ERTH 3020: METAMORPHIC REACTIONS

1
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
2
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!!)
3
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   at equilibrium, for specified P-T-X,
the stable mineral assemblage (P,,C) can be
predicted by the phase rule in combination with
thermodynamics
4
ERTH 3020 METAMORPHIC REACTIONS
How are stable mineral assemblages
recognized?   - textural criteria -
compositional criteria - thermodynamic criteria
5
ERTH 3020 METAMORPHIC REACTIONS
Textural criteria - no relict or overprinting
minerals or textures - no obvious reaction
textures - mutual contacts among phases and grain
boundary network - textural equilibrium
(e.g., granoblastic textures)  
6
ERTH 3020 METAMORPHIC REACTIONS
Textural criteria - no relict or overprinting
minerals or textures - no obvious reaction
textures - mutual contacts among phases and grain
boundary network - textural equilibrium
(e.g., granoblastic textures)   Compositional
criteria - minerals (and rock) chemically
homogeneous on all scales -
compatibility with phase rule (chemographic
diagrams)  
7
ERTH 3020 METAMORPHIC REACTIONS
Textural criteria - no relict or overprinting
minerals or textures - no obvious reaction
textures - mutual contacts among phases and grain
boundary network - textural equilibrium
(e.g., granoblastic textures)   Compositional
criteria - minerals (and rock) chemically
homogeneous on all scales - compatibility
with phase rule (chemographic diagrams)   Thermody
namic criteria - compatibility with assemblages
predicted from thermodynamics
8
ERTH 3020 METAMORPHIC REACTIONS
In practice - much easier to recognize textural
/ mineralogical disequilibrium than to
demonstrate equilibrium - most rocks contain at
least some relict / overprinting minerals or
textures, or compositional heterogeneities - try
to recognise domains or subdomains (rock or thin
section scale) that may approach equilibrium
9
ERTH 3020 METAMORPHIC REACTIONS
Changes in mineral assemblages   a) with time -
for a given volume of rock, T and P change with
time - burial, heating, exhumation, cooling -
prograde ( T ? ) and retrograde ( T ? )
reactions and resulting assemblages - P-T-t-d
paths
10
ERTH 3020 METAMORPHIC REACTIONS
Changes in mineral assemblages   b) with space -
spatial variations in lithology (X) or P-T
conditions - lithological variations considered
in terms of metamorphic facies (sets of
assemblages) - P-T conditions may reflect
proximity to plutons, regional variations in
P-T are related to orogenic processes, etc. -
for a given lithology, map isograds based on
distribution of assemblages
11
ERTH 3020 METAMORPHIC REACTIONS
PHASE RULE IN METAMORPHISM (Winter Ch.
24.1)   What is the phase rule?
12
ERTH 3020 METAMORPHIC REACTIONS
PHASE RULE IN METAMORPHISM (Winter Ch.
24.1)   What is the phase rule?
F C - F 2 (or F C - P 2)
F ? C ? F ? why 2 ?
13
ERTH 3020 METAMORPHIC REACTIONS
PHASE RULE IN METAMORPHISM (Winter Ch.
24.1)   What is the phase rule?
F C - F 2 (or F C - P 2)
what is a system? what is a phase? what is a
component?
14
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic
systems phases - physically separable parts of
system (minerals, fluids) components -
chemical constituents of phases
15
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic
systems phases - physically separable parts of
system (minerals, fluids) components -
chemical constituents of phases
SiO2 Al2SiO5 KAlSi3O8 (K,Na)AlSi3O8 KAl3Si3O10(OH)
2 (K,Na)Al3Si3O10(OH)2
16
ERTH 3020 METAMORPHIC REACTIONS
PHASE RULE IN METAMORPHISM (Winter Ch.
24.1)   What is the phase rule?
F C - F 2 (or F C - P 2)
On a P-T phase diagram where F 2 - divariant
field F 1 - univariant line F 0 -
invariant point
17
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
1-component systems
components? phases?
C 1 ? F
18
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
2-component systems
Or
Ab
Or60Ab40
components? phases?
C 2 ? F
19
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
2-component systems
MgO
H2O
Mg(OH)2
C 2 ? F
components? phases?
20
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
3-component systems
CaO
SiO2
CO2
components? phases?
C 3 ? F
P-T diagram
21
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
3-component systems
chemographic (compatibility) diagram
components? phases?
C 3 ? F
P-T diagram
22
ERTH 3020 METAMORPHIC REACTIONS
F C - F 2 In general F 2  (multivariant
assemblages) F C (mineralogical phase
rule)  F C (divariant assemblage)    
where F lt C solid solution     where F gt
C F lt 2 or disequilbrium or C estimated
incorrectly
23
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
chemographic (compatibility) diagram
3 component system (apices of triangle) phases
do not exhibit solid solution (plot as points)
24
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
chemographic (compatibility) diagram
3 component system (apices of triangle) phases
do not exhibit solid solution (plot as
points) tie lines link coexisting
phases (assemblages)
25
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
chemographic diagrams can be used along with P-T
diagrams to illustrate reactions
tie lines join coexisting phases change in
arrangement of tie lines indicates reaction
involving phases at ends of tie lines
26
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
chemographic diagrams can be used along with P-T
diagrams to illustrate reactions
"tie-line flip" reaction
tie lines join coexisting phases change in
arrangement of tie lines indicates reaction
involving phases at ends of tie lines
27
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
chemographic (compatibility) diagram
3 component system (apices of triangle) phases
exhibit solid solution (plot as fields) tie
lines form arrays linking coexisting solid
solutions
28
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
chemographic (compatibility) diagram
more than 3 components? (e.g., 4 tetrahedron)
must project from one phase or component onto a
face of the tetrahedron projecting
phases/components must be present in assemblage
considered "in excess"
x'
29
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
commonly used chemographic diagrams
ACF diagram used for metabasites and calcareous
rocks (will be considered later in term)
30
ERTH 3020 METAMORPHIC REACTIONS
Phases and components in metamorphic systems
commonly used chemographic diagrams
AFM diagram used for pelites (will be
considered later in term)
31
ERTH 3020 METAMORPHIC REACTIONS
REACTIONS THE PHASE RULE (W Ch. 26)
Types of metamorphic reactions - based on
effect - exchange vs net-transfer based on
reaction progress - continuous vs discontinuous
based on phases involved - solid-solid -
net-transfer, polymorphic - devolatilisation -
dehydration, decarbonation - mixed volatile -
terminal vs tie-line flip - ion exchange