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## PowerPoint Slideshow about 'Metamorphic Phase Diagrams' - sora

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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
- 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

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

450 C: One Tie Line Replaced by Another(Quartz + Dolomite -> Calcite + Talc)

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)

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