1. Isograds for a single shale unit in southern Vermont
Which side reflects a higher grade, or higher P/T environment?

2. Systems Rock made of different minerals
Metamorphic agents of change beat on it  metamorphic reactions occur
A closed system does not gain or lose material of any kind
An open system can lose stuff – liquids, gases especially
Outside
world
Hunk o’ rock

3. Phase diagrams Tool for ‘seeing’ phase transitions H2Oice  H2Oliquid
Reaction (line) governed
by DG=DH – TDS
Phase Rule:
P+F=C+2
Phases coexisting + degrees of freedom = number of components + 2
Degree of freedom  2= either axis can change and the phase stays the same  where??

4. Phase diagrams
Let’s think about what happens to water as conditions change…
P+F=C+2
Point A?
Point B?
Point C? A B C

5. 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
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.)

7. The Phase Rule in Metamorphic Systems
Consider the following three scenarios:
C = 1 (Al2SiO5)
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 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.

8. Representing Mineral Reactions
albite  jadeite + quartz

9. From Hacker, B.R.,

10. Let’s put it all together…

11. Metamorphic facies
P-T conditions, presence of fluids induces different metamorphic mineral assemblages (governed by thermodynamics/ kinetics)
These assemblages are lumped into metamorphic facies (or grades)

13. What if we had staurolite and andalusite together
What if we had staurolite and andalusite together? What conditions would that indicate?