Chemographic Analysis

1
Chemographic Analysis
(Net Transfer Reactions)
2
The Phase Rule in Metamorphic Systems
F C P 2

• F number of degrees of freedom
• C minimum number of components needed to define
  system
• P number of phases

Divariant
Univariant
Mineralogical Phase Rule
F C P
Invariant
Pressure and temperature not specified
3
Divariant Mineral Assemblages in a Three
Component System
(Components in this representation are extensive
parameters)
B, C, and E are compositionally degenerate
4
Implication of Solid Solution in a Three
Component System
D
E
Bulk rock composition
B
A
C
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J.B. Thompsons 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

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The A(K)FM Projection

• A Al2O3 - 3K2O (Ms projection)
• A Al2O3 - K2O (Kfs projection)

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Pelitic minerals on an AFM Diagram
Projection from muscovite
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Representation of a mineral assemblage on an AFM
diagram
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A(K)FM Projection from Muscovite
Mg-enrichment typically in the order cordierite
gt chlorite gt biotite gt chloritoid gt staurolite gt
garnet
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• The ACF Diagram

A,C, F are pseudo-components calculated on an
atomic basis A Al2O3 Fe2O3 Na2O K2O C
CaO 3.3 P2O5 F FeO MgO MnO
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Mineral Assemblages on an ACF Diagram
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The AKF Diagram
A,K, F are pseudo-components calculated on an
atomic basis A Al2O3 Fe2O3 Na2O K2O -
CaO K K2O F FeO MgO MnO
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Mineral Assemblages on an AKF Diagram
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Reactions in a 2-Component System

• Fo Qtz En Mg2SiO4 SiO2 Mg2Si2O6

• En Per Fo Mg2Si2O6 2 MgO 2 Mg2SiO4

• Per Qtz Fo or En

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Reactions in a 3-Component System
Crossing Tie-Line Reaction
Below the isograd
A B C D
At the isograd
Above the isograd
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The Cordierite-K-feldspar-in Reaction
Biotite Muscovite Cordierite K-feldspar H2O
2 K(Fe,Mg)3AlSi3O10(OH)2 6 KAl3Si3O10(OH)2 15
SiO2 3 (FeMg)2Al4Si5O18 8 KAlSi3O8 8
H2O
Orijärvi Ms-Cd-Bt Ms-Kf-Bt
Oslo Ms-Kf-Cd Kf-Bt-Cd
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Reactions in a 3-Component System
Terminal Reaction
Below the isograd
A B D E
At the isograd
Above the isograd
Above the isograd
E
E
Bulk rock composition
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AFM Potting positions
A
St
Crd
Cld
Gt
Chl
F
M
Bt
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Crossing Tie-Line Reaction
A
St
Crd
Cld
Gt
Chl
F
M
Bt
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Terminal Reaction
A
St
Crd
Cld
Gt
Chl
F
M
Bt