Chemographic Analysis
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Transcript of Chemographic Analysis
Chemographic Analysis
(Net Transfer Reactions)
The Phase Rule in Metamorphic SystemsThe Phase Rule in Metamorphic Systems
F = C – P + 2 F – number of degrees of freedomC – minimum number of components needed to define systemP – number of phases
Mineralogical Phase Rule
F = C – PUniva
riantDivariant
Invariant
Pressure and temperature not specified
Divariant Mineral Assemblages in a Three Component System
Bulk rock composition (A, B, C, D, E)
A
B
C
DE
B, C, and E are compositionally degenerate
(Components in this representation are extensive parameters)
Implication of Solid Solution in a Three Component System
Bulk rock composition
A
B
C
D
E
J.B. Thompson’s A(K)FM Diagram for MetapelitesJ.B. Thompson’s A(K)FM Diagram for Metapelites
Most pelitic rocks can be described by the system K2O-Al2O3-FeO-MgO-SiO2-H2O
Quartz is an excess phase and buffers the activity of SiO2
Muscovite and K-feldspar are ubiquitous phases above and below the K-feldspar-aluminosilicate isograd, respectively
H2O is mobile and is therefore an intensive parameter of the system
The A(K)FM Projection
A = AlA = Al22OO33 - 3K - 3K22O (Ms projection)O (Ms projection)
A = AlA = Al22OO33 - K - K22O (Kfs projection) O (Kfs projection)
Pelitic minerals on an AFM Diagram
Projection from muscovite
Representation of a mineral assemblage on an AFM diagram
Mg-enrichment typically in the order: cordierite > chlorite > biotite > chloritoid > staurolite > garnet
A(K)FM Projection from Muscovite
The ACF DiagramThe ACF DiagramA,C, F are pseudo-components calculated on an atomic basis:A = Al2O3 + Fe2O3 – Na2O – K2O; C = CaO – 3.3 P2O5
F = FeO + MgO + MnO
Mineral Assemblages on an ACF Diagram
The AKF DiagramThe AKF DiagramA,K, F are pseudo-components calculated on an atomic basis:A = Al2O3 + Fe2O3 – Na2O – K2O - CaO; K = K2OF = FeO + MgO + MnO
Mineral Assemblages on an AKF Diagram
MgO SiO2
Per Fo En Qtz
Fo + Qtz = EnFo + Qtz = En Mg Mg22SiOSiO44 + SiO + SiO22 = Mg = Mg22SiSi22OO66
En + Per = FoEn + Per = Fo Mg Mg22SiSi22OO66 + 2 MgO = 2 Mg + 2 MgO = 2 Mg22SiOSiO44
Per + Qtz = Fo or EnPer + Qtz = Fo or En
Reactions in a 2-Component System
Above the isograd
Below the isograd
At the isograd
A + B = C + D
Crossing Tie-Line Reaction
Reactions in a 3-Component System
Biotite +Muscovite = Cordierite + K-feldspar + H2O
2 K(Fe,Mg)2 K(Fe,Mg)33AlSiAlSi33OO1010(OH)(OH)22 + 6 KAl + 6 KAl33SiSi33OO1010(OH)(OH)22 + 15 SiO + 15 SiO22
= 3 (FeMg)= 3 (FeMg)22AlAl44SiSi55OO1818 + 8 KAlSi + 8 KAlSi33OO88 + 8 H + 8 H22OO
The Cordierite-K-feldspar-in Reaction
Orijärvi: Orijärvi: Ms-Cd-Bt Ms-Cd-Bt Ms-Kf-BtMs-Kf-Bt
Oslo: Ms-Kf-Cd Kf-Bt-Cd
Above the isograd
E
E
Below the isograd
At theisograd
Above theisograd
Terminal Reaction
A + B + D = E
Bulk rock composition
Reactions in a 3-Component System
A
F M
St
Gt
Cld
Bt
Chl
Crd
AFM Potting positions
A
F M
St
Gt
Cld
Bt
Chl
Crd
Crossing Tie-Line Reaction
A
F M
St
Gt
Cld
Bt
Chl
Crd
Terminal Reaction