Themes of the Day:

• Graphical Representations of Mineral Assemblages
• Metamorphic Variables: Pressure, Temperature, and Fluid
• Types of Metamorphic Reactions

Graphical Representations of Mineral Assemblages

• Geometric Diagrams that show compositional variability under fixed P-T conditions
  • Example #1: CaO-Al₂O₃-SiO₂ (excess CO₂, no H₂O) - See Figure 20-6, p. 404 in Blatt & Tracy
  • Example #2: ACF and A'KF - See Figure 20-7, p. 405 in Blatt & Tracy
  • Example #3: AFM (projected from muscovite or K-feldspar) - See Figures 20-8 and 20-9, p. 406-407 in Blatt & Tracy
  • Example #4: Metapelites in ACF, A'KF, and AFM - See Figure 20-10, p. 408 in Blatt & Tracy

Controls on Metamorphic Reactions

• Temperature - Earth's internal heat, radioactive decay
• Pressure - confining vs. directed, hydrostatic vs. lithostatic
• Fluids - Mainly H₂O, CO₂ - partial pressure - fluid inclusions

Types of Metamorphic Reactions

• Reactions - reactants (low grade) on left, products (high grade) on right (generally)
• Discontinuous (Univariant)
  • Switching tie lines - ex., Grt + Chl + Mus = St + Bt + Qtz + H₂O (Fig. 21-9)
  • Appearance or Disappearance of a mineral - ex., St + Mus + Qtz = Grt + Bt + Sil + H₂O (Fig. 21-5)
• Continuous (Divariant) - ex., Chl + Mus + Qtz = Mg-richer Chl + Grt + Bt + H₂O (Fig. 21-10)
• Solid-solid - ex., Ky = Sil = And (Fig. 21-11)
• Dehydration - ex., Pyr = Als + Qtz + H₂O (Fig. 21-11)
• Decarbonation - ex. Cal + Qtz = Wol + CO₂ (Fig. 21-12)
• Clausius-Clapeyron Equation - ΔP/ΔT = (S/V)_products/(S/V)_reactants