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Chemographic Analysis

Chemographic Analysis. (Net Transfer Reactions). 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.

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Chemographic Analysis

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  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. D E C B A Bulk rock composition (A, B, C, D, E) 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

  5. J.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

  6. The A(K)FM Projection • A = Al2O3 - 3K2O (Ms projection) • A = Al2O3 - K2O (Kfs projection)

  7. Pelitic minerals on an AFM Diagram Projection from muscovite

  8. Representation of a mineral assemblage on an AFM diagram

  9. A(K)FM Projection from Muscovite Mg-enrichment typically in the order: cordierite > chlorite > biotite > chloritoid > staurolite > garnet

  10. 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 The ACF Diagram

  11. Mineral Assemblages on an ACF Diagram

  12. 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

  13. Mineral Assemblages on an AKF Diagram

  14. 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

  15. Reactions in a 3-Component System Crossing Tie-Line Reaction Below the isograd A + B = C + D At the isograd Above the isograd

  16. 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

  17. 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

  18. AFM Potting positions A St Crd Cld Gt Chl F M Bt

  19. Crossing Tie-Line Reaction A St Crd Cld Gt Chl F M Bt

  20. Terminal Reaction A St Crd Cld Gt Chl F M Bt

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