Lecture 13 Silicate Magmas Basic structural unit of silicates solid amp liquid is the silica tetrahedron These are variously joined by shared or bridging oxygens to form various structures in solids and liquids ID: 588436
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Slide1
Thermodynamic Models of Magmas
Lecture 13 Slide2
Silicate Magmas
Basic structural unit of silicates (solid & liquid) is the silica tetrahedron
These are variously joined by shared, or bridging, oxygens, to form various structures in solids and liquids.
Basic difference between solids and liquids is lack of long-range structure in the latter.
Liquids structure can be studied by quenching them to glass.Slide3
Liquid Structures
Bridging oxygens and joining tetrahedra results in
polymerization
of the melt, changing its properties.
Al3+, Ti, and Fe3+ can promote polymerization and, along with Si, are called network-forming ions.Other ions, Ca2+, Mg
2+, Fe2+, Na+, K+
, and H
+
tend to break up this structure and are called network modifiers.Slide4
Modeling Silicate Liquids
Problems are:
Decide on the components
Determine the nature of the model
Ghiorso et al. adopt a regular solution model for their MELTS model.Determine the interaction parameters from experimental data.The resulting program then iteratively computes free energy of the liquid plus free energy of all possible precipitating solids and calculates the equilibrium assemblage based on the principles that
the stable assemblage is the one with the lowest free energy.The chemical potentials of components in coexisting phases are equal.
Silicate liquids are complex solutions of many components.
Solids crystallizing from them are generally solutions themselves.
Generally these solutions cannot be treated as ideal.
Crystallization (or melting) occurs over a wide range of T (400-500˚C)
.Slide5
MELTS Model
Free energy of the liquid solution is:
Activity coefficients calculated as:
For network modifiers,
Ghiroso chose silicate components such as CaSiO3, Mg2SiO4, Na
2SiO3, KAlSiO4, etc. because mole fractions of individual oxides tend to be small numbers, reducing influence of interaction parameters.
Network formers generally just the oxides (e.g., Al
2
O3
).
Eleven components, plus water treated separately.Slide6
Components & Interaction ParametersSlide7
Will Plagioclase Precipitate?
For
anorthite
, reaction of interest is:
CaSiO3(l) + Al2O3(l) + SiO2(l
) ⇋ CaAl2Si2O
8(
plag
)But
plag
is usually a solid solution, so:
x[½NaSiO
3
+ ½Al
2
O
3(l) + 2½SiO2(
l)] + y[CaSiO3(
l) + Al2O3(l) + SiO2(l)]
⇋
[yCaAl
2
Si
2
O
8
–xNaAlSi
3
O
8
]
(
plag
)
What thermodynamic condition must be met for plagioclase to precipitate?
∆G
r
is negative.
The negative of
∆G
r
is often referred to (particularly in MELTS lingo) as the affinity of reaction,
A
r
.
Melts calculates affinities for all possible reactions.
If a plagioclase crystal has precipitated from a magma under equilibrium conditions, what can we say about the component?
Chemical potentials of components must be equal in both solid and liquid.Slide8
pMELTS
Predicted and actual pyroxene compositions in lavas.
Predicted and actual SiO
2
concentrations in experimental melts of peridotite as a function of melt percent.