The importance of volatiles in supervolcano systems

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The importance of volatiles in supervolcano
systems John Stix McGill University 3 February
2006
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Volatile elements in supervolcano systems

• This presentation examines three aspects of
  volatile behaviour in supervolcano-related
  magmas
• Degassing behaviour of magmas
• Rheological response of magmas to volatiles and
  degassing
• The influence of dissolved and exsolved volatiles
  during magma interactions
• With some understanding of the parameters above,
  we can start to predict the behaviour of natural
  magmas as they are transported and stored beneath
  supervolcanoes.

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1. Degassing behaviour of magmas

• In the next four slides, I illustrate different
  pathways of degassing. The two main variables
  are
• Degassing of magma as it ascends and decompresses
• Degassing of magma when it ponds in a magma
  reservoir
• These contrasting styles are important, because
  we know magmas both rise and pond at various
  times in their histories.
• Another crucial variable is whether the magma
  contains CO2 or not, because the degassing paths
  are significantly different. Examine these
  differences carefully in the following slides.

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Decompression degassing of a H20-CO2-bearing
magma note the strong depletion of CO2 but only
small reduction of H2O during depressurization
Saturation curves of H2O and CO2 solubility in
rhyolite
From http//volcanoes.usgs.gov/staff/jlowenstern
/Melt20Inc20Page/fluidsat.html
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Decompression degassing of CO2-free magma H2O
strongly depleted during depressurization
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Isobaric degassing during crystallization of an
H2O-CO2 bearing magma note the CO2 depletion
and H2O enrichment (with attendant rheological
effects)
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Isobaric degassing during crystallization of a
CO2-absent magma here the water content of the
magma is buffered at a constant value
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2. Rheological response of magmas to volatiles
and degassing
Volatiles, principally water, are probably the
most important control on a magmas viscosity.
Small changes in water contents, particularly at
low values, can have a huge effect upon a magmas
rheology. In the following slides, I will
illustrate the contrasting and very interesting
role of volatiles (a) at deep crustal levels, and
(b) as a magma ascends from deep levels to erupt
at the surface.
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Magma viscosities at high water contents
Consider a magma at 200-300 MPa (2-3 kbar)
pressure. Lets assume that the magma is
CO2-free, which means that the dissolved water
contents can build to their maximum values of
around 7 wt . Lets also consider that the magma
is hot at about 1000ºC (since its deep) and
crystal-poor. Under these conditions, the
viscosity of the magma is slightly less than 1000
Pa s. Contrast this value with the viscosity of
basaltic magma, which typically ranges from
100-1000 Pa s. The conclusion to be drawn here
is that under certain conditions, i.e., rhyolite
magmas stored in mid-crustal reservoirs, the
viscosity difference between rhyolitic and
basaltic magma is small.
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Magma ascent
Once magma starts to rise, its rheology will
change significantly. This is because volatile
solubility in magma, again principally water, is
a strong function of depth and pressure. So as a
magma rises, it essentially dewaters. As it
dewaters, its rheology changes significantly. And
the changes will be most pronounced at shallow
depths, since water solubility is highly
sensitive to depth and pressure here.
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Changing magma rheology during magma ascent

• The dewatering effect during magma ascent and
  decompression causes three things to happen in
  terms of a magmas rheology
• As the melt phase loses dissolved volatiles, its
  viscosity increases.
• The dewatering process causes the magmas
  liquidus temperature to increase, resulting in
  crystallization. The formation of crystals
  (typically microlites) further increases the
  magmas viscosity.
• Crystallization may drive the residual melt
  towards a more silica-rich composition. The
  increase in silica causes the magma to become
  polymerized and hence more viscous.

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The products of slow magma ascent
crystallization and eruption
This photomicrograph of 1986 Mt St Helens dacite
lava shows the highly crystallized nature of this
rock, as a result of slow magma ascent through
the crust. This is a good example of a
dewatered rock.
http//www.answersingenesis.org/tj/v10/i3/argon.as
p
The factors discussed on the previous slide
cause a rheological stiffening of the magma as
it ascends. In essence, the magma becomes less
eruptible, and if it does erupt, there is a
greater chance it will do so explosively rather
than effusively. I have not discussed the effect
of vesicles, which is an interesting topic in
itself. A vesicular magma in a conduit may be
more fragile than a non-vesicular magma. A
stiffened vesicular magma may be particularly
prone to erupting explosively.
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3. The influence of dissolved and exsolved
volatiles during magma interactions
With some understanding of volatile solubilities
in magmas and the attendant rheological effects,
we can start to think about magma interactions,
particularly if one magma is more volatile-rich
than another magma. Consider the injection of a
rhyolite magma into a resident magma reservoir
consisting of rhyolite with identical bulk
composition. The intruding magma is hotter and
more volatile-rich than the resident magma. It is
thus more buoyant than the resident magma and
will tend to rise. Consider three things. (1)
What will be the effects if the intruding magma
is degassing as it intrudes (it is ascending and
decompressing)? (2) How will the the intruding
magma behave if it ascends (a) through liquid
magma, and (b) through mushy magma? (3) What is
the nature of the magma interactions???
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Injection of gas plumes into a magma reservoir
Here, an experimental bubble plume is injected
into a reservoir. These experiments were designed
for basaltic magmas, but it is important to think
about similar processes for silicic magma systems.
Sequence of photographs showing the development
of the turbulent bubble plume and the return flow
in the experimental tank. The upper 1 cm of the
fluid in the tank was dyed red to enable
visualisation of the return flow. The bubble
plume is viewed along axis in these photographs,
and appears anomalously white relative to the
surrounding fluid. From Phillips and Woods 2001,
EPSL 186, 297-309.
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Gas sparging percolation of gas through a magma
Bachman and Bergantz 2003, Geology 31, 789-792
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Concluding remarks
1. How are volatiles transferred from one magma
to another? Is the process mainly one of exsolved
gas moving as bubbles through the system? Or is
there an important component of volatile
diffusion, whereby volatiles are able to diffuse
from one melt to the other? 2. What are the
implications of these processes for excess sulfur
emitted by volcanoes? What proportion of this
sulfur is derived from resident magma, and what
proportion from replenishing magma?
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