Themes of the Day:

• Mineral Assemblages and Equilibrium
• Reactions, the Phase Rule, and the Petrogenetic Grid
• Metamorphic Field Gradients and P-T-t paths
• Graphical Representations of Mineral Assemblages
• Metamorphic Variables: Pressure, Temperature, and Fluid

Mineral Assemblages and Equilibrium

• Mineral Assemblages - thermodynamically stable group of minerals
• System (often a hand sample) homogeneous in pressure (P), temperature (T), and composition (X)
  • Closed systems - no mass transfer (fluids free to move in and out)
  • Open systems - mass transfer (e.g., skarns)
• Equilibrium - lowest energy state
• Metastability and activation energy
• Textural evidence for equilibrium:
  • all phases in contact
  • no zoning or armoring
  • number of phases does not exceed maximum allowed by the Phase Rule
• Prograde and Retrograde reactions: Mineral assemblage generally records highest equilibrium T

Reactions, the Phase Rule, and the Petrogenetic Grid

• Interpreting Metamorphic Reaction Progress from Mineral Assemblages
• The Phase Rule: F = C - P + 2
  • P, T, X are potential variables
  • in P-T phase diagrams X is assumed to be fixed
  • Divariant Fields, Univariant Lines, and Invariant Points
• Petrogenetic Grid
  • Aluminosilicates
  • KFMASH system

Metamorphic Field Gradients and P-T-t paths

• Facies series - Metamorphic Field Gradients
  • based on rocks exposed at the surface
  • originally though to preserve geothermal gradients - NOT SO!
  • some widely recognized MFG's
  • specific examples
• P-T-t paths
  • Clockwise vs. counterclockwise paths
  • Isobaric heating and cooling
  • Isothermal pressurization and depressurization
  • Where do we lock in the metamorphic mineral assemblage?
• Thermal Conditions of Contact Metamorphism - dry, wet
• Tectonic Settings of Metamorphism
  • paired metamorphic belts - Japan, Pacific Rim
  • subduction zone cross section
  • Continental Collisions - Alps, New England

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