Stromboli annual average
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Volatile budget for basaltic fissure eruptions
1,3 Thordarson Self (1993) Bull Vocanol 93
and (1996) JVGR 74 2 Thordarson et al.,
(2001), JVGR, 108
p-tephra quenched melts indicative of magma
degassing during during ascent

• M.I. and W.R. have comparable composition
• gt95 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)
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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 (gt2000 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 gt 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
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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. ?
lt10 of magma is extruded given that
quiescent degassing contributes to 95 total SO2
degassing (Allard et al., 2008)
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Unresolved questions and directions for future
studies
Benbow (Ambrym, Vanuatu)
31

• 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

32

• 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