The Role of Magmatic Volatiles in Arc Magmas

1
The Role of Magmatic Volatiles in Arc
Magmas Paul Wallace University of Oregon
2
Volatile Recycling Subduction Zone Magmatism
Components in downgoing slab Sediment Altered
oceanic crust Serpentinized upper mantle (?)
3
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.
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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!
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How do we measure volatile concentrations in
magmas?

• Melt inclusions
• Submarine pillow glasses
• Experimental petrology

100 mm
Moore Carmichael (1998)
Phase equilibria for basaltic andesite
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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
7
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.
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Experimental and natural polyhedral olivine with
melt inclusions (slow cooling)
100 mm
Keanakakoi Ash, Kilauea, Hawaii
Experimental natural skeletal (hopper
morphology) olivine with melt inclusions (faster
cooling)
Keanakakoi Ash
Paricutin, Mexico
500 mm
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Post-Entrapment Modification of Melt Inclusions
Ascent Eruption
Slow Cooling
Inclusion entrapment
Vapor bubble
Crystal
Diffusive exchange
Melt inclusion
Crystallizaton along melt crystal interface
Volatile leakage if inclusion ruptures
Crystallization possible further leakage
10
Volcanic gases - another way to get information
on volatiles
Ground airborne remote sensing
Satellite-based remote sensing Direct sampling
analysis
COSPEC at Masaya
TOMS data for El Chichon Pinatubo
Sampling gases at Cerro Negro
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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.
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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)
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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
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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 lt0.25 wt Cl and thus are not
saturated with hydrosaline melt
15
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
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Sulfur Solubility
S solubility is more complicated because of
multiple oxidation states Dissolved S occurs as
either S2- or S6 Solubility is limited by
satn with pyrrhotite, Fe-S melt, anhydrite, or
CaSO4 melt S in vapor phase occurs primarily as
H2S and SO2
Basaltic glasses
Minerals
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
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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
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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-.
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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
20
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 Earths atmosphere
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VaporMelt 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)
Isopleths of Constant Svapor / Smelt
Temperature (C)
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S Contents of Magmatic Vapor Phase for
Intermediate to Silicic Magmas
From Wallace (2003)
STotal (mol) in vapor
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
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• 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

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Degassing of low-H2O basaltic magma (Kilauea)
Fraction remaining (C / Cinitial)
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.
25
Degassing of H2O-rich rhyolitic magma
Fraction remaining (C / Cinitial)
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
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Volatile contents of mafic arc magmas based on
melt inclusions
100 mm
100 mm
Jorullo volcano, Mexico
Blue Lake Maar, Oregon Cascades
Photos by Emily Johnson, Univ. of Oregon
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H2O CO2 in Melt Inclusions from Jorullo
Volcano, Mexico
Vapor saturation isobars from Newman Lowenstern
(2002)
All data by FTIR
CO2 (ppm)
Avg. error
H2O (wt.)
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
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Degassing Paths During Magma Ascent
Crystallization
Degassing paths calculated using Newman
Lowenstern (2002)
Initial melt
CO2 (ppm)
H2O (wt.)
Johnson et al. (in press)
Some data cannot be explained by simple
degassing models
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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.
30
Mariana arc
H2O contents of arc basaltic magmas are quite
variable CO2 contents are lower than estimates
based on global arc CO2 flux
31
Arc basaltic magmas CO2 0.61.3 wt.
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
32
Arc basaltic magmas CO2 0.61.3 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
33
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
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Fluid Inclusions in Eclogites as Analogues for
Subduction Zone Fluids
Data from Philippot et al. (1998)
Low Salinity Fluids 3.14.0 NaCl
Eclogites from exhumed subduction complexes
contain fluid inclusions that represent samples
of fluids released during dehydration of
metabasalt
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S contents of arc magmas are typically higher
than for MORB, but in most cases not nearly as
enriched as is observed for Cl
36
Sulfur concentrations in melt inclusions
submarine basaltic glasses
5970
S (ppm)
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)
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Comparing inputs and outputs of volatiles in
subduction zones
Measuring volatile fluxes from arc volcanism -
one method
Modified from Fischer et al. (2002)
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
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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
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Fluxes of Major Volatiles from Subduction-related
Magmatism
Gas Flux Composition
W
Assuming 24 km3/yr magma flux
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Input vs. Output for Major Volatiles in
Subduction Zones
Amount recycled to surface reservoir by
magmatism H2O 40120 of dike/gabbro H2O
2080 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)
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CO2 Input vs. Output for Individual Arcs
Data from Hilton et al. (2002)
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How does addition of H2O to the mantle wedge
cause melting?
Experimental determinations of the effect of H2O
on the peridotite solidus From Grove et
al. (2006)
Wet solidus
Dry solidus
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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)
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Effect of H2O on Isobaric Partial Melting of
Peridotite
Hirschmann et al. (1999)
1 GPa
Mariana Trough data from Stolper Newman (1994)
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Effect of H2O on Isobaric Partial Melting of
Peridotite
To get the high H2O contents of arc magmas, H2O
must be added to the mantle either by aqueous
fluid or hydrous melt
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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)
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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
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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)
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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