Magmas and their physical properties

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Magmas and their physical properties

• definition of magma
• field evidence of diversity in behavior of magmas
• range of compositions of magmas
• atomic/molecular structures of silicate liquids,
  network-forming vs. network modifying components
• physical properties -- viscosity (P,T,X) density
  (P,T,X)
• graphical representation of chemical composition
• norms and classification

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Definition of magma
Naturally occurring mobile rock material,
generated within the earth and capable of
intrusion and extrusion, through which igneous
rocks are thought to have been derived through
solidification and related processes. It may or
may not contain suspended solids (such as
crystals and rock fragments) and/or gas phases.
Glossary of Geology, 3rd edition.
Magmas are highly diverse in their physical
properties, as manifested by their eruptive
styles. What factors lead to this diversity?
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Hawaiian lava flows - 1
note the highly fluid nature of these lavas (50
SiO2)
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Hawaiian lava flows - 2
pahoehoe
aa
even among Hawaiian lava flows (or even in a
single flow) there are significant differences in
flow properties
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Ol Doinyo Lengai volcano, Tanzania
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Ol Doinyo Lengai summit crater
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Ol Doinyo Lengai eruptions and lava flows - 1
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Ol Doinyo Lengai eruptions and lava flows - 2
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Panum Crater and lava dome

• very viscous silicic magma (75 SiO2) forms a
  thick,
• -steep-sided flow (called coulees in the Mono
  craters)

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pyroclastic flows associated with lava domes - 1
Unzen volcano, 1990s
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pyroclastic flows associated with lava domes - 2
Unzen volcano, 1993
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Mont Pelée spine
60 SiO2
In 1903 the spine grew up to 50ft/day to 1,020ft
above the crater.
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Montserrat spine
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Fire fountains and explosions of basaltic magma
Eldfell, Heimaey, Iceland
Kilauea
Stromboli
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Plinian eruptions
Anak Krakatau, 1979
Mt. Spurr, Alaska, 1992
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pyroclastic flows from Plinian eruptions
Mayon Volcano, Philippines, 1984
Mont Pelée, 1902
simulation
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we will now try to relate some of these phenomena
to the relationships between physical properties
of melts and melt composition, temperature, and
pressure
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what is the distribution of igneous rock
compositions, and how do physical properties vary
with composition?
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the Daly gap
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viscosity is the physical property that most
influences the observed eruptive behavior of
magmas

• viscosity depends on chemical composition (most
  strongly correlated with SiO2 content and
  volatile content which obviously has a major
  influence on eruptive style as well),
  temperature, pressure, and strain rate

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definition of viscosity
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why should melt viscosity depend on composition?

• need to think about molecular level melt
  structure to understand the connection

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cartoon of crystalline and molten SiO2 structure
fully polymerized crystal
fully polymerized melt
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what are the effects of compositional variation
away from SiO2? - 1
balanced addition of alkalis and aluminum retains
the fully polymerized network
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what are the effects of compositional variation
away from SiO2? - 2
but addition of network modifiers (e.g., H2O,
Na2O, MgO, etc.) leads to depolymerization
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contrasting structures of melts rich in
network-forming and network-modifying components
fully polymerized
depolymerized
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dependence of liquid viscosity on chemical
composition
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viscosity variations of residual liquids from
fractionation
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effect of dissolved water on melt viscosity
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pressure dependence of water solubility in
rhyolitic melt
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why is water solubility quadratic in pressure?
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Panum Crater and lava dome
explosive eruption of initially volatile-rich
(lower viscosity) magma followed by a
volatile-poor (high viscosity) dome the low
volatile content could be due to degassing or to
a zoned magma chamber
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note that CO2 solubility increases linearly with
pressure

• CO2 dissolves as CO2 molecules
• so the solubility reaction is CO2 (vapor)
  CO2 (melt)with an equilibrium constant
  K1 XCO2 (melt)/pCO2 and XCO2 (melt) a
  pCO2
• Henrys law
• knowledge of molecular level processes makes a
  difference, as the contrast with water solubility
  demonstrates

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effect of pressure on viscosity
depolymerized melts
polymerized melts
possible explanations coordination change with
pressure? open structure of highly polymerized
structures?
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effects of temperature
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effects of crystallinity
could this play a role in the formation of the
Mount Pelée spine
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Newtonian vs. non-Newtonian viscosity
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non-Newtonian behavior of highly polymerized melts
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effect of crystallinity on yield strength
could this play a role in the formation of the
Mount Pelée spine? in the transport of xenoliths
from great depth by magmas?
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melt density another critical physical property

• how does it vary with composition, temperature,
  and pressure?

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the contrast in density between melt and crystals
is a critical control on fractionation
melt segregation
crystal settling
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density varies significantly with melt composition
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systematics in variations of density with
composition
r m/V

• it turns out that if we define the compositional
  components based on a single oxygen basis (e.g.,
  Si1/2O rather than SiO2, Al2/3O vs. Al2O3, etc.,
  most silicate melts have roughly the same partial
  molar volume(i.e., the volume occupied by an
  oxygen ion is roughly the same in all magmas we
  can think of these liquids as packings of oxygens
  with cations in the spaces between them), so the
  denominator is roughly constant (N.B. the
  alkalis are exceptions, they are very fluffy.)
• but the molecular weights are very different
  (e.g., Mi is 30 g/mol for Si1/2O and 72 g/mol
  for FeO), so the key control on density is the
  molecular weight (per unit oxygen) of the
  component added or subtracted from the melt
  I.e., is the component heavy or light

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fractionation densities
VI coordinated
IV coordinated
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variations in melt density due to crystallization
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density and viscosity variations of residual
liquids
olivine
oxides
pyroxene plagioclase
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have we found a way to explain the Daly gap?
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could density or viscosity variations of residual
liquids help to explain the Daly gap?
too dense to erupt
crustal density
eruptible
too viscous to erupt
viscosity decreases due to build up in H2O
critical viscosity
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the effect of temperature on density is
relatively small
aP 1/V (?V/?T)P isobaric thermal
expansivity or the isobaric coefficient of
thermal expansion 2(10-5) C-1 in melts
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the effect of pressure on melt density is large
bT -1/V (?V/?P)T isothermal
compressibility 0.005 kbar-1 in melts
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compressibility of melts is much larger than than
of crystals
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why do melts compress more than solids?
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petrological importance of melt compression
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melt densities at higher pressures
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significance for a terrestrial magma ocean?
might this contribute to the olivine-rich nature
of the upper mantle?
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does the higher compression of the melt continue
indefinitely?
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if pressure gets high enough, the solid density
eventually overtakes that of the liquid

• coordination change of melt nears completion
• phase changes in the solid

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How do we show composition graphically?

• Suppose we want to express compositions that
  mixtures of silica (SiO2) and sodium aluminate
  (NaAlO2 ).
• Choosing Si4O8 (equivalent to SiO2) and Na4Al4O8
  (equivalent to NaAlO2 ) as components, we can
  express the compositions of mixtures by the
  concentration of NaAlO2 in the mixture
  X(Na4Al4O8) n(Na4Al4O8)/n(Na4Al4O8)
  n(Si4O8), where n(Na4Al4O8) and n(Si4O8) are
  the number of units (e.g.,. grams, moles, etc.)
  in the mixture.
• For example, consider albite (NaAlSi3O8)
  NaAlSi3O8 0.25 Na4Al4O8 0.75 Si4O8or
  
  X(Na4Al4O8) 0.25.
• Note that even though there are two components,
  X(Si4O8) 1 - X(Na4Al4O8), so only one
  compositional coordinate is needed to express
  composition (e.e., compositional variation is
  one-dimensional.

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the silica-sodium aluminate join-1
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the lever ruledetermining composition
graphically
0
1
0.25
0.33
0.50
Si4O8 (SiO2) quartz
Na4Al4O8 (NaAlO2) sodium aluminate
NaAlSi3O8 albite
Na4/3Al4/3Si8/3O8 (NaAlSi2O6) jadeite
Na2Al2Si2O8 (NaAlSiO4) nepheline
X(Na4Al4O8) a/b
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suppose we want to know how much jadeite and
nepheline would be in a rock with 40 NaAlO2?
X(Na4Al4O8)
Qtz
NA
Ab
Jd
Ne
X
0.40
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we could also do it algebraically
starting with theinitial coordinates nNA ( of
moles of Na4Al4O8) and nQtz ( of moles of
Si4O8) we want to convert to nNe ( of moles of
Na2Al2Si2O8) and nJd ( of moles of
Na4/3Al4/3Si8/3O8) of Nas (or Als) 4nNA
2nNe (4/3)nJd of Sis
4nQtz 2nNe (8/3)nJd solving for nNe and
nJd, we obtain nJd 3(nQtz - nNA) and nNe
4nNA - 2nQtz or XNe (4nNA - 2nQtz)/(nNA
nQtz) or (6nNA - 2) if as in this example nNA
nQtz 1 we can check if it works for Ne, nNA
0.5, plugging in gives XNe 1
for Jd, nNA 1/3, plugging
in gives XNe 0 we only need to check these two
to make sure it is correct
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we have just developed the concept of the norm
and how it is determined
X(NA)
Qtz
Ab
Jd
Ne
0
1
X
NA
1
0
X(Jd)

• given a chemical composition (usually as oxides),
  the idea is to recast the composition in terms of
  hypothetical minerals
• the first step is to choose the minerals into
  which the composition will be recast
• the next step is to figure out which minerals
  surround the composition to be recast
• the final step is to change the basis vectors
  (either algebraically or graphically) from the
  original components to the mineral components of
  interest
• there are usually gtgt 2 components, but the
  concept is the same

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the graphical representation of composition is
readily extended to 3 or more components
XA nA/(nAnBnC) XB nB/(nAnBnC) XC
nC/(nAnBnC) XA XB XC 1 therefore only
2-d, even though there are 3 components choose
any two components as basis vectors (and a unit
of quantity), then any composition can be
expressed as a vector sum Note just as with the
compositional line, points can have negative
coordinates (i.e., plot outside the triangle)
C
XB0.5 XC0.3
XC
A
B
XB
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variations on the theme
C
XB0.5 XC0.3
the axes need not be at 60 and the basis vectors
do not need to be the same length
XC
B
A
XB
C
XB0.5 XC0.3
compositions can be determined graphically by
decomposing a point into a vector sum of vectors
from the origin along the axes
XC
A
B
XB
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the norm and classification - 1
Qtz
SiO2
Ab
pyroxene Na2SiO3
X
X
olivine Na4SiO4
Ne
Al2O3
Na2O
NaAlO2
Co
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the norm and classification - 2mafic rocks
Qtz
SiO2
normative qtz (quartz tholeiite, andesite,
dacite, rhyolite)
Ab
normative olivine and pyroxene (olivine tholeiite)
pyroxene Na2SiO3
olivine Na4SiO4
normative nepheline (alkaline)
Ne
critical plane of silica saturation
Na2O
Al2O3
NaAlO2
Co
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the norm and classification - 3felsic rocks
Qtz
SiO2
granite/rhyolite
Ab
syenite/trachyte
pyroxene Na2SiO3
nepheline syenite/ phonolite
olivine Na4SiO4
Ne
Na2O
Al2O3
NaAlO2
Co