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Metamorphic Reactions

Metamorphic Reactions. Today. Updates: Review on Wednesday Final next Monday , 10.00 am Lecture outline: Finish up Reaction types Example for metapelites. Chemical Equilibrium in Metamorphism. http:// www.earth.ox.ac.uk/~sem/SEM+Probe_pics/fras_gt_mini.gif.

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Metamorphic Reactions

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  1. Metamorphic Reactions

  2. Today Updates: Review on Wednesday Final next Monday, 10.00 am Lecture outline: • Finish up • Reaction types • Example for metapelites

  3. Chemical Equilibrium in Metamorphism http://www.earth.ox.ac.uk/~sem/SEM+Probe_pics/fras_gt_mini.gif

  4. Types of Metamorphic Reactions Types: Phase transformations Exsolution Solid-solid net transfer Devolitalization Ion-exchange In the field recognized as isograd: First occurrence of new mineral P-T-X dependent Given X, estimate P-T

  5. Phase Transformations: Al2SiO5 • Isochemical • Only f(P,T) • Common in Al rich rocks (metapelites) • Rough P-T indicator: • Andalusite suggests low P • Kyanite = high P • Sillimanite = high P and T

  6. Exsolution

  7. Solid-Solid Net-Transfer Reactions

  8. Net-Transfer Reactions and Volatiles

  9. Devolatilization Reactions Other volatiles besides H2O-CO2 systems Example: KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O Ms Qtz Kfs Sill W Dependence: P, T Partial pressure : Not all “pores” filled => Pfluid < Plithostatic Other volatiles in fluid: Plith = PH2O + PCO2 +…

  10. Devolatilization Reactions

  11. Ion Exchange Mg2+ Fe2+

  12. Discontinuous reaction: geotherm crosses reaction curve at 1 P & T, reaction takes place at those constant P & T Continuous reaction: reaction occurs over range in P & T similar to melting in solid solution Pseudomorphs: small change in entropy: need fair bit of energy from heat (T): reaction over small T range Pseudomorphs and (Dis)ContinuousReactions

  13. A(K)FM Diagram Project from a phase that is present in the mineral assemblages to be studied to make a triangular plot (AFM) Figure 24-18. AKFM Projection from Mu. After Thompson (1957). Am. Min. 22, 842-858.

  14. Why we ignored SiO2 in the ACF and AKF diagrams: assumed present in all rocks • AFM: A = Al2O3 - (3)K2O • Subtraction for pseudo-components: A = Al2O3 + Fe2O3 - Na2O - K2O (AKF) • Na, K typically combined with Al in Fsp • In the ACF diagram only interested in Al2O3 outside Fsp • Since the ratio of Al2O3 to Na2O or K2O in feldspars is 1:1, we subtract from Al2O3 an amount equivalent to Na2O and K2O in the same 1:1 ratio Projections

  15. Plotting in A(K)FM Diagram • Biotite (from Ms): KMg2FeSi3AlO10(OH)2 • A = Al2O3-3K2O • = 0.5 - 3 (0.5) =  - 1 F = FeO = 1 M = MgO = 2 • To normalize we multiply each by 1.0/(2 + 1 - 1) = 1.0/2 = 0.5 • Thus A = -0.5 • F = 0.5 • M = 1 • MgO/FeO+MgO=.67

  16. Medium P-T path for Metapelites

  17. Upper Biotite zone

  18. Garnet zone

  19. Kyanite zone

  20. Sillimanite zone

  21. Granulite facies

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