Transcript Chapter 21: Metamorphism - Central Washington University

• Quiz 2 is on Thursday, Nov. 14
• Exam 2 is on Thursday, Nov. 21
• HW 2 is due on Tuesday, Nov. 26, 5PM
Metamorphic Assemblages,
Reactions, and Equilibrium
• MUST MUST MUST read Chapter 19
Stable Mineral Assemblages in
Metamorphic Rocks
• What does equilibrium mean? Describe a
system in a state of equilibrium.
• How is equilibrium different? What are
examples of metastable systems on Earth’s
surface?
Equilibrium Mineral Assemblages
• At equilibrium, the mineralogy (and the
composition of each mineral) is determined by
T, P, and X (composition)
• Concept is important because many
metamorphic mineral assemblages appear to be
in equilibrium--so examining them can tell us
about P-T (and possibly X).
How Do We Evaluate Mineral
Assemblages and Equilibrium?
Chemographic Diagrams
Chemographics refers to the graphical representation
of the chemistry of mineral assemblages
A simple example: the MgO-SiO2 system as a linear
C = 2 plot:
Per
Fo
MgO
Mg2SiO4
En
MgSiO3
Qtz
SiO2
Example: CaO-MgO-SiO2 diagram
C =3 = ternary diagram!
Example: CaO-MgO-SiO2 diagram
Additional Information:
P, T fixed so this diagram is called
isothermal, isobaric diagram.
OR range of P, T small
Now consider:
Bulk composition (6
possible equilibrium
mineral assemblages)
Types of Metamorphic Reactions
A metamorphic reaction represents a change in
mineral assemblage that is brought on by a change in
pressure, temperature and/or composition.
The easiest way to understand what reaction is
occurring is to examine chemographic diagrams.
We will examine two types of metamorphic
reactions: tie-line switch, and terminal
appearance/disappearance
Types of Metamorphic Reactions:
Discontinuous or Univariant Reaction
Discontinuous reactions are recognized by distinct
changes in the field (in metamorphic zones) through
the appearance and disappearance of minerals
Two types of discontinuous reactions:
1. Terminal Reaction
2. Tie-Line Switch
Discontinuous Reaction: Terminal reaction
a
a
Consider a metamorphic system of bulk composition a. For this system,
H2O and CO2 are part of the assemblage but not plotted on the diagram.
The stable assemblage at the start of the reaction is:
Quartz + Talc + Dolomite + H2O + CO2
Discontinuous Reaction: Terminal reaction
a
a
At the end of the reaction, the mineral assemblage is:
Quartz + Tremolite + Dolomite + H2O + CO2
So the reaction is approximately:
Quartz + Talc + Dolomite --> Tremolite
Discontinuous Reaction: Terminal reaction
a
a
Note that you can determine the three mineral assemblage after the reaction
is completed by examining where the bulk composition lies within the
triangle.
In this case, you know the assemblage includes quartz-dolomite-tremolite
because the bulk composition, labeled “a”, sits inside that three phase or
three mineral triangle.
Discontinuous Reaction: Terminal reaction
a
a
Note that composition “a” contains more dolomite and quartz than talc.
Once talc is consumed, the reaction stops, leaving newly formed tremolite,
plus the left-over quartz and dolomite.
Called a terminal reaction because this marks the terminal appearance (or
disappearance) of tremolite for any bulk composition.
It is discontinuous because a mineral disappears and a new one appears.
Discontinuous Reaction: Tie-line switch
Type 2: A tie-line switch reaction involves changes in mineral
compatibility.
We will examine the same system as it is metamorphosed to higher
temperature.
Discontinuous Reaction: Tie-Line Switch
x
x
Now consider a metamorphic system of bulk composition x. H2O and CO2
are part of the assemblage but not plotted on the diagram.
The stable assemblage at the start of the reaction is:
Quartz + Calcite + Dolomite + H2O + CO2
Discontinuous Reaction:Tie-Line Switch
x
x
At the end of the reaction, the mineral assemblage is:
Tremolite + Calcite + Dolomite + H2O + CO2
So the reaction is approximately:
Quartz + Dolomite --> Tremolite + Calcite
Discontinuous Reaction:Tie-Line Switch
x
x
This reaction, which occurs when calcareous rocks are metamorphosed at
low-medium grade, is thought to be responsible for the first appearance of
tremolite in MgO-, SiO2-poor calcareous rocks.
Called a tie-line switch because the tie line connecting dolomite and quartz
switches to a tie-line connecting calcite and tremolite.
It is discontinuous because a mineral disappears and a new one appears.
Discontinuous Reaction:Tie-Line Switch
x
x
Finally, note that you can determine the three mineral assemblage by
examining where the bulk composition lies within the triangle.
In this case, you know the assemblage includes calcite-dolomite-tremolite
because the bulk composition, labeled x, sits inside that three phase or three
mineral triangle.
How can we tell if an assemblage is
in equilibrium?
• Theoretical analysis of thermodynamics
• Textures--some textures reflect equilibrium
states
• Experiments
The Phase Rule in Metamorphic Systems
• Phase rule, as applied to systems at equilibrium:
F=C-f+2
the phase rule
f = the number of phases in the system
C = the number of components: the minimum
number of chemical constituents required to
specify every phase in the system
2 = typically represents P and T of system
F = the number of degrees of freedom: the
number of independently variable parameters of
state
The Phase Rule in Metamorphic Systems





Think of components as ingredients (flour, sugar,
butter)
Think of phases as products (different types of
cookies)
So in a metamorphic system, components can be
SiO2, CaO, Al2SiO5
Phases are particular minerals like kyanite,
quartz
Different F --> how many parameters can
change?
Three Cases
F = 2 is the most common situation; the phase
rule tells us:
F = C - f + 2; then
f = C (case 1)
Number of components and phases equal in
a system where there are 2 degrees of
freedom (typically P and T can vary)
The Phase Rule in Metamorphic Systems
For case 1 (f = C):

The standard divariant situation

The rock probably represents an equilibrium
mineral assemblage from within a
metamorphic zone

We will see what this means on a P-T
diagram in a minute
The Phase Rule in Metamorphic Systems
F =1 (case 2)
The sample is collected from a location
right on a univariant reaction curve
(isograd).
F =0 (case 3)
The sample is collected from a location
right on a invariant point (e.g., triple
point).
The Phase Rule in Metamorphic Systems
Consider the following three scenarios:
C=1
 f = 1 common
 f = 2 rare
 f = 3 only at the
specific P-T
conditions of the
invariant point
(~ 0.37 GPa and
500oC)
Figure 21-9. The P-T phase diagram for the system Al2SiO5
calculated using the program TWQ (Berman, 1988, 1990,
1991). Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
The Phase Rule in Metamorphic Systems
2) Equilibrium has not been attained

The phase rule applies only to systems at
equilibrium, and there could be any number of
minerals coexisting if equilibrium is not attained
Chemographic Diagrams for
Metamorphic Rocks
• Most common natural rocks contain the major elements:
SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO, MnO and
H2O such that C = 9
• Three components is the maximum number that we can
easily deal with in two dimensions
• What is the “right” choice of components?
• Several “standard” ternary diagrams applied to
metamorphic rocks.
• Goal is to understand these; you will not have to derive
these…….but understand how to use them!
The ACF Diagram
• Illustrate metamorphic mineral assemblages in mafic rocks
on a simplified 3-C triangular diagram
• Concentrate only on the minerals that appeared or
disappeared during metamorphism, thus acting as
indicators of metamorphic grade
• So SOME minerals are not shown, but are ASSUMED to
be part of assemblage (e.g., quartz, muscovite). In these
cases, mineral names will be provided.
Figure 24-4. After Ehlers and Blatt
(1982). Petrology. Freeman. And
Miyashiro (1994) Metamorphic
Petrology. Oxford.
The ACF Diagram
• The points of ternary are defined as
components
• Calculated on an atomic basis:
A = Al2O3 + Fe2O3 - Na2O - K2O
C = CaO - 3.3 P2O5
F = FeO + MgO + MnO
The ACF Diagram
A = Al2O3 + Fe2O3 - Na2O - K2O
Why the subtraction?
• Na and K in the average mafic rock are typically
combined with Al to produce Kfs and Albite
• In the ACF diagram, we are interested only in the other Kbearing metamorphic minerals, and thus only in the
amount of Al2O3 that occurs in excess of that combined
with Na2O and K2O (in albite and K-feldspar)
• Because the ratio of Al2O3 to Na2O or K2O in feldspars is
1:1, we subtract from Al2O3 an amount equivalent to Na2O
and K2O in the same 1:1 ratio
The ACF Diagram
C = CaO - 3.3 P2O5
F = FeO + MgO + MnO
The ACF Diagram
By creating these “combined” components, Eskola reduced
the number of components in mafic rocks from 8 to 3
• Water is omitted under the assumption that it is perfectly
mobile
• Note that SiO2 is simply ignored

We shall see that this is equivalent to projecting from quartz
• In order for a projected phase diagram to be truly valid,
the phase from which it is projected must be present in
the mineral assemblages represented.
• What this means is that QUARTZ MUST BE PRESENT
for use of the ACF diagram.
How do we use this diagram?
Different bulk
compositions:
equilibrium
assemblage or no?
(Lab)
Figure 24-5. After
Turner (1981).
Metamorphic Petrology.
McGraw Hill.
Choosing the Appropriate Chemographic Diagram
• Variations in metamorphic mineral assemblages result
from:
1) Differences in bulk chemistry
2) differences in intensive variables, such as T, P, PH2O,
etc (metamorphic grade)
• A good chemographic diagram permits easy
visualization of the first situation
• The second can be determined by a balanced reaction in
which one rock’s mineral assemblage contains the
reactants and another the products
• These differences can often be visualized by comparing
separate chemographic diagrams, one for each grade