The Role of Magmatic Volatiles in Arc Magmas

Paul WallaceUniversity of Oregon

Volatile Recycling & Subduction Zone Magmatism

Oceanic lithosphere

Crust

Mantle flow

Volcanic arc output

Material transfer from slab

Sediment

Returned to mantle

Forearc fluid output

Lithospheric mantle

Asthenosphere

Crust

Components in downgoing slab• Sediment• Altered oceanic crust• Serpentinized upper mantle (?)

Breeding et al. (2004)

Complex reaction zone at slab-wedge interface

Outline

• How do we measure magmatic volatile concentrations?

• Review of experimental studies of volatile solubility

• Volatile contents of basaltic arc magmas based on melt inclusion data

• A comparison of volatile inputs and outputs in subduction zones

• Effect of H2O on melting of the mantle wedge, and a brief look at how fluids and melts move through the wedge.

Problem of Magma Degassing

• Solubility of volatiles is pressure dependent

• Volatiles are degassed both during eruption & at depth before eruption

• Bulk analysis of rock & tephra are not very useful!

• Melt inclusions

• Submarine pillow glasses

• Experimental petrology

How do we measure volatile concentrations in magmas?

100 m

Phase equilibria for basaltic andesite

Moore & Carmichael (1998)

How do we analyze glasses & melt inclusions for volatiles?

• Secondary ion mass spectrometry (SIMS or ion microprobe) H2O, CO2, S, Cl, F

• Fourier Transform Infrared (FTIR) spectroscopy H2O, CO2

• Electron microprobe Cl, S, F

• Nuclear microprobe CO2

• Larger chips of glass from pillow rims or experimental charges can be analyzed for H2O and CO2 using bulk extraction techniques e.g., Karl-Fischer titration, manometry

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 encased in the growing crystal.

• This can occur from a variety of mechanisms, including:1. skeletal or other irregular growth forms due to strong undercooling or

non-uniform supply of nutrients2. formation of reentrants by resorption followed by additional crystallization3. 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 and entrapment of that phase along with silicate melt

• Melt inclusions can be affected by many post-entrapment processes:1. Crystallization along the inclusion-host interface2. Formation of a shrinkage bubble caused by cooling, which depletes the included melt in CO2.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

100 m

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

Keanakakoi Ash, Kilauea, Hawaii

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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

Paricutin, Mexico500 m

Keanakakoi Ash

Post-Entrapment Modification of Melt Inclusions

Crystal

Meltinclusion

Inclusion entrapment

Cooling

Vaporbubble

Crystallizaton alongmelt – crystal interface

Diffusiveexchange

Ascent &Eruption

Volatile leakageif inclusion ruptures

Slow Cooling

Crystallization &possible further

leakage

• Ground & airborne remote sensing

• Satellite-based remote sensing

• Direct sampling & analysis

COSPEC at MasayaTOMS data for El Chichon & Pinatubo

Sampling gases at Cerro Negro

Volcanic gases - another way to get information on volatiles

Review of Experimentally Measured Solubilities for Volatiles

• Volatile components occur as dissolved species in silicate melts, but they can also be present in an exsolved vapor phase if a melt is vapor saturated.

• In laboratory experiments, it is possible to saturate melts with a nearly pure vapor phase (e.g., H2O saturated), though the vapor always contains at least a small

amount of dissolved solute.

• In natural systems, however, multiple volatile components are always present (H2O, CO2, S, Cl, F, plus less abundant volatiles like noble gases).

• 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 is never pure and always contains more than one volatile component.

Some key things to remember:

H2O and CO2 solubilities measured by experiment

• Solubilities are strongly pressure dependent

• Solubilities do not vary much with composition

• CO2 has very low solubility compared to H2O (~30x lower)

Solubilities with more than 1 volatile component present

• In natural systems, melts are saturated with a multicomponent vapor phase

• 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)

Chlorine Solubility

• In this simplified experimental system, basaltic melts are either saturated with H2O-Cl vapor or molten NaCl with dissolved H2O (hydrosaline melt)

• Real basaltic melts typically have <0.25 wt% Cl and thus are not saturated with hydrosaline melt

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

Chlorine in rhyolitic meltsNote: x and y axes have been switched from previous figure

• Cl solubility is much lower in rhyolitic melts compared to basaltic melts

• Some rhyolitic melts (e.g., Augustine volcano) have high enough dissolved Cl for the melt to be saturated with hydrosaline melt before eruption

Sulfur Solubility

• S solubility is more complicated because of multiple oxidation states• Dissolved S occurs as either S2- or S6+

• Solubility is limited by sat’n with pyrrhotite, Fe-S melt, anhydrite, or CaSO4 melt

• S in vapor phase occurs primarily as H2S and SO2

• Fortunately we can measure the oxidation state of S in minerals & glasses by measuring the wavelength of S K radiation by electron microprobe

Basaltic glassesMinerals

From Jugo et al. (2005)

Effect of oxygen fugacity on S speciation in silicate melts

From Jugo et al. (2005)

• A rapid change from mostly S2- to mostly S6+ occurs over the oxygen fugacity range that is typical for arc magmas

Jugo et al. (2005)

Effect of oxygen fugacity on S solubility

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

Sulfur solubility – effects of temperature, pressure & composition

S solubility at low oxygen fugacityS2- is the dominant species

Solubility of both S2- and S6+ aretemperature dependent

S solubility in intermediate to silicic melts

• Because of strong temperature dependence of S solubility, low temperature magmas like dacite and rhyolite have very low dissolved S.

• This led earlier workers to erroneously conclude that eruptions of such magma would release little SO2 to Earth’s atmosphere

°

Vapor–Melt Partitioning of Sulfur

• Experiments show strong partitioning of S into vapor (Scaillet et al., 1998; Keppler, 1999)

• Thermodynamic modeling allows calculation of vapor-melt partitioning at high fO2

SO2 (vapor) + O2– (melt) + 0.5 O2 (vapor) = SO42– (melt)

750

800

850

900

950

1000

0 1 2 3

Isopleths of Constant S vapor / Smelt

Tem

pe

ratu

re (

°C)

Relative oxygen fugacity (²NNO)

10

50

100

200

500

1000 PTotal = 2.2 kbar

Pinatubo

Tem

pera

ture

(°C

)

Isopleths of Constant Svapor / Smelt

S Contents of Magmatic Vapor Phase for Intermediate to Silicic Magmas

0

2

4

6

8

10

700 750 800 850 900 950

STo

tal (

mo

l%)

in v

ap

or

Temperature (°C)

El Chichón

MSH

Redoubt (a)

Ruiz

Bishop

FishCanyon

Pinatubo

Krakatau

Redoubt (r)

KatmaiToba

ST

otal (

mol

%)

in v

apor

From Wallace (2003)

• Because S strongly partitions into the vapor phase at lower temperatures, most of the SO2 released from eruptions of intermediate to silicic magma comes from a pre-eruptive vapor phase

What can melt inclusions tell us about volatiles if magmas are generally vapor saturated?

• Only part of the story – melt inclusions tell us the concentrations of dissolved volatiles

• Information captured by melt inclusions depends on the vapor / melt partition coefficient, and thus is different for each volatile component

• Melt inclusions also provide information on magma storage depths

and vapor phase compositions (e.g., use of H2O vs. CO2 diagram)

• Diagrams in the next two figures show how much of the initial amount of each volatile is still dissolved at the time inclusions are trapped

0.0

0.2

0.4

0.6

0.8

1.0

1 10 100 1000

Pressure (bars)

SummitReservoir

H2O

CO2

SCl

Fra

ctio

n re

mai

ning

(C

/ C

initi

al)

Degassing of low-H2O basaltic magma (Kilauea)

• When olivine crystallizes in the magma chamber beneath the summit of of Kilauea, most of the original dissolved CO2 has already been degassed from the melt.

0.0

0.2

0.4

0.6

0.8

1.0

0 1000 2000 3000 4000 5000

Pressure (bars)

H2O

CO2

SCl

CrystalGrowth

Degassing of H2O-rich rhyolitic magma

Fra

ctio

n re

mai

ning

(C

/ C

initi

al)

• When rhyolitic melt inclusions are trapped in quartz or feldspar at typical magma chamber depths, most of the original CO2 and S has been degassed

Volatile contents of mafic arc magmas based on melt inclusions

100 m

Jorullo volcano, Mexico

100 m

Blue Lake Maar, Oregon Cascades

Photos by Emily Johnson, Univ. of Oregon

0

200

400

600

800

1000

0 1 2 3 4 5 6

CO

2 (

ppm

)

H2O (wt.%)

H2O & CO2 in Melt Inclusions from Jorullo Volcano, Mexico

Vapor saturation isobars from Newman & Lowenstern (2002)

• Early – wide range of olivine crystallization pressures (mid-crust to surface)

• Middle & Late – all olivine crystallized at very shallow depths

• Degassing and crystallization occurred simultaneously during ascent

EarlyMiddleLate

4 kb

0.5 kb

I kb

3 kb

2 kb

Avg.error

All data by FTIR

Johnson et al. (in press)

0

200

400

600

800

1000

0 1 2 3 4 5 6

4 kb

0.5 kb

I kbClosedsystemdegassing

Closed system1% initial gas

3 kb

2 kb

CO

2 (

ppm

)

H2O (wt.%)

Degassing Paths During Magma Ascent & Crystallization

Initialmelt

• Some data cannot be explained by simple degassing models

Degassing paths calculated using Newman & Lowenstern (2002)

Johnson et al. (in press)

Effects of degassing

• 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.

• S can also be affected by this variable degassing, but Cl and F solubilities are so high that they tend to stay dissolved in the melt.

• From a large number of analyzed melt inclusions (preferably 15-25), the highest analyzed volatile values provide a minimum estimate of the primary volatile content of the melt before any degassing.

• The data shown on the following slides are for the least degassed melt inclusions from a number of different volcanoes.

Minimum for arc magmasbased on global CO2 flux

Mariana arc

• H2O contents of arc basaltic magmas are quite variable

• CO2 contents are lower than estimates based on global arc CO2 flux

Arc basaltic magmas CO2 = 0.6–1.3 wt.%

Estimate based on magma flux & CO2

flux

Minimum for arc magmasbased on global CO2 flux

Melts from mantle wedge +subducted sediment

Mariana arc

Melts from mantle wedge +subducted oceanic crust

Arc basaltic magmas CO2 = 0.6–1.3 wt.%

• Subducted oceanic crust and sediments contain abundant C in the form of carbonate sediment/limestone and buried organic C

• This figure shows simple mass balance for bulk addition of H2O & CO2 from slab to wedge, and for addition of H2O-rich, CO2-poor fluid to the wedge from the slab

Minimum for arc magmasbased on global CO2 flux

Arc basaltic magmas CO2 = 0.6–1.3 wt.%

Melts from mantle wedge +low-CO2 fluid from slab

Mariana arc

• Subducted oceanic crust and sediments contain abundant C in the form of carbonate sediment/limestone and buried organic C

• This figure shows simple mass balance for bulk addition of H2O & CO2 from slab to wedge, and for addition of H2O-rich, CO2-poor fluid to the wedge from the slab

Melts from mantle wedge + subducted sediment

Melts from mantle wedge + subducted oceanic crust

Chlorine in Arc and Back-arc Basaltic Magmas

• Cl contents in arc and back-arc magmas (Lau Basin, Marianas) are much higher than in MORB• This indicates substantial recycling of seawater-derived Cl into the mantle wedge

Fluid Inclusions in Eclogites as Analogues for Subduction Zone Fluids

Data from Philippot et al. (1998) High Salinity Fluids17–45 % NaCl

Low Salinity Fluids3.1–4.0 % NaCl

• Eclogites from exhumed subduction complexes contain fluid inclusions that represent samples of fluids released during dehydration of metabasalt

Melts from mantle wedge + subducted oceanic crust + sediment

• S contents of arc magmas are typically higher than for MORB, but in most cases not nearly as enriched as is observed for Cl

0

1000

2000

3000

4 6 8 10 12 14 16 18 20

S (

pp

m)

FeOT (wt.%)

MORB

Kilauea

Mexico

Cascades

Sulfur in Basaltic Magmas

Aeolian Is.& Italy

Etna

Luzon

Data sources: Anderson (1974); Wallace & Carmichael (1992);Métrich et al. (1996; 1999); Cervantes & Wallace (2002)

S (

ppm

)5970

Sulfur concentrations in melt inclusions & submarine basaltic glasses

• The higher S contents of arc magmas relative to MORB are even more clear on this plot

Measuring volatile fluxes from arc volcanism - one method

Volcanic Gases• Measure SO2 flux by remote sensing• Collect & analyze fumarole gases• Use fumarole gas ratios (e.g., CO2/SO2) to calculate fluxes of other components

Modified from Fischer et al. (2002)

Comparing inputs and outputs of volatiles in subduction zones

Measuring volatile fluxes - another method

Melt Inclusions• Use magmatic volatile concentrations in melt inclusions• Combine with magma flux (mantle to crust) estimates from:

– seismic studies of intraoceanic arcs– isotope systematics for crustal growth– geochronology & field mapping

Oceanic lithosphere

Crust

Mantle flow

Back-arcoutput

Volcanic arcoutput

Material transferfrom slab

Sediment

Returnedto mantle

Forearc fluidoutput

Lithospheric mantle

Asthenosphere

Fluxes of Major Volatiles from Subduction-related Magmatism

Gas Flux & Composition

W

Assuming 2–4 km3/yr magma flux

Input vs. Output for Major Volatiles in Subduction Zones

• Inputs include structurally bound volatiles in subducted sediment & altered oceanic crust (Hilton et al., 2002; Jarrard, 2003)

Amount recycled to surface reservoir by magmatismH2O 40–120% of dike/gabbro H2O

20–80% of totalCO2 ~ 50%

S ~ 20%Cl ~ 100%

CO2 Input vs. Output for Individual Arcs

Data from Hilton et al. (2002)

How does addition of H2O to the mantle wedge cause melting?

Experimental determinationsof the effect of H2O on theperidotite solidus

From Grove et al. (2006)

Dry solidusWet solidus

Xitle

Effect of H2O on Isobaric Partial Melting of Peridotite

Hirschmann et al. (1999)

1 GPa

• Increasing H2O has a linear effect on degree of melting (Hirose & Kawamoto, 1995; Hirschmann et al., 1999)

Mariana Trough data from Stolper & Newman (1994)

Effect of H2O on Isobaric Partial Melting of Peridotite

Hirschmann et al. (1999)

1 GPa

Max. H2O for amphibole-bearing peridotite

• To get the high H2O contents of arc magmas, H2O must be added to the mantle either by aqueous fluid or hydrous melt

Effect of H2O on Isobaric Partial Melting of Peridotite

A model for hydrous flux melting of the mantle wedge

• Fluids and/or hydrous melts percolate upward through the inverted thermal gradient inthe mantle wedge

• A small amount of very H2O-rich melt forms when temperatures reach the wet peridotite solidus

• This wet melt continues to rise into hotter parts of the wedge, and becomes diluted with basaltic components melted from the peridotite

• H2O-poor magmas form by upwelling induced decompression melting driven by corner flow

From Grove et al. (2006)

From slab to surface – some complications

• Hydrous minerals are also stable in the mantle wedge just above the slab & act like a ‘sponge’

• H2O released from the slab migrates into the wedge, reacts, & gets locked up in these phases

• Chlorite is stable to ~135 km depth, then breaks down & again releases H2O upwards

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Do fluids and melts move vertically upward through the mantle wedge?

From Cagnioncle et al. (2006)

No, solid mantle flow deflects hydrousfluids from buoyant vertical migrationthrough the wedge

Solid mantle flow also deflects partialmelts formed in the hottest part of thewedge back towards the trench

And finally, mafic arc magmas have enough H2O to cause

explosive eruptions (violent strombolian, sub-plinian, andoccasionally plinian) that produce large amounts of ash and lapilli