Metamorphic Phase Diagrams

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# Metamorphic Phase Diagrams - PowerPoint PPT Presentation

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. Phase Diagram for Water. The Phase Rule (Gibbs, 1928). Phases (Distinct Materials or States) +

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### 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

The Phase Rule (Gibbs, 1928)

Phases (Distinct Materials or States)

+

Degrees of Freedom (Independent Variables)

=

Components

+

2

Degrees of Freedom
• Pressure
• Temperature
• PH2O
• PCO2
• pH
• Oxygen fugacity
• Eh
Simplifying Degrees of Freedom
• Generally ignore pH, Eh, Oxygen fugacity for most rocks
• Important for sulfide systems
• Assume PH2O = Ptotal for silicates, PCO2 = 0
• Assume PCO2 = Ptotal for carbonates, PH2O = 0
• Assume PH2O = 0 and PCO2 = 0 in some cases
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
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
Simplifying Components
• Components reduced to Al2O3, FeO, CaO, MgO, K2O
• ACF graphs Al2O3, CaO, (FeO+ MgO)
• A’KF 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
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
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
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
A’KF 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
AFM Diagram
• Graphs Al2O3, FeO, MgO, K2O
• Cross-section through a tetrahedron
• Used where MgO and FeO don’t fully substitute
• Must include K2O because of micas
• A = (Al2O3-3K2O)/(Al2O3-3K2O+FeO+MgO)
• M = MgO/ (FeO + MgO)
• F = FeO/ (FeO + MgO)