Transcript Metamorphic Phase Diagrams • Differ from Igneous Phase Diagrams • Show a snapshot of all compositions at given T,P • Rock remains at same.

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)
+
Degrees of Freedom (Independent
Variables)
=
Components
+
2
Phase Rule and Phase Diagrams
Phase Diagram for Al2SiO5
Phase Diagram for Ice
Phase Diagram of Silica
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
Carbonate Rocks at 450C
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
Carbonate Rocks at Low T
400 C: New Phase on Tie Line
(Quartz + Brucite -> Talc)
450 C: One Tie Line Replaced by Another
(Quartz + Dolomite -> Calcite + Talc)
500 C: New Phase in Middle of Field
(Quartz + Talc + Calcite -> Tremolite)
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
The ACF 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
The A’KF Diagram
ACF and A’KF Diagrams are
often paired
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)
AFM Diagram
AFM Diagram
CaO-MgO-SiO2 Diagram