Concept of Index Minerals

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Concept of Index Minerals
Index Minerals Chlorite, biotite, garnet,
kyanite, sillimanite
Need to exist over narrow P-T range
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The Phase Rule in Metamorphic Systems

• Phase rule, as applied to systems at equilibrium
• F C - f 2

f the number of phases in the system C the
number of components the minimum number of
chemical constituents required to specify every
phase in the system F the number of degrees of
freedom the number of independently variable
intensive parameters of state (such as
temperature, pressure, the composition of each
phase, etc.)
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1. Phase Transformations
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The Phase Rule in Metamorphic Systems

• Consider the following three scenarios

• C 1
• f 1 common
• f 2 rare
• f 3 only at the specific P-T conditions of the
  invariant point
• ( 0.37 GPa and 500oC)

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Chemographic Diagrams

• 3- component mineral compositions are plotted on
  a triangular chemographic diagram as shown in
  Fig. 24-2

Plot the following x, y, z, xz, xyz, and yz2
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• Note that this subdivides the chemographic
  diagram into 5 sub-triangles, labeled (A)-(E)

• x-xy-x2z
• xy-xyz-x2z
• xy-xyz-y
• xyz-z-x2z
• y-z-xyz

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Chemographic Diagrams

• Valid compatibility diagram must be referenced to
  a specific range of P-T conditions, such as a
  zone in some metamorphic terrane, because the
  stability of the minerals and their groupings
  vary as P and T vary

• At different grades the diagrams change
• Other minerals become stable
• Different arrangements of the same minerals
  (different tie-lines connect different coexisting
  phases)

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Below the isograd
Bulk rock composition
A B ? C D
At the isograd
Above the isograd
This is called a tie-line flip, and results in
new groupings in the next metamorphic zone
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• A diagram in which some minerals exhibit solid
  solution

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• If bulk rock composition (Xbulk) on a tie-line

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• Xbulk in 3-phase triangles F 2 (P T) so Xmin
  fixed

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Chemographic Diagrams for Metamorphic Rocks

• Most common natural rocks contain the major
  elements SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO,
  MnO and H2O such that C  9
• Three components is the maximum number that we
  can easily deal with in two dimensions
• What is the right choice of components?
• Some simplifying methods

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• The ACF Diagram

• The three pseudo-components are all calculated on
  an atomic basis
• A Al2O3 Fe2O3 - Na2O - K2O
• C CaO - 3.3 P2O5
• F FeO MgO MnO

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• A typical ACF compatibility diagram, referring to
  a specific range of P and T (the kyanite zone in
  the Scottish Highlands)

Green area is bulk composition of metabasaltic
rocks
What are the common assemblages for kyanite zone
(amphibolite facies)??