• Isograds for a single shale unit in southern Vermont

• Which side reflects a higher grade, or higher P/T environment?

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

Hunk o’ rock

Outsideworld

Phase diagrams• Tool for ‘seeing’ phase transitions

• H2Oice H2Oliquid

• Reaction (line) governed by G=H – TS

• 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??

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

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

The Phase Rule in Metamorphic Systems

• Phase rule, as applied to systems at equilibrium:

F = C - P + 2 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.)

The Phase Rule in Metamorphic SystemsConsider 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 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.

Representing Mineral Reactions

• albite jadeite + quartz

From Hacker, B.R.,

Let’s put it all together…

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)

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