Metamorphic Phase Diagrams

1
Metamorphic Phase Diagrams

• Differ from Igneous Phase Diagrams
• Show a snapshot of all compositions at given T,P
• Rock remains at same point but diagram changes

2
Phase Diagram for Water
3
The Phase Rule (Gibbs, 1928)

• Phases (Distinct Materials or States)
• Degrees of Freedom (Independent Variables)
• Components
• 2

4
Phase Rule and Phase Diagrams
5
Phase Diagram for Al2SiO5
6
Phase Diagram for Ice
7
Phase Diagram of Silica
8
Degrees of Freedom

• Pressure
• Temperature
• PH2O
• PCO2
• pH
• Oxygen fugacity
• Eh

9
Simplifying Degrees of Freedom

• Generally ignore pH, Eh, Oxygen fugacity for most
  rocks
• Important for sulfide systems
• Usually care about T,P
• Assume PH2O Ptotal for silicates, PCO2 0
• Assume PCO2 Ptotal for carbonates, PH2O 0
• Assume PH2O 0 and PCO2 0 in some cases

10
Simplifying Degrees of Freedom

• P F C 2
• If F 2, then P C
• Number of phases number of components
• Components SiO2, Al2O3, Fe2O3, FeO, CaO, MgO,
  Na2O, K2O, TiO2, Cr2O3, MnO, BaO, SrO, P2O5, H2O,
  CO2, F, Cl

11
Simplifying Components

• We can plot a maximum of three components using
  triangle diagrams
• Ignore SiO2(excess), H2O (excess or 0), CO2
  (excess or 0)
• Assume P2O5 goes into apatite, Na2O into albite,
  TiO2 into rutile or ilmenite, Cr2O3, into
  chromite, F into fluorite or apatite, Cl into
  halite
• Include MnO with FeO, BaO and SrO with CaO, Fe2O3
  with Al2O3 or FeO, Cl, F with OH
• Subtract major elements as necessary

12
Simplifying Components

• Components reduced to Al2O3, FeO, CaO, MgO, K2O
• ACF graphs Al2O3, CaO, (FeO MgO)
• AKF graphs Al2O3, (FeO MgO), K2O
• AFM graphs Al2O3, FeO, MgO, K2O in a tetrahedron
• SiO2 CaO MgO used for carbonate systems
• Plot Molar amounts, not weights

13
Metamorphic Phase Diagrams

• Tie lines denote two coexisting phases
• Triangular subfields denote three coexisting
  phases
• Phase Diagram is snapshot of mineral combinations
  under given T,P conditions
• Evolution tracked by changes in diagrams
• Not really interested in amounts

14
Carbonate Rocks at 450C
15
Metamorphic Phase Diagrams

• Normally F 2, C 3, P 3
• On a reaction curve, F 1, P 4
• How to get 4 Phases Together
• New Phase Appears in Middle of Field
• New Phase Appears on Tie Line
• Tie Line Breaks and New One Forms
• Changes in Metamorphism
• New Minerals Appear
• Old Minerals Disappear
• Compatibilities Shift

16
Carbonate Rocks at Low T
17
400 C New Phase on Tie Line(Quartz Brucite -gt
Talc)
18
450 C One Tie Line Replaced by Another(Quartz
Dolomite -gt Calcite Talc)
19
500 C New Phase in Middle of Field(Quartz
Talc Calcite -gt Tremolite)
20
ACF Diagram

• Probably most versatile and instructive diagram
• A (Al2O3 Fe2O3) (Na2O, K2O)
• Subtract Al2O3 in K-spar and albite
• C CaO - 3.3 P2O5
• Subtract Ca in apatite
• F FeO MgO MnO

21
The ACF Diagram
22
AKF Diagram

• A (Al2O3 Fe2O3) (Na2O, K2O) variable
  Ca (epidote, garnet, anorthite)
• K K2O
• F FeO MgO MnO amount in diopside or
  hornblende
• Distinguishes K-feldspar and micas

23
The AKF Diagram
24
ACF and AKF Diagrams are often paired
25
AFM Diagram

• Graphs Al2O3, FeO, MgO, K2O
• Cross-section through a tetrahedron
• Used where MgO and FeO dont fully substitute
• Must include K2O because of micas
• A (Al2O3-3K2O)/(Al2O3-3K2OFeOMgO)
• M MgO/ (FeO MgO)
• F FeO/ (FeO MgO)

26
AFM Diagram
27
AFM Diagram
28
CaO-MgO-SiO2 Diagram