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Phase Diagrams

Phase Diagrams. Best, Chapter 14. Gibbs Phase Rule. F = 2 + C -  F = degrees of freedom (P-T-X) C= components  = phases. Degrees of Freedom. Rule applies to a phase or assemblage Divariant indicates two degrees of freedom Univariant means one degree of freedom

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Phase Diagrams

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  1. Phase Diagrams Best, Chapter 14

  2. Gibbs Phase Rule F = 2 + C -  F = degrees of freedom (P-T-X) C= components  = phases

  3. Degrees of Freedom • Rule applies to a phase or assemblage • Divariant indicates two degrees of freedom • Univariant means one degree of freedom • Invariant means there are no degrees of freedom

  4. Petrogenetic Grid • The grid define stability limits • End-member minerals • Mineral assemblages • More thermodynamic data is needed to construct a useful grid

  5. Anhydrous Phase Diagrams • Solid-solid reactions • Governed by Clapeyron equation • dP/dT = 10 H/T V = S/V • H is the heat of reaction • S is the change in entropy • V is the change in volume • The slope of the stability is dP/dT

  6. Jadeite + Quartz = Albite

  7. System Open to H2O • Dehydration curves • Example of the general case • Specific minerals • Breakdown of chlorite, muscovite, biotite, etc

  8. System Open to CO2 • Decarbonation curves • General case example • Specific minerals • Breakdown of calcite, dolomite, etc

  9. Univariant Curves • Curves that define reactions with one degree of freedom • In P-T space this means that if T is changed, than P must also change to maintain equilibrium • Many important metamorphic reactions are defined by these curves

  10. Important Reactions • Al2O3 phase stability • Dehydration curves

  11. Stability of Iron Oxides • PO2 (fO2 ) vs. Temp. • Main phases • Hematite • Magnetite • Fayalite • Native Iron/Wustite

  12. Miyashiro’s Facies Series • Low geothermal gradient • Zeolite, pumpellyite-prehnite, blueschist • Intermediate geothermal gradient • Barrow’s zones • High geothermal gradient • Andalucite present in pelitic rocks

  13. Relation to Geotherms • High pressure series • Medium P/T series • High temperature series

  14. Facies in P-T Space

  15. Granite Solidus (Wet)

  16. Metamorphic Path

  17. Polymetamorphism • Sometimes there are repeated episodes of metamorphism • The last event may be weak or of short duration • Polymetamorphism is common in post tectonic environments and in contact aureoles

  18. Material Transport Diffusion Infiltration

  19. Diffusion • Materials move through crystal lattices or a stationary pore fluid • Rate of movement controlled by a diffusion coefficient (Fick’s Law) Q = k (C/ x) • Material moves about 1 cm/m.y.

  20. Infiltration • Passive mass transport of a solute in a moving fluid medium • Driven by fluid pressure • Microfractures are important • Reaction-enhanced permeability • Volume reduction due to reactions • Dilatency pumping

  21. Reaction Textures olivine + plagioclase = hypersthene + diopside+ spinel

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