Volatile Abundances in Basaltic Magmas & Their Degassing Paths Tracked by Melt

Inclusions

Nicole MétrichLaboratoire Pierre Sue

CNRS-CEA, France

Paul WallaceDept. of Geological SciencesUniversity of Oregon, USA

Volcan Colima, MexicoPhoto 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)

100 m

Experimental and natural polyhedral olivine with melt inclusions (slow cooling)

Keanakakoi Ash, Kilauea, HawaiiFaure & Schiano (2005)

Experimental & natural skeletal (hopper morphology) olivine with melt inclusions (faster cooling)

Paricutin, Mexico500 m

Keanakakoi Ash

Faure & Schiano (2005)

100 m

Jorullo volcano, Mexico

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

100 m

Blue Lake Maar, Oregon Cascades

Experimental and natural closed dendritic olivine with melt inclusions (very fast cooling)

Stromboli Volcano

Faure & Schiano (2005)

Faure & Schiano (2005)Experiments in CMAS system.

Effect of Growth Rate on Trapped Melt Compositions

• 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

• 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.

Data from Johnson et al. (2008)

Post-Entrapment Modification of Melt Inclusions

Diffusive loss of H2 or molecular H2O

Crystal

Meltinclusion

Inclusion entrapment

Cooling

Shrinkage vaporbubble

Crystallization alongmelt – crystal interface

Fe

• 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

• 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.

Some key things to remember:

Solubilities with 2 Volatile Components Present

• H2O and CO2 contribute the largest partial pressures, so people often focus

on these when comparing pressure & volatile solubility

Solid lines show solubility atdifferent constant total pressures

Dashed lines show the vaporcomposition in equilibrium withmelts of different H2O & CO2

From Dixon & Stolper (1995)

Estimating Vapor-Saturation Pressures for Melt Inclusions

Etna 3900 BP eruption Melt inclusions (12-14wt% MgO) in olivine Fo91

(Kamenetsky et al., Geology 2007)

Etna 2001,2002

Ca,Mg-bearing carbonates

Arc basalts

(Walla

ce 2005)

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.

• As much as 80% of the initial CO2 can be

transferred to a shrinkage bubble over a

cooling interval of ~ 100°C.

Carbonate crystals lining bubble walls

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.

Cervantes et al., (2002)

Chlorine Solubility in Basaltic Melts

• 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.

From Webster et al., (1995)

Vapor saturated

Hydrosaline melt (brine) saturated

Continuous transition from vapor to hydrosaline melt as Cl concentrationin vapor (% values) rapidly increases

2 kbar

Cl (wt%)

H2O

(w

t%)

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Jugo et al. (2005)

Sulfur Solubility

• Changes in fO2 have a strong effect on solubility because S6+ is much more soluble than S2-.

Basalt

Trachyandesite

Sulfide saturated Sulfate saturated

• Sulfur solubility depends on temperature, pressure, melt composition & oxygen fugacity.

• Thermodynamic model of Scaillet & Pichavant (2004) relates these variables to fS2.

The record of magma degassing preserved in melt inclusions &

the effect of H2O loss on magma crystallization

Popocatépetl, Mexico

• 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)

Melt inclusions from Keanakakoi Ash, Kilauea, Hawaii (Hart & Wallace, unpublished)

H2O and CO2 Variations in Basaltic Melt Inclusions

Closed Open

Open-system degassing Exsolved gas is continuously separated from melt

Closed-system degassing Exsolved gas remains entrained in melt & maintains equilibrium.

H2O and CO2 Contents of Basaltic Magmas

• 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.

Wallace (2005)

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

2.230.07 vs 2.120.10 wt%

Newman et al., 2000 G-cubed1

! 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: Exsolved gas remains entrained in melt & maintains equilibrium.

Volatilecalc (Newman & Lowenstern 2002) computations assuming equilibrium conditions

Spilliaert et al. 2006 JGR

Etna : 2002 Lava fountain activity

Photos: P

. Allard

Closed system ascent of magma coexisting with a CO2-rich gas phase

at 400 MPa

Closed-system degassing and gas fluxing

Etna (Sicily) 2002 flank eruption

1

10

100

Rb Th Nb La Pb Nd Hf Eu Dy Yb Lu

Normalized data/PM

CO2 diffusion in bubble

Bulk rocksMelt inclusions

Etna 2002

Métrich et al., 2007

Both major and trace elements of natural inclusions in Fo~82, match those of the basalt-trachybasalt bulk-rocks

Not a pure closed-system degassing a two-stage (multi-stage) process?

Closed-system degassing and gas fluxing

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

Spilliaert et al., 2006, JGR

CO2-flushed magma ponding zone

Enhanced magma dehydration 2.6 wt% H2O

1175 ppm CO22.7 wt% H2O

1140 ppm CO2

0

100

200

300

400

500

0 1 2 3 4 5

H2O wt%

CO

2 ppm

Irazu-MI-Ol [1]Irazu-MI-cpx [1]Arenal-MI-Ol [2]Ar model parent [2]

Fo79Fo80-81

Fo73

Fo76-77

Fo85-87

Fo87

Fo76-79

20

50

100

150

200

300

CSD 2%

CSD 1%

Closed-system degassing and gas fluxing

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-

1045°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

Wade et al. 2006

In both cases, the highest CO2 and H2O contents are preserved in M.I .hosted in Mg-rich olivines

Not a pure closed-system degassing CO2-rich gas fluxing

Gas fluxing, H2O loss and crystallization

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

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).

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

Jorullo (Mexico) monogenic basaltic cinder cone

H2O loss and crystallization

Johnson et al., 2008 , EPSL 269

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)

H2O loss and crystallization

Sulfur and halogen degassing

140 MPa140 MPa

(1) P = 58.96x(S/Cl) 1.369

R2 = 0.92

0

100

200

300

400

500

0.0 0.5 1.0 1.5 2.0 2.5

S/Cl (wt)

Pressure (MPa)

140 MPa

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)

0

1000

2000

3000

4000

5000

0 1 2 3 4 5H2O wt%

Irazu-MI-Ol [1]

Irazu-MI-Cpx [1]

Arenal-MI-Ol [2]

AR model parent [2]

Sulfide saturation

~150 MPa

Irazù: Benjamin et al. 2007, JVGR,168, 68-92Arenal: Wade et al. 2006, JVGR,157, 94-120 Etna: Spilliaert et al., 2006, EPSL, 248, 772-786

0

1000

2000

3000

4000

5000

0 1 2 3 4H2O wt%

S ppm

140 MPaEtna [3]

Sulfi

de s

atur

atio

n

Eruption styles and degassing budget

Information from melt inclusions

What are the recent improvements?

Stromboli - 2006

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

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.

Pre-requisite: no differential transfer of gas

S = CS(M.I.) – CS(res)

Wallace 2005, JVGR

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

Petrologic estimates of the sulfur output

[1,3] Thordarson &Self: (1993) Bull Vocanol 93 and (1996) JVGR 74; [2] Thordarson et al., (2001), JVGR, 108

Fissure eruptions Magma Duration Magma S conc. M.I. Total SO 2 output Ref.

Composition vol. in km 3 ppm 106 tons

1783-84 Laki eruption (Iceland) Qz-norm. Thol. 8 months 15.1 1675 ± 225 122 1934AD Eldgjà eruption (Iceland) Trans. Basalt ~3-8 years 19.6 2155 ± 165 220 2Rosa Columbia River basalt (USA) Qz-norm. Thol. ~10 years 1300 1965 ± 110 12,420 3

Eldgjà [2] Laki [1]

Melt inclusions

p-tephra*

s-tephra

lava

M.I. and W.R. have comparable composition >95% of initial sulfur releasedSulfur 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

*p-tephra : quenched melts indicative of magma degassing during during ascent

Approach used for assessing the impact of large flood basalts on the atmosphere (Self et al; 2008 Science)

Volatile budget for basaltic fissure eruptions

Petrologic estimates commonly used for assessing the degassing budget of other volatiles in particular Cl and F

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: 6.9108 kg (Petrologic estimates, Spilliaert et al. 2006) / 8.6108 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)

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?

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)

(Wade et al., 2007)

Volatile budget for basaltic fissure eruptions

Differential transfer of gas bubbles – Excessive degassing

- 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)….

Stromboli Magma supply rate is assessed to be 0.001 km3 y-1,

154 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)

Excessive degassing at persistently active basaltic volcanoes such as:

e.g. Jaupart et Vergniolle, 1988, Vergniolle, 1996; Philips and Wood 1998

Differential transfer of gas bubbles

MI data used for assessing the mass (volume) of unerupted magma when combined with gas flux measurements

Qm = SO2 /2SQm : Mass flux of magma2S = SO2 degassed from the magma

SO2 = SO2 flux measured by COSPEC or other techniques

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

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

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.