Metamorphic Reactions

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Metamorphic Reactions
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Today

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

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Chemical Equilibrium in Metamorphism
http//www.earth.ox.ac.uk/sem/SEMProbe_pics/fras
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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

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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

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Exsolution
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Solid-Solid Net-Transfer Reactions
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Net-Transfer Reactions and Volatiles
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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
• gt Pfluid lt Plithostatic
• Other volatiles in fluid
• Plith PH2O PCO2

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Devolatilization Reactions
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Ion Exchange
Mg2 Fe2
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Pseudomorphs and (Dis)ContinuousReactions
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
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• The ACF Diagram

• Mafic rocks
• Concentrate only on minerals that appeared /
  disappeared
• from 8 to 3 pseudo
• -components
• A Al2O3 Fe2O3
• - Na2O - K2O
• C CaO - 3.3 P2O5
• F FeO MgO
• MnO

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• The AKF Diagram

Pelitic sediments high in Al2O3, K2O

• In the AKF diagram, the pseudo-components are
• A Al2O3 Fe2O3
• - Na2O - K2O CaO
• K K2O
• F FeO MgO
• MnO

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• 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.
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• Projections

• 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 11, we subtract from Al2O3 an
  amount equivalent to Na2O and K2O in the same 11
  ratio

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• 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/FeOMgO.67

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Medium P-T path for Metapelites
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Upper Biotite zone
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Garnet zone
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Kyanite zone
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Sillimanite zone
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Granulite facies