ERTH 3020: METAMORPHIC REACTIONS

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ERTH 3020 METAMORPHIC REACTIONS
Winter, Ch. 24 - Stable Mineral
Assemblages Winter, Ch. 26 - Metamorphic
Reactions Winter, Ch. 27 - Thermodynamics of
Reactions

• What is a mineral assemblage?
• What determines relative stabilities of
  chemically
• equivalent mineral assemblages?
• What controls metamorphic reactions?
• How are reaction boundaries determined?
• How do we evaluate P-T stability of mineral
• assemblages?

Thermodynamics website (very useful!)
http//serc.carleton.edu/research_education/equili
bria/index.html
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ERTH 3020 METAMORPHIC REACTIONS
MINERAL ASSEMBLAGES (Winter ch.
24.1)   Metamorphic grade defined on basis of
stable mineral assemblages (equilibrium) - for a
given X, stable assemblage controlled by T, P -
changes in T,P ? changes in mineral assemblage
via metamorphic reactions
mineral assemblage - set of minerals present in a
rock and inferred to have coexisted in chemical
equilibrium (not necessarily all the minerals now
present in the rock!!)
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ERTH 3020 METAMORPHIC REACTIONS
Mineral assemblage defined by - kinds of
minerals present (phases) - their relative
proportions (modes) - their chemical
compositions (components in solid solutions)
- NOT defined based on texture
(but commonly recognised from textures)   at
equilibrium, for specified P-T-X, the stable
mineral assemblage (P, , C) can be predicted by
the phase rule in combination with
thermodynamics
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ERTH 3020 METAMORPHIC REACTIONS

• What controls metamorphic reactions?
• Why determines the position and form of a
  reaction
• boundary in P-T space?
• How do we use reactions and assemblages to
• work out P-T conditions?
• 1. Thermodynamic Controls (W 27.1)
•   
• 2. Calculating Reaction Boundaries (W 27.1)
•    
• 3. P-T Determinations
• a) petrogenetic grids (W 26.11)
• b) thermobarometry (W 27.4)

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ERTH 3020 METAMORPHIC REACTIONS

• How do we use reactions and assemblages to
• work out P-T conditions?
• P-T Determinations
• a) petrogenetic grids (W 26.11)
• b) thermobarometry (W 27.4)

a) What is a petrogenetic grid? How are they
constructed? How are they used? How do
they differ from "pseudosections"?
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ERTH 3020 METAMORPHIC REACTIONS
how can we calculate the position of a reaction
in P-T space? (Winter Ch. 27.1)
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ERTH 3020 METAMORPHIC REACTIONS
how can we calculate the position of a reaction
in P-T space? (Winter Ch. 27.1)
how can we use the calculated reactions to
estimate P and T?
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ERTH 3020 METAMORPHIC REACTIONS
Method 1. compare observed assemblage with
stability field on P-T diagram
jadeite quartz albite jadeite quartz albite
andalusite kyanite sillimanite
kyanite andalusite kyanite sillimanite
univariant, or better yet, invariant assemblages
provide the best P-T constraints
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ERTH 3020 METAMORPHIC REACTIONS
Method 1. compare observed assemblage with
stability field on P-T diagram
adding more reactions to simple phase diagram
further restricts possible P-T range
A
C
Assemblage andalusite Kfs Qtz kyanite Ms
Qtz sillimanite Ms Qtz andalusite
sillimanite Kfs Qtz
B
D
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ERTH 3020 METAMORPHIC REACTIONS
Method 1. compare observed assemblage with
stability field on P-T diagram
adding more reactions to simple phase diagram
further restricts possible P-T range
A
C
Assemblage andalusite Kfs Qtz (B) kyanite
Ms Qtz (C) sillimanite Ms Qtz
(A) andalusite sillimanite Kfs Qtz (D)
B
D
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a specific
rock type
normally show all possible reactions for a
specified chemical system (e.g.,
Na2O-CaO-MgO-Al2O3-SiO2-H2O)
petrogenetic grid for metabasites
Winter Fig. 26-19
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a specific
rock type
normally show all possible reactions for a
specified chemical system (e.g.,
K2O-Na2O-FeO-MgO-Al2O3-SiO2-H2O)
petrogenetic grid for pelites
Winter Fig. 28-2
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a specific
rock type
normally show all possible reactions for a
specified chemical system (e.g.,
Na2O-CaO-MgO-Al2O3-SiO2-H2O)
petrogenetic grid for metabasites
how do you deal with something like this????
Winter Fig. 26-19
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a specific
rock type
normally show all possible reactions for a
specified chemical system (e.g.,
Na2O-CaO-MgO-Al2O3-SiO2-H2O)

• how to simplify?
• limit to a certain P-T
• range of interest
• limit number of
• components
• limit number of phases
• build up grid one reaction
• at a time

Winter Fig. 26-19
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a specific
rock type

• how to simplify?
• limit to a certain P-T
• range of interest
• limit number of
• components
• limit number of phases
• build up grid one reaction
• at a time

Al2SiO5 Kfd H2O
Ky
Ms Qtz
Sil
And
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ERTH 3020 METAMORPHIC REACTIONS
What is a pseudosection and how does it
differ from a petrogenetic grid?
petrogenetic grid considers all reactions in
specified system pseudosection considers
only those relevant to a specified bulk
composition
can be plotted as T-X diagrams P-T diagrams
Winter Fig. 28-12
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ERTH 3020 METAMORPHIC REACTIONS
What is a pseudosection and how does it
differ from a petrogenetic grid?
petrogenetic grid considers all reactions in
specified system pseudosection considers
only those relevant to a specified bulk
composition
can be plotted as T-X diagrams P-T diagrams
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ERTH 3020 METAMORPHIC REACTIONS
What is a pseudosection and how does it
differ from a petrogenetic grid?
Al2O3-SiO2-H2O
pseudosection - reactions for specified
composition
petrogenetic grid - all reactions
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a specific
rock type
simplified petrogenetic grid for pelites
K2O-FeO-Al2O3-SiO2-H2O
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a specific
rock type
simplified petrogenetic grid for pelites
Fe-staurolite
K2O-FeO-Al2O3-SiO2-H2O
Fe-cordierite
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ERTH 3020 METAMORPHIC REACTIONS
Petrogenetic Grids (W Ch. 26.11)
P-T diagrams for multicomponent systems that
show a set of reactions, generally for a
specific rock type
Na2O-CaO-MgO-Al2O3-SiO2-H2O
"simplified" petrogenetic grid for metabasites
Grt Cpx Qz Opx Plag
Hbl Ep Qz Cpx Plag H2O
Grt Hbl Qz Opx Plag H2O
Hbl Qz Cpx Opx Plag H2O
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ERTH 3020 METAMORPHIC REACTIONS
petrogenetic grid example using WINTWQ
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ERTH 3020 METAMORPHIC REACTIONS