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

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metamorphic phase diagrams

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
The Phase Rule (Gibbs, 1928)

Phases (Distinct Materials or States)

+

Degrees of Freedom (Independent Variables)

=

Components

+

2

degrees of freedom
Degrees of Freedom
  • Pressure
  • Temperature
  • PH2O
  • PCO2
  • pH
  • Oxygen fugacity
  • Eh
simplifying degrees of freedom
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 freedom10
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
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 components12
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 diagrams13
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 diagrams15
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
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’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
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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