Chapter 26 - Metamorphic Reactions - Faculty web pages

Chapter 26: Metamorphic Reactions
If we treat isograds as reactions, we can:
Understand what physical variables might
affect the location of a particular isograd
We may also be able to estimate the P-T-X P
conditions that an isograd represents

Types of Metamorphic Reactions
1. Phase Transformations:
Phase transformations (the polymorphs of SiO 2 or
Al 2 SiO 5 or graphite-diamond or calcite-aragonite)
aragonite)
are the simplest to deal with
The transformations depend on temperature and
pressure only

1. Phase Transformations
Figure 26-1. A portion of the
equilibrium boundary for the calcitearagonite
phase transformation in the
CaCO 3
system. After Johannes and
Puhan (1971), Contrib. Mineral. Petrol.,
31, 28-38. Winter (2001) An
Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.

Figure 26-15. The P-T phase diagram
for the system Al 2
SiO 5
showing the
stability fields for the three
polymorphs andalusite, kyanite, and
sillimanite. Calculated using the
program TWQ (Berman, 1988, 1990,
1991). Winter (2001) An Introduction
to Igneous and Metamorphic
Petrology. Prentice Hall.
1. Phase Transformations

1. Phase Transformations
Little driving force for phase transformations to proceed
→ common metastable relics of one polymorph in the
stability field of another
Coexisting polymorphs may therefore represent non-
equilibrium states

Figure 6-16. T-X phase
diagram of the system
albite-orthoclase at 0.2
GPa H 2
O pressure. After
Bowen and Tuttle (1950). J.
Geology, 58, 489-511. Winter
(2001) An Introduction to
Igneous and Metamorphic
Petrology. Prentice Hall.
2. Exsolution

3. Solid-Solid Net-Transfer Reactions
Involve solids only
Differ from polymorphic transformations: involve
solids of differing composition, and thus material
must diffuse from one site to another for the
reaction to proceed

3. Solid-Solid Net-Transfer Reactions
Examples:
NaAlSi 2 O 6 + SiO 2 = NaAlSi 3 O 8
Jd
Qtz
MgSiO 3 + CaAl 2 Si 2 O 8
En
An
Ab
4 (Mg,Fe)SiO 3 + CaAl 2 Si 2 O 8 =
Opx
Plag
= CaMgSi 2 O 6 + Al 2 SiO 5
Di
And
(Mg,Fe) 3 Al 2 Si 3 O 12 + Ca(Mg,Fe)Si 2 O 6 + SiO 2
Gnt
Cpx
Qtz

Figure 27-1. Temperature-pressure phase diagram for the reaction: Albite = Jadeite + Quartz calculated using the
program TWQ of Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice
Hall.

3. Solid-Solid Net-Transfer Reactions
If minerals contain volatiles, the volatiles must be
conserved in the reaction so that no fluid phase is
generated or consumed
For example, the reaction:
Mg 3 Si 4 O 10 (OH) 2 + 4 MgSiO 3 = Mg 7 Si 8 O 22 (OH) 2
Tlc
En
Ath
involves hydrous phases, but conserves H 2 O
It may therefore be treated as a solid-solid net-
transfer reaction

3. Solid-Solid Net-Transfer Reactions
When solid-solution solution is limited, solid-solid net-
transfer reactions are discontinuous reactions
Discontinuous reactions run to completion at a
single temperature and at a particular pressure
There is thus an abrupt, discontinuous change
from the reactant assemblage to the product
assemblage at the reaction isograd

4. Devolatilization Reactions
Among the most common metamorphic reactions
H 2 O-CO
2 systems are most common
Reactions dependent not only upon temperature
and pressure, but also upon the partial pressure of
the volatile species

4. Devolatilization Reactions
For example the location on a P-T P T phase diagram of the
dehydration reaction:
KAl 2 Si 3 AlO 10 (OH) 2 + SiO 2 = KAlSi 3 O 8 + Al 2 SiO 5 + H 2 O
Ms
Qtz
Kfs
Sill
depends upon the partial pressure of H 2 O (p(
H2O )
This dependence is easily demonstrated by applying Le
Châtelier’s s principle to the reaction at equilibrium
W

4. Devolatilization Reactions
The equilibrium curve
represents equilibrium
between the reactants and
products under water-
saturated conditions
(p H2O = P Lithostatic )
P-T phase diagram for the reaction Ms + Qtz
= Kfs + Al 2
SiO 5
+ H 2
O showing the shift in
equilibrium conditions as p H2O
varies
(assuming ideal H 2
O-CO 2
mixing). Calculated
using the program TWQ by Berman (1988,
1990, 1991). After Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.

KAl 2 Si 3 AlO 10 (OH) 2 + SiO 2 = KAlSi
AlO 10
Ms
Qtz
= KAlSi 3 O 8 + Al 2 SiO 5 + H 2 O
Kfs
Sill
W
Suppose H 2 O is withdrawn from the system at some point
on the water-saturated equilibrium curve: p H2O < P lithostatic
According to Le Châtelier’s s Principle, removing water at
equilibrium will be compensated by the reaction running
to the right, thereby producing more water
This has the effect of stabilizing the right side of the
reaction at the expense of the left side
So as water is withdrawn the Kfs + Sill + H 2 O field
expands slightly at the expense of the Mu + Qtz field, and
the reaction curve shifts toward lower temperature

Figure 26-2. P-T phase
diagram for the reaction
Ms + Qtz = Kfs + Al 2
SiO 5
+
H 2
O showing the shift in
equilibrium conditions as
p H2O
varies (assuming ideal
H 2
O-CO 2
mixing).
Calculated using the program
TWQ by Berman (1988, 1990,
1991). Winter (2001) An
Introduction to Igneous and
Metamorphic Petrology.
Prentice Hall.

4. Devolatilization Reactions
p H2O
P fluid
H2O can become less than P Lith by either of two ways
fluid < P Lith by drying out the rock and reducing the
fluid content
P fluid = P Lith , but the water in the fluid can become
diluted by adding another fluid component, such as
CO 2 or some other volatile phase

4. Devolatilization Reactions
The temperature of an isograd based on a devolatilization
reaction is sensitive to the partial pressure of the volatile
species involved
T-X fluid phase diagram
Because H 2 O and CO 2 are by far the most common
metamorphic volatiles, the X in T-X T X diagrams is usually
the mole fraction of CO 2 (or H 2 O) in H 2 O-CO
2 mixtures
Because pressure is also a common variable, a T-XT
fluid
diagram must be created for a specified pressure

Figure 26-4. T-X H2O
phase
diagram for the reaction Ms
+ Qtz = Kfs + Sil + H 2
O at
0.5 GPa assuming ideal
H 2
O-CO 2
mixing, calculated
using the program TWQ by
Berman (1988, 1990, 1991).
Winter (2001) An Introduction
to Igneous and Metamorphic
Petrology. Prentice Hall.
4. Devolatilization Reactions

4. Devolatilization Reactions
Figure 26-2. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Figure 26-4. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.

4. Devolatilization Reactions
Shape of ~ all dehydration curves on T-XT
fluid
diagrams is similar to the curve below
Maximum temperature is at the pure H 2 O end, and
slope gently at high X H2O , but steeper toward low
X H2O , becoming near vertical at very low X H2O

4. Devolatilization Reactions
Decarbonation reactions
CaCO 3
Cal
+ SiO 2 = CaSiO 3 + CO 2 (26-6)
6)
Qtz
Wo
Can also be shown on a T-XT
CO2 diagram
Maximum thermal stability of the carbonate
mineral assemblage occurs at pure X CO2

4. Devolatilization Reactions
Figure 26-1. A portion of the equilibrium boundary for the calcitearagonite
phase transformation in the CaCO 3
system. After
Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, 28-38.
Winter (2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Figure 26-5. T-X CO2
phase diagram for the reaction Cal + Qtz
= Wo + CO 2
at 0.5 GPa assuming ideal H 2
O-CO 2
mixing,
calculated using the program TWQ by Berman (1988, 1990,
1991). Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.

5 types of
devolatilization
reactions, each with a
unique general shape
on a T-X T X diagram
Reaction #3:
dehydration-
decarbonation
Ca 2 Mg 5 Si 8 O 22 (OH) 2 + 3 CaCO 3 + 2 SiO 2
Tr
Cal
Qtz
= 5 CaMgSi 2 O 6 + 3 CO 2 + H 2 O
Di
Figure 26-6. Schematic T-X CO2
phase diagram illustrating the general
shapes of the five types of reactions involving CO 2
and H 2
O fluids. After
Greenwood (1967). In P. H. Abelson (ed.), Researches in Geochemistry. John
Wiley. New York. V. 2, 542-567. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.

5. Continuous Reactions
Continuous reactions involve members of a solid solution
series.
Most metamorphic reactions.
Partitioning of chemical elements between phases changes
progressively during reactions over range of P and T OR
with the production of new phases
e.g., Ca-rich garnet grows accompanied by reduction in An
content of plagioclase as it reacts and fractionates into the
garnet

5. Continuous Reactions
Continuous reactions occur when the reactants and
products coexist over a temperature (or grade) interval
The composition of solid solution phases vary
across the interval, and the proportions of the
minerals changes until one of the reactants
disappears with increasing grade

5. Continuous Reactions
Discontinuous reactions occur at a constant grade
Do not involve solid solution series
The P-T P T path crosses the reaction at a single grade
Take place instantaneously at specified P and T
e.g., tie line switch reaction – caused disappearance
of particular mineral only from certain rock
composition; or disappearance of mineral from all
rock compositions

6. Ion Exchange Reactions
Reciprocal exchange of components between 2 or
more minerals
MgSiO 3 + CaFeSi 2 O 6 = FeSiO 3 + CaMgSi 2 O 6
Annite + Pyrope = Phlogopite + Almandine
Expressed as pure end-members, but really
involves Mg-Fe (or other) exchange between
intermediate solutions
Basis for many geothermobarometers
Causes rotation of tie-lines on compatibility
diagrams

Figure 27-6. AFM projections showing the relative distribution of Fe and Mg in garnet vs.
biotite at approximately 500 o C (a) and 800 o C (b). From Spear (1993) Metamorphic Phase Equilibria and
Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. MSA. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.

6. Redox Reactions
Involves a change in oxidation state of an element
6 Fe 2 O 3 = 4 Fe 3 O 4 + O 2
2 Fe 3 O 4 + 3 SiO 2 = 3 Fe 2 SiO 4 + O 2
At any particular pressure these become oxygen
buffers
Fig. 26-10. Isobaric T-f O2
diagram showing the location of
reactions (26-13) - (26-15) used
to buffer oxygen in
experimental systems. After Frost
(1991), Rev. in Mineralogy, 25, MSA,
pp. 469-488. Winter (2001) An
Introduction to Igneous and
Metamorphic Petrology. Prentice
Hall.

7. Reactions Involving Dissolved Species
Minerals plus ions plus neutral molecules dissolved in
a fluid
One example is hydrolysis:
2 KAlSi 3 O 8 + 2 H + + H 2 O = Al 2 Si 2 O 5 (OH) 4 +
SiO 2 + 2 K +
Kfs + aq. species = kaolinite + aq. . species

Reactions and Chemographics
We can use chemographics to infer reactions
Per Fo En Qtz
MgO SiO 2
Fo + Qtz = En
Mg 2 SiO 4 + SiO 2 = Mg 2 Si 2 O 6
En + Per = Fo
Mg 2 Si 2 O 6 + 2 MgO = 2 Mg 2 SiO 4
Per + Qtz = Fo or En
If we know the chemographics we can determine
that a reaction is possible

Reactions and Chemographics
What reaction is possible between A-B-C-D? A
A chemographic
diagram for some
metamorphic zone
Fig. 26-14a. From Winter (2001) An
Introduction to Igneous and
Metamorphic Petrology. Prentice
Hall.

Below the
isograd
Fig. 26-14. From Winter (2001) An
Introduction to Igneous and
Metamorphic Petrology. Prentice
Hall.
A + B = C + D
At the isograd
Above the
isograd
This is called a tie-line flip, , and
results in new groupings in the
next metamorphic zone

Petrogenetic Grids
P-T T diagrams for multicomponent systems that show a
set of reactions, generally for a specific rock type
Petrogenetic grid
for mafic rocks
Fig. 26-19.
Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined
univariant reactions in the CaO-MgO
MgO-Al
2
O 3
-SiO
2
-H 2
O-(Na
2
O) system (“C(N)MASH(
C(N)MASH”). ). Winter (2001) An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall.