1. Introduction to Metamorphism 2
IN THIS LECTURE
Importance of Understanding Metamorphic Mineral Assemblages
Progressive Nature of Metamorphism
Stable Mineral Assemblages
The Phase Rule in Metamorphic Systems
The MgO-H2O system

2. What’s so Important About Metamorphic Mineral Assemblages
Equilibrium mineral assemblages can tell us about P-T conditions
P-T conditions tell us about tectonic environments
P-T conditions combined with geochronology tells us about how tectonic environments evolve through time
Optical Mineralogy is the starting point!!

3. The Progressive Nature of Metamorphism
A rock at a high metamorphic grade probably progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today
All rocks that we now find must also have cooled to surface conditions. Therefore, at what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate?
The preserved zonal distribution of metamorphic rocks suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature)
Metamorphic rocks usually maintain equilibrium as grade increases
A rock at a high metamorphic grade thus probably progressed through a sequence of mineral assemblages as it passed through all of the mineral changes necessary to maintain equilibrium with increasing temperature and pressure, rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today

4. The Progressive Nature of Metamorphism
Prograde reactions are endothermic and easily driven by increasing T
Devolatilization reactions are easier than reintroducing the volatiles
Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature
Retrograde metamorphism is of only minor significance, and is usually detectable by observing textures, such as the incipient replacement of high-grade minerals by low-grade ones at their rims

5. Stable Mineral Assemblages in Metamorphic Rocks
Equilibrium Mineral Assemblages
At equilibrium, the mineralogy (and the composition of each mineral) is determined by T, P, and X
“Mineral paragenesis” refers to such an equilibrium mineral assemblage
Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated
“Mineral assemblage” is used by some as a synonym for paragenesis, conventionally assuming equilibrium for the term
Impossible to prove that a mineral assemblage now at the Earth’s surface represents thermodynamic (chemical) equilibrium at prior elevated metamorphic conditions
Indirect textural and chemical support for such a conclusion is discussed in the text
In short, it is typically easy to recognize non-equilibrium minerals (retrograde rims, reaction textures, etc.)
We shall assume equilibrium mineral assemblages in the following discussion (will ignore retrograde…)

6. The Phase Rule in Metamorphic Systems
Phase rule, as applied to systems at equilibrium:
F = C - P the phase rule
P is the number of phases in the system
C is the number of components: the minimum number of chemical constituents required to specify every phase in the system
F is the number of degrees of freedom: the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.)
Remember we’ve seen this already with igneous systems

7. The Phase Rule in Metamorphic Systems
Pick a random point anywhere on a phase diagram
Likely point will be within a divariant field and not on a univariant curve or invariant point
The most common situation is divariant (F = 2), meaning that P and T are independently variable without affecting the mineral assemblage
In complex natural systems there may be one or more compositional variables as well, so that F may be greater than two
The common occurrence of certain metamorphic mineral assemblages worldwide supports this contention that F  2, since such assemblages are much more likely to represent variable P-T-X conditions than more restricted situations

8. Remember this diagram?

9. The Phase Rule in Metamorphic Systems
If F  2 is the most common situation, then the phase rule may be adjusted accordingly such that
F = C - P + 2  2
and therefore P  C
This is Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule
The above simplified phase rule states that in the most common situation for a rock at equilibrium the number of phases is equal to or greater than the number of components
It has been called Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule
It is useful in evaluating whether or not a rock is at equilibrium

10. The Phase Rule in Metamorphic Systems
If C has been determined for a particular rock then there are three potential situations according to the phase rule
P=C
This is the standard divariant situation in metamorphic rocks
The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone
P<C
A situation that commonly arises in systems that display solid solution.
We’ve seen this already with the binary phase diagrams for the albite-anorthite system
To some authors the mineralogical phase rule is phi = C

11. Albite-Anorthite Phase Diagram

12. The Phase Rule in Metamorphic Systems
If C has been determined for a particular rock then there are three potential situations according to the phase rule
1. P = C
This is the standard divariant situation in metamorphic rocks
The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone
2. P < C
A situation that commonly arises in systems that display solid solution.
We’ve seen this already with the binary phase diagrams for the albite-anorthite system
3. P > C
A more interesting situation, and at least one of three situations must be responsible
To some authors the mineralogical phase rule is phi = C

13. The Phase Rule in Metamorphic Systems
For P > C then the following three situations could apply
F < 2
Equilibrium not attained
Choice of C not correct
If (1) applies then the sample was
collected from a location right on
a univariant reaction curve or
invariant point
To some authors the mineralogical phase rule is phi = C
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.

14. The Phase Rule in Metamorphic Systems
2. Equilibrium is not attained
The situation with (2) is more important in terms of optical mineralogy.
The phase rule only applies to systems that are in equilibrium.
If equilibrium is not attained or maintained then there could be any number of minerals co-existing
Unfortunately, this is often the case, especially in rocks that have been partially retrogressed or rocks in the blueschist and eclogite facies.
Optical Mineralogy is the main tool for decided which minerals represent the equilibrium mineral assemblage
Graywackes typically contain fragments of a number of different rocks in a matrix of grains derived from a rapidly eroding source
Dozens of minerals may be present, but they are not in equilibrium with one another
A number of igneous and metamorphic rocks also have retrograde reactions that begin, but do not run to completion

15. The Phase Rule in Metamorphic Systems
3. The number of components was not correct
Some guidelines for an appropriate choice of C
Begin with a 1-component system, such as CaAl2Si2O8 (anorthite), there are 3 common types of major/minor components that we can add
a)  Components that generate a new phase
Adding a component such as CaMgSi2O6 (diopside), results in an additional phase: in the binary Di-An system diopside coexists with anorthite below the solidus
b) Components that substitute for other components
Adding a component such as NaAlSi3O8 (albite) to the 1-C anorthite system would dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below the solidus
Fe and Mn commonly substitute for Mg
Al may substitute for Si
Na may substitute for K
Graywackes typically contain fragments of a number of different rocks in a matrix of grains derived from a rapidly eroding source
Dozens of minerals may be present, but they are not in equilibrium with one another
A number of igneous and metamorphic rocks also have retrograde reactions that begin, but do not run to completion

16. The Phase Rule in Metamorphic Systems
3. The number of components was not correct (cont.)
c) “Perfectly mobile” components
Either a freely mobile fluid component or a component that dissolves in a fluid phase and can be transported easily
The chemical activity of such components is commonly controlled by factors external to the local rock system
They are commonly ignored in deriving C for metamorphic systems
Graywackes typically contain fragments of a number of different rocks in a matrix of grains derived from a rapidly eroding source
Dozens of minerals may be present, but they are not in equilibrium with one another
A number of igneous and metamorphic rocks also have retrograde reactions that begin, but do not run to completion

17. The Phase Rule in Metamorphic Systems
Consider the very simple metamorphic system, MgO- H2O
Possible natural phases in this system are periclase (MgO), aqueous fluid (H2O), and brucite (Mg(OH)2)
How we deal with H2O depends upon whether water is perfectly mobile or not
A reaction can occur between the potential phases in this system:
MgO + H2O  Mg(OH)2 Per + Fluid = Bru
As written this is a retrograde reaction (occurs as the rock cools and hydrates)

18. The System MgO-H2O
Cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O  Mg(OH)2
1) Suppose H2O is mobile and we ignore it as a component
If we don’t treat a constituent as a component, we cannot treat it as a phase either, so we ignore the aqueous pore fluid in this case as well (the fluid phase is usually gone by the time we look at the rock anyway)
Begin with periclase above the reaction temperature
phi = 1 (water doesn’t count) and C = 1 (MgO), so the mineralogical phase rule holds
As we cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O  Mg(OH)2

19. The System MgO-H2O Reaction: periclase coexists with brucite:
P = C + 1
F = 1 (2nd reason to violate the mineralogical phase rule)
To leave the curve, all the periclase must be consumed by the reaction, and brucite is the solitary remaining phase
F = 1 and C = 1 again
Reaction: univariant conditions periclase coexists with brucite (phi = C + 1) so F = 1 (the second reason to violate the mineralogical phase rule stated above)
End: “Perfectly mobile” means that H2O behaves such that it can be added as it is needed, and any excess will leave as it is produced
Water is simply an external reservoir, available in sufficient quantity that it enables the immobile magnesium to exist as either periclase or brucite, depending on which is stable under the given hydrous P-T conditions
Imagine the diagram to be a field area, and the reaction an isograd
For any common sample phi = 1 (since we ignore H2O and don’t really see the fluid in the rock), and phi = C in accordance with the mineralogical phase rule
Only an uncommon sample, collected exactly on the isograd, will have phi > C

20. The Phase Rule in Metamorphic Systems
Once the water is gone, the excess periclase remains stable as conditions change into the brucite stability field
Thus periclase can be stable anywhere on the whole diagram, if water is present in insufficient quantities to permit the reaction to brucite to go to completion
As a general rule, reactions such as this do indeed represent the absolute stability boundary of a phase such as brucite, because it is the only reactant in the prograde reaction (brucite  periclase + water)
The reaction is not the absolute stability boundary of periclase, or of water, because either can be stable across the boundary if the other reactant is absent
Of course this is true for any reaction involving multiple phases

21. The Phase Rule in Metamorphic Systems
At any point (other than on the univariant curve itself) we would expect to find two phases, not one
P = brucite + periclase below the reaction curve (if water is limited), or periclase + water above the curve
To the right of the curve we appear to have only periclase, but if we count water as a component, we must also include it as a phase: the fluid phase

22. The Phase Rule in Metamorphic Systems
How do you know which way is correct?
The rocks should tell you
The phase rule is an interpretive tool, not a predictive tool, and does not tell the rocks how to behave
If you only see low-P assemblages (e.g. Per or Bru in the MgO- H2O system), then some components may be mobile
If you often observe assemblages that have many phases in an area (e.g. periclase + brucite), it is unlikely that so much of the area is right on a univariant curve, and may require the number of components to include otherwise mobile phases, such as H2O or CO2, in order to apply the phase rule correctly