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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.

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Presentation on theme: "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."— Presentation transcript:

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 Al 2 SiO 5

6 Phase Diagram for Ice

7 Phase Diagram of Silica

8 Degrees of Freedom Pressure Temperature PH 2 O PCO 2 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 PH 2 O = P total for silicates, PCO 2 = 0 –Assume PCO 2 = P total for carbonates, PH 2 O = 0 –Assume PH 2 O = 0 and PCO 2 = 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 = SiO 2, Al 2 O 3, Fe 2 O 3, FeO, CaO, MgO, Na 2 O, K 2 O, TiO 2, Cr 2 O 3, MnO, BaO, SrO, P 2 O 5, H 2 O, CO 2, F, Cl

11 Simplifying Components We can plot a maximum of three components using triangle diagrams Ignore SiO 2 (excess), H 2 O (excess or 0), CO 2 (excess or 0) Assume P 2 O 5 goes into apatite, Na 2 O into albite, TiO 2 into rutile or ilmenite, Cr 2 O 3, into chromite, F into fluorite or apatite, Cl into halite Include MnO with FeO, BaO and SrO with CaO, Fe 2 O 3 with Al 2 O 3 or FeO, Cl, F with OH Subtract major elements as necessary

12 Simplifying Components Components reduced to Al 2 O 3, FeO, CaO, MgO, K 2 O ACF graphs Al 2 O 3, CaO, (FeO+ MgO) A’KF graphs Al 2 O 3, (FeO + MgO), K 2 O AFM graphs Al 2 O 3, FeO, MgO, K 2 O in a tetrahedron SiO 2 – 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 -> Talc)

18 450 C: One Tie Line Replaced by Another (Quartz + Dolomite -> Calcite + Talc)

19 500 C: New Phase in Middle of Field (Quartz + Talc + Calcite -> Tremolite)

20 ACF Diagram Probably most versatile and instructive diagram A = (Al 2 O 3 + Fe 2 O 3 ) – (Na 2 O, + K 2 O) –Subtract Al 2 O 3 in K-spar and albite C = CaO * P 2 O 5 –Subtract Ca in apatite F = FeO + MgO + MnO

21 The ACF Diagram

22 A’KF Diagram A’ = (Al 2 O 3 + Fe 2 O 3 ) – (Na 2 O, + K 2 O) – variable Ca (epidote, garnet, anorthite) K = K 2 O F = FeO + MgO + MnO – amount in diopside or hornblende Distinguishes K-feldspar and micas

23 The A’KF Diagram

24 ACF and A’KF Diagrams are often paired

25 AFM Diagram Graphs Al 2 O 3, FeO, MgO, K 2 O Cross-section through a tetrahedron Used where MgO and FeO don’t fully substitute Must include K 2 O because of micas A = (Al 2 O 3 -3K 2 O)/(Al 2 O 3 -3K 2 O+FeO+MgO) M = MgO/ (FeO + MgO) F = FeO/ (FeO + MgO)

26 AFM Diagram

27

28 CaO-MgO-SiO 2 Diagram


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