Transcript EHaz_Volatiles_Wallace. ppt

The Role of Magmatic Volatiles in Arc Magmas
Paul Wallace
University of Oregon
Volatile Recycling & Subduction Zone Magmatism
Volcanic arc output
Forearc fluid
outpu t
Sediment
Crust
Oceanic lithosphere
Crus t
Lithospheric mantle
Asthenosphere
Mantle flow
Components in downgoing slab
• Sediment
• Altered oceanic crust
• Serpentinized upper mantle (?)
Complex reaction zone
at slab-wedge interface
Breeding et al. (2004)
Material transfer
from sla b
Returned
to mantle
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!
How do we measure volatile concentrations in magmas?
• Melt inclusions
• Submarine pillow glasses
• Experimental petrology
Moore & Carmichael (1998)
Phase equilibria for basaltic andesite
100 mm
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 nutrients
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 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 interface
2. Formation of a shrinkage bubble caused by cooling, which depletes the
included melt in CO2.
Experimental and natural polyhedral olivine with melt inclusions (slow cooling)
100 mm
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Keanakakoi Ash, Kilauea, Hawaii
Experimental & natural skeletal (hopper morphology) olivine with melt inclusions (faster cooling)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Keanakakoi Ash
500 mm
Paricutin, Mexico
Post-Entrapment Modification of Melt Inclusions
Ascent &
Cooling
Eruption
Slow Cooling
Inclusion entrapment
Vapor
bubble
Crystal
Diffusive
exchange
Melt
inclusion
Crystallizaton along
melt – crystal interface
Volatile leakage
Crystallization &
if inclusion ruptures possible further
leakage
Volcanic gases - another way to get information on volatiles
• Ground & airborne remote sensing
• Satellite-based remote sensing
• Direct sampling & analysis
TOMS data for
El Chichon
& Pinatubo
Sampling gases at Cerro Negro
COSPEC at Masaya
Review of Experimentally Measured Solubilities for Volatiles
Some key things to remember:
• 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.
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
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)
• 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
Chlorine Solubility
Vapor saturated
Continuous transition from vapor to
hydrosaline melt as Cl concentration
in vapor (% values) rapidly increases
Hydrosaline melt (brine) saturated
From Webster et al., (1995)
• 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
Chlorine in rhyolitic melts
Note: 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
Minerals
Basaltic glasses
From Jugo et al. (2005)
• Fortunately we can measure the oxidation state of S in minerals & glasses
by measuring the wavelength of S K radiation by electron microprobe
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
Effect of oxygen fugacity on S solubility
Jugo et al. (2005)
• 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 fugacity
S2- is the dominant species
Solubility of both S2- and S6+ are
temperature 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)
IsoplethsofofConstant
ConstantSSvapor
Smelt
Isopleths
/ S/ melt
vapor
1000
10
50
900
100
(°C)
Temperature
Temperature(°C)
950
850
200
800
Pinatubo
500
1000
PT otal= 2.2 kbar
750
0
1
2
Relative oxygen fugacity (²NNO)
3
S Contents of Magmatic Vapor Phase for Intermediate to Silicic Magmas
From Wallace (2003)
S
(mol%)
STotal
(mol%)ininvapor
vapor
Total
10
8
Fish
Canyon
El Chichón
6
Bishop
Pinatubo
Redoubt (a)
4
Redoubt (r)
2
Katmai
Krakatau
Toba
0
700
750
800
850
MSH
Ruiz
900
950
Temperature (°C)
• 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
Fraction remaining (C / Cinitial)
Degassing of low-H2O basaltic magma (Kilauea)
1.0
H2O
0.8
S
Cl
0.6
Summit
Reservoir
0.4
CO 2
0.2
0.0
1
10
100
1000
Pressure (bars)
• 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.
Fraction remaining (C / Cinitial)
Degassing of H2O-rich rhyolitic magma
1.0
0.8
H2O
0.6
Cl
Crystal
Growth
0.4
S
CO 2
0.2
0.0
0
1000
2000
3000
4000
5000
Pressure (bars)
• 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 mm
100 mm
Blue Lake Maar, Oregon Cascades
Jorullo volcano, Mexico
Photos by Emily Johnson, Univ. of Oregon
H2O & CO2 in Melt Inclusions from Jorullo Volcano, Mexico
Vapor saturation isobars from Newman & Lowenstern (2002)
4 kb
3 kb
1000
Early
Middle
Late
All data by FTIR
2 kb
CO2 (ppm)
800
600
I kb
400
0.5 kb
Avg.
error
200
0
0
1
2
3
H2O (wt.%)
4
5
6
Johnson et al. (in press)
• 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
Degassing Paths During Magma Ascent & Crystallization
Degassing paths calculated using Newman & Lowenstern (2002)
4 kb
3 kb
1000
2 kb
Closed
system
1% initial
gas
800
CO2 (ppm)
Initial
melt
600
I kb
Closed
system
degassing
400
0.5 kb
200
0
0
1
2
3
H2O (wt.%)
4
5
6
Johnson et al. (in press)
• Some data cannot be explained by simple degassing models
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.
Arc basaltic magmas
CO2 = 0.6–1.3 wt.%
Estimate based on
magma flux & CO2
flux
Minimum for arc magmas
based 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.%
Melts from mantle wedge +
Melts from mantle wedge +
subducted sediment
subducted oceanic crust
Minimum for arc magmas
based on global CO2 flux
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
Arc basaltic magmas
CO2 = 0.6–1.3 wt.%
Melts from mantle wedge +
low-CO2 fluid from slab
Minimum for arc magmas
based on global CO2 flux
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
Chlorine in Arc and Back-arc Basaltic Magmas
Melts from mantle wedge +
subducted sediment
Melts from mantle wedge +
subducted oceanic crust
• 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
High Salinity Fluids
17–45 % NaCl
Data from Philippot et al. (1998)
Low Salinity Fluids
3.1–4.0 % 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
Sulfur concentrations in melt inclusions & submarine basaltic glasses
Sulfur in Basaltic Magmas
5970
Etna
3000
(ppm)
SS(ppm)
Aeolian Is.
& Italy
2000
MORB
Luzon
Mexico
1000
Cascades
Kilauea
0
4
6
8
10
12
14
16
18
20
FeO T (wt.%)
• The higher S contents of arc magmas relative to MORB are even more clear on this plot
Data sources: Anderson (1974); Wallace & Carmichael (1992);
Métrich et al. (1996; 1999); Cervantes & Wallace (2002)
Comparing inputs and outputs of volatiles in subduction zones
Measuring volatile fluxes from arc volcanism - one method
Volcanic Gases
Modified from Fischer et al. (2002)
• Measure SO2 flux by remote sensing
• Collect & analyze fumarole gases
• Use fumarole gas ratios (e.g., CO2/SO2) to calculate fluxes of
other components
Measuring volatile fluxes - another method
Volcanic arc
output
Forearc fluid
output
Sediment
Back-arc
output
Crust
Oceanic lithosphere
Lithospheric mantle
Asthenosphere
Mantle flow
Material transfer
from slab
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
Returned
to mantle
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
Amount recycled to surface reservoir by magmatism
H2O 40–120% of dike/gabbro H2O
20–80% of total
CO2 ~ 50%
S ~ 20%
Cl ~ 100%
• Inputs include structurally bound volatiles in subducted sediment
& altered oceanic crust (Hilton et al., 2002; Jarrard, 2003)
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 determinations
of the effect of H2O on the
peridotite solidus
Wet solidus
Dry solidus
From Grove et al. (2006)
Effect of H2O on Isobaric Partial Melting of Peridotite
Hirschmann et al. (1999)
1 GPa
Xitle
• Increasing H2O has a linear effect on degree of melting
(Hirose & Kawamoto, 1995; Hirschmann et al., 1999)
Effect of H2O on Isobaric Partial Melting of Peridotite
Hirschmann et al. (1999)
1 GPa
Mariana Trough data from Stolper & Newman (1994)
Effect of H2O on Isobaric Partial Melting of Peridotite
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
A model for hydrous flux melting of the mantle wedge
• Fluids and/or hydrous melts
percolate upward through the
inverted thermal gradient in
the 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
Do fluids and melts move vertically upward through the mantle wedge?
No, solid mantle flow deflects hydrous
fluids from buoyant vertical migration
through the wedge
Solid mantle flow also deflects partial
melts formed in the hottest part of the
wedge back towards the trench
From Cagnioncle et al. (2006)
And finally, mafic arc magmas have enough H2O to cause
explosive eruptions (violent strombolian, sub-plinian, and
occasionally plinian) that produce large amounts of ash and lapilli